# RF24-70 & RF15-35 Reviews



## LesC (Sep 26, 2019)

Digital Camera world have just posted reviews. Seems both are good but edge sharpness could be better ...









Canon RF 24-70mm f/2.8L IS USM review


The EOS R's long-awaited standard zoom is here. Is the Canon RF 24-70mm good enough to win over the naysayers?




www.digitalcameraworld.com













Canon RF 15-35mm f/2.8L IS USM review


Canon’s EOR R system gets a pro-spec constant-aperture f/2.8 ultra-wide zoom – it's a must-have lens for pros, but at a price




www.digitalcameraworld.com


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## Viggo (Sep 26, 2019)

So both new RF zoom’s have average edge sharpness? Coming from the 50 and 85 and all the noise around those releases I’m disappointed the zooms aren’t as mind blowing (compared to their EF counterpart, not the primes.)


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## Maximilian (Sep 26, 2019)

Truely somewhat disappointing. Esp. as they match the results of TDP.
Not bad - but not as good as the RF primes seemed to have promised. 
(And the EF zoom counterparts, although IS is included now).


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## SecureGSM (Sep 26, 2019)

according to reviewers, the 24-70 lens is a very quiet focusing one and proved to be parfocal.
a parfocal 24-70 FF video ready Canon L zoom at RRP $2,200.00? ok. I see some logic here.


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## mjg79 (Sep 26, 2019)

I think we might really be now well into the law of diminishing returns. The EF 24-70 2.8L II was such an amazing lens, the equal of a whole range of f/2.8 primes that to expect a big step forward in terms of sharpness was unlikely. Consider that since it was released Nikon released the 24-70/2.8VR, the (mirrorless) 24-70/2.8S and Sony released the (mirrorless) 24-70GM, while Tamron and Sigma both put out highly regarded 24-70 2.8s... And not a single one of them is as sharp as the EF Canon lens. SLR or mirrorless, it doesn't seem to make much difference with the size/quality of 24-70s.

Then when one considers that the new lens has IS and how that must complicate lens design and layout then perhaps hoping for more was unrealistic. I have the EF 24-70 2.8L II on the adapter to my R and it works flawlessly so given that it seems there won't be any huge optical improvements (though the closer minimum focus distance would be nice) I will likely wait a while for prices on the RF to fall. If you need IS then I think buying the RF lens you would be unlikely to be disappointed especially as the appearance now of an entire holy trinity with IS make me wonder if Canon will bring IBIS anytime soon. If they were planning to then I would expect the 15-35 and 24-70 to both be without IS.

As for the RF 15-35L, that will be the more investing one to see as more tests come in. The very low distortion at 15mm is welcome. Probably the key question for many will be, given it is a sharp lens, what is the vignetting like? That was the achilles' heel of the EF 16-35 2.8L III and in particular the area where the Sony 16-35GM beat it. If the RF 15-35L is much better in that regard it may well be a good choice for astro use, the larger size being a reasonable trade off for the extra millimetre in focal length.

One final thing I'll add is that Canon clearly regards 2.8 zooms as their professional bread and butter. Things like autofocus performance, build quality, repairability etc will all factor as high as image quality but don't tend to feature in the same way in most reviews. I am confident they will get such lenses right.


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## AlP (Sep 26, 2019)

I wouldn't take the two reviews cited above as a reference for judging the performance of these lenses. There isn't a lot of information about the methodology used for sharpness measurements, so it could well be that the results depend on image processing. It might also well be that vignetting affects the measurement. It is surprising that center sharpness reaches its maximum at f/4, while edge sharpness reaches its maximum at f/11 (!) for the 15-35 at 15 mm. Most recent Canon designs (or maybe even all of them) didn't need that much stopping down to reach peak performance even in the corners, and it would be very surprising if that changed with the RF lenses. Something is a bit off in my opinion.

Also, the TDP results for the 15-35 do not look disappointing to me. The comparison with the EF lenses tested on a 5 DSR is difficult, as confirmed by Bryan Carnathan in his comments to the review. He also suggests comparing it to the RF 85 tested on the EOS R. The comparison is then between a UWA zoom lens and a 85 prime (possibly the best one available), and that doesn't look too bad.

Another aspect is distortion: The 15-35 has significantly lower distortion than all EF 16-35 designs, and it has also lower distortion than the EF 24-70 in the overlapping focal range. Software algorithms are getting better and better, but I'd argue that having slightly less edge sharpness but significantly lower distortion might still lead to better detail in the corners than with a lens requiring a significant amount of distortion correction in post.

There is a video "review" of the 15-35 here: 



It's not a scientific test, but it compares the RF and EF lenses in real-world situations and there are some interesting aspects: The 15-35 seems to be only slightly less sharp at 15 mm than the EF 16-35 III at 16 mm, but at 15 mm there will be "fewer pixels available for the same amount of information", meaning that with identical sharpness a 15 mm lens would look worse than a 16 mm one on the same sensor, if the 15 mm image is cropped such that it covers the same fov. So, it's possible that the two lenses would have the same sharpness at 16 mm.
What also seems to be good is coma performance. There's just one image, so again not a scientific test, but the corner performance of the RF lens at 15 mm looks at least as good as the EF 16-35 at 16 mm if not better. If that turns out to be true it would be great!
Vignetting is difficult to judge, but it doesn't seem to be worse than the EF lens. That one is rather bad concerning vignetting though, so this might be a less than optimal result. But then, the 15-35 lens allows the use of front filters and has a not so critically exposes front element while other <16 mm f/2.8 UWA zooms with less vignetting wouldn't.


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## YuengLinger (Sep 26, 2019)

Do engineering changes needed to accommodate IS affect image quality?


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## SecureGSM (Sep 27, 2019)

mjg79 said:


> I think we might really be now well into the law of diminishing returns. The EF 24-70 2.8L II was such an amazing lens, the equal of a whole range of f/2.8 primes that to expect a big step forward in terms of sharpness was unlikely. Consider that since it was released Nikon released the 24-70/2.8VR, the (mirrorless) 24-70/2.8S and Sony released the (mirrorless) 24-70GM, while Tamron and Sigma both put out highly regarded 24-70 2.8s... And not a single one of them is as sharp as the EF Canon lens. SLR or mirrorless, it doesn't seem to make much difference with the size/quality of 24-70s.
> 
> Then when one considers that the new lens has IS and how that must complicate lens design and layout then perhaps hoping for more was unrealistic. I have the EF 24-70 2.8L II on the adapter to my R and it works flawlessly so given that it seems there won't be any huge optical improvements (though the closer minimum focus distance would be nice) I will likely wait a while for prices on the RF to fall. If you need IS then I think buying the RF lens you would be unlikely to be disappointed especially as the appearance now of an entire holy trinity with IS make me wonder if Canon will bring IBIS anytime soon. If they were planning to then I would expect the 15-35 and 24-70 to both be without IS.
> 
> ...


A slight correction: Sigma 24-70 Art isn’t all that hot. CA levels are excessive to say the least. Tamron 24-70 G2 and 70-200 G2 are a very compelling proposition though.


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## Maximilian (Sep 27, 2019)

YuengLinger said:


> Do engineering changes needed to accommodate IS affect image quality?


I suppose yes.
At least the additional optical elements of the IS are additional glass in the path of light.
So more possibilities to lower the IQ.

Do I remenber right that this was an argument from Canon to release the EF24-70/2.8 II and not an IS version?
Or was this just forum talk?


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## YuengLinger (Sep 27, 2019)

Maximilian said:


> I suppose yes.
> At least the additional optical elements of the IS are additional glass in the path of light.
> So more possibilities to lower the IQ.
> 
> ...



It gets harder for me to track down things I think I remember...


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## bhf3737 (Sep 27, 2019)

YuengLinger said:


> Do engineering changes needed to accommodate IS affect image quality?


IS usually requires a couple of seconds to kick in. So it has negative effect if shooting before IS settles down. 
From what I know, the IS mechanism may cause a slight blur due to added elements bending the light but it is much lesser than what is caused by the motion of camera and lens for longer focal lengths. So it reduces the "overall" blur.


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## koenkooi (Sep 27, 2019)

bhf3737 said:


> IS usually requires a couple of seconds to kick in. So it has negative effect if shooting before IS settles down.
> From what I know, the IS mechanism may cause a slight blur due to added elements bending the light but it is much lesser than what is caused by the motion of camera and lens for longer focal lengths. So it reduces the "overall" blur.



On R and M cameras IS is always running in live view, it only shuts down during playback and entering the menus.


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## BillB (Sep 27, 2019)

Maximilian said:


> I suppose yes.
> At least the additional optical elements of the IS are additional glass in the path of light.
> So more possibilities to lower the IQ.
> 
> ...


I do remember something about Canon saying IS wasn't a high priority for 24-70 when they did an assessment of likely users, compared to resolution, weight and I don't know what else, but that would have been a while ago now. It is true that IS doesn't help a lot if you have moving subjects or the camera is on a tripod.


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## Viggo (Sep 27, 2019)

koenkooi said:


> On R and M cameras IS is always running in live view, it only shuts down during playback and entering the menus.


My R kept the IS running also in menus.


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## YuengLinger (Sep 27, 2019)

bhf3737 said:


> IS usually requires a couple of seconds to kick in. So it has negative effect if shooting before IS settles down.
> From what I know, the IS mechanism may cause a slight blur due to added elements bending the light but it is much lesser than what is caused by the motion of camera and lens for longer focal lengths. So it reduces the "overall" blur.



This does make sense; it's complex. I wonder if some focal lengths might be more problematic, or if implementation and tolerances might make for better or worse IS.

Just from my limited experience, on the ef 70-200mm's, and on the ef 100-400mm II, the IS seems amazing. And I'm really impressed by the older ef 100mm f/2.8L IS. Maybe it's just my copy, but the IS on the ef 85mm 1.4L helps, but sometimes seems to make the AF miss in odd ways. (Looking through the OVF when IS engages, I see the smoothing and steadying right away, but the pics never quite live up to expectations.)

The ef 16-35mm f/4L IS works great too, but the focal-lengths, for still photos, don't really need all that much help.

Interestingly, the RF 50mm f/1.2, combined with the EOS R's Servo AF, produce astounding results with no IS in the mix, even at slower shutter speeds. My theory here is that the Servo AF is so good it acts almost like IS for the slight forward and backward swaying or shaking that would otherwise cause havoc with the shallow DoF, but of course it does nothing for lateral movement. The balance of the lens and camera also have something to do with it, and it helps that the lens is so sharp to begin with. But I am getting better results with Servo AF than with One Shot.


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## Random Orbits (Sep 27, 2019)

YuengLinger said:


> This does make sense; it's complex. I wonder if some focal lengths might be more problematic, or if implementation and tolerances might make for better or worse IS.
> 
> Just from my limited experience, on the ef 70-200mm's, and on the ef 100-400mm II, the IS seems amazing. And I'm really impressed by the older ef 100mm f/2.8L IS. Maybe it's just my copy, but the IS on the ef 85mm 1.4L helps, but sometimes seems to make the AF miss in odd ways. (Looking through the OVF when IS engages, I see the smoothing and steadying right away, but the pics never quite live up to expectations.)



The 85 experience you're describing... is that on the DSLR or the R?


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## YuengLinger (Sep 27, 2019)

Random Orbits said:


> The 85 experience you're describing... is that on the DSLR or the R?



On both. Same results, 5DIV and R. AFMA is fine on the dSLR, seems to work just fine on the R, but just never get the expected IQ at any shutter speed. Maybe just my expectations and not the IS, but, like I said, I don't think the IS is as good as on other lenses. We are talking one copy here, one photographer.

Yes, my keeper rate was a slightly higher than on the 85 1.2L II which I sold, but I think my technique was pretty good with the older lens. (Wish I could say the same about my technique with the 135mm f/2!)

Maybe I should have sent it to CPS for a check, but they seem to have a fairly easy going attitude about what is "in spec."


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## mjg79 (Sep 27, 2019)

SecureGSM said:


> A slight correction: Sigma 24-70 Art isn’t all that hot. CA levels are excessive to say the least. Tamron 24-70 G2 and 70-200 G2 are a very compelling proposition though.



Well that is true, though a few people, notably Dustin Abbott, have praised its rendering you're quite right it falls short in measurable terms. Which again I feel re-inforces that the EF 24-70 2.8L II was just one of those lenses that set a new standard and somehow found something special previously missed. I don't say it as a Canon fanboy either. Probably the best example from recent years of such a lens was Nikon's 14-24. I wasn't alone in using it adapted to a Canon SLR back when Canon offered very little with high quality in the wide angle arena. All these years later and Sigma brings out a 14-24 2.8 Art that isn't really that much different. Canon never did bring one out a 14-24 2.8 and Tamron only got to 15mm.

Every now and then you get those kind of lenses that it takes years for others to catch up and I think the EF 24-70 2.8L II was just such a lens.


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## privatebydesign (Sep 28, 2019)

The Nikon 14-24 is a dogs breakfast of a lens, at the time the Canon option was the 15mm 2.8 fisheye defished, it has better image quality and a wider fov. Now the 11-24 at 11 comfortably outperforms the Nikon at 14. The poor Nikon always made far to many compromises to be an f2.8 at which it is basically unusable if anything is outside the center of the image. 



mjg79 said:


> Well that is true, though a few people, notably Dustin Abbott, have praised its rendering you're quite right it falls short in measurable terms. Which again I feel re-inforces that the EF 24-70 2.8L II was just one of those lenses that set a new standard and somehow found something special previously missed. I don't say it as a Canon fanboy either. Probably the best example from recent years of such a lens was Nikon's 14-24. I wasn't alone in using it adapted to a Canon SLR back when Canon offered very little with high quality in the wide angle arena. All these years later and Sigma brings out a 14-24 2.8 Art that isn't really that much different. Canon never did bring one out a 14-24 2.8 and Tamron only got to 15mm.
> 
> Every now and then you get those kind of lenses that it takes years for others to catch up and I think the EF 24-70 2.8L II was just such a lens.


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## Del Paso (Sep 28, 2019)

YuengLinger said:


> This does make sense; it's complex. I wonder if some focal lengths might be more problematic, or if implementation and tolerances might make for better or worse IS.
> 
> Just from my limited experience, on the ef 70-200mm's, and on the ef 100-400mm II, the IS seems amazing. And I'm really impressed by the older ef 100mm f/2.8L IS. Maybe it's just my copy, but the IS on the ef 85mm 1.4L helps, but sometimes seems to make the AF miss in odd ways. (Looking through the OVF when IS engages, I see the smoothing and steadying right away, but the pics never quite live up to expectations.)
> 
> ...



I can only confirm!
I've been using the 100-400 quite often at speeds like 1/30th sec. at 400 mm with success! (and some failures, of course...)
Canon's IS works extremely well, and, as you wrote, servo AF is indeed a great help, also for "windy" macro shots, not speaking of hurricanes, obviously!
PS: I'm in love with the 100-400 , it's become along with the 35 mm Leica M my most used lens!


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## SecureGSM (Sep 29, 2019)

mjg79 said:


> Well that is true, though a few people, notably Dustin Abbott, have praised its rendering you're quite right it falls short in measurable terms. Which again I feel re-inforces that the EF 24-70 2.8L II was just one of those lenses that set a new standard and somehow found something special previously missed. I don't say it as a Canon fanboy either. Probably the best example from recent years of such a lens was Nikon's 14-24. I wasn't alone in using it adapted to a Canon SLR back when Canon offered very little with high quality in the wide angle arena. All these years later and Sigma brings out a 14-24 2.8 Art that isn't really that much different. Canon never did bring one out a 14-24 2.8 and Tamron only got to 15mm.
> 
> Every now and then you get those kind of lenses that it takes years for others to catch up and I think the EF 24-70 2.8L II was just such a lens.


the Sigma 14-24/2.8 --is a great lens wide open. the issue: it suffers from a substantial focus shift once stopped down to F4 and beyond. yes, one can notice that even with such a deep DoF at super wide focal lengths. I was going to obtain one for my low light crowd shots.. 14mm sounds tempting.. but.. why, SIgma, why...


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## wockawocka (Sep 30, 2019)

I can't see myself moving to these RF zooms in a hurry when the EF versions are just as sharp, lighter, cheaper and aren't focus by wire.


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## AlP (Oct 2, 2019)

TDP now also published vignetting measurements for the 15-35.

At 15 mm and f/2.8 it looks a bit better than the EF 16-35 at 16 mm and f/2.8 (https://www.the-digital-picture.com...sComp=1073&CameraComp=979&FLIComp=0&APIComp=3)

At the other focal lengths the RF lens has more vignetting than the EF lens. And, the situation doesn't improve that much when stopping down.

Looks like low vignetting wasn't the top priority for this design.


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## LesC (Oct 6, 2019)

These customers seem very happy with the RF15-35 ...








Canon RF 15-35mm f/2.8L IS USM Lens


Buy Canon RF 15-35mm f/2.8L IS USM Lens featuring RF-Mount Lens/Full-Frame Format, Aperture Range: f/2.8 to f/22, Three Aspherical Elements, Two Ultra-Low Dispersion Elements, Air-Sphere and Fluorine Coatings, Nano USM AF System, Optical Image Stabilizer, Customizable Control Ring, Rounded...




www.bhphotovideo.com


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## Del Paso (Oct 8, 2019)

LesC said:


> These customers seem very happy with the RF15-35 ...
> 
> 
> 
> ...



Could there be a discrepancy between real world pictures and sterile MTF/chart pictures?
Leica M lenses almost always disappoint in "objective" tests, but convince when taking photographs...
Quote from Leica Camera: "Lenses are made for taking pictures, not to photograph charts..."


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## Michael Clark (Oct 9, 2019)

YuengLinger said:


> Do engineering changes needed to accommodate IS affect image quality?



Lens based IS works by causing an intentional (slight) misalignment of the IS element/group. So yeah, it does affect optical image quality.

In a different way, IBIS moves the center of the imaging sensor away from the center of the lens's optical axis, so edge performance on one side will improve, but on the other side it will degrade.

Alas, TANSTAAFL.


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## Michael Clark (Oct 9, 2019)

Viggo said:


> So both new RF zoom’s have average edge sharpness? Coming from the 50 and 85 and all the noise around those releases I’m disappointed the zooms aren’t as mind blowing (compared to their EF counterpart, not the primes.)





Maximilian said:


> Truely somewhat disappointing. Esp. as they match the results of TDP.
> Not bad - but not as good as the RF primes seemed to have promised.
> (And the EF zoom counterparts, although IS is included now).





mjg79 said:


> Well that is true, though a few people, notably Dustin Abbott, have praised its rendering you're quite right it falls short in measurable terms. Which again I feel re-inforces that the EF 24-70 2.8L II was just one of those lenses that set a new standard and somehow found something special previously missed. I don't say it as a Canon fanboy either. Probably the best example from recent years of such a lens was Nikon's 14-24. I wasn't alone in using it adapted to a Canon SLR back when Canon offered very little with high quality in the wide angle arena. All these years later and Sigma brings out a 14-24 2.8 Art that isn't really that much different. Canon never did bring one out a 14-24 2.8 and Tamron only got to 15mm.
> 
> Every now and then you get those kind of lenses that it takes years for others to catch up and I think the EF 24-70 2.8L II was just such a lens.





Del Paso said:


> Could there be a discrepancy between real world pictures and sterile MTF/chart pictures?
> Leica M lenses almost always disappoint in "objective" tests, but convince when taking photographs...
> Quote from Leica Camera: "Lenses are made for taking pictures, not to photograph charts..."



Lens design is always a compromise.

Do you want a lens that can do better on the edges shooting flat test charts at the expense of pleasing out of focus areas when taking photos of a 3D world?

Or would you rather leave a little of the field curvature and spherochromatism uncorrected/undercorrected so that the bokeh is smooth and creamy?


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## Viggo (Oct 9, 2019)

Michael Clark said:


> Lens design is always a compromise.
> 
> Do you want a lens that can do better on the edges shooting flat test charts at the expense of pleasing out of focus areas when taking photos of a 3D world?
> 
> Or would you rather leave a little of the field curvature and spherochromatism uncorrected/undercorrected so that the bokeh is smooth and creamy?


I’ll take both please, both my RF’s does both really, really well. And one of the reasons I really disliked the 50 L is field curvature, calibrated center focusing point was way frontfocused at the edges. And sharpness in the corners is a lot more important than bokeh for uwa imo  Another argument for the excellent corner sharpness of for example the RF85 is that combined with the AF-point spread of the R, makes for an incredible freedom when composing.


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## Maximilian (Oct 9, 2019)

Michael Clark said:


> Maximilian said:
> 
> 
> > Truely somewhat disappointing. Esp. as they match the results of TDP.
> ...


As I stated in my post you've quoted I compare a new lens design to its predecessor or similar lenses (EF vs. RF).
And I want a NEW one to be BETTER or at least AS GOOD as those. (or to be cheaper)
And the IQ improvement of the RF primes (compared to the EF counterparts) made me (and other people) hope that the zooms would continue that.
So somewhat disappointing - but still not bad.


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## Michael Clark (Oct 10, 2019)

Viggo said:


> I’ll take both please, both my RF’s does both really, really well. And one of the reasons I really disliked the 50 L is field curvature, calibrated center focusing point was way frontfocused at the edges. And sharpness in the corners is a lot more important than bokeh for uwa imo  Another argument for the excellent corner sharpness of for example the RF85 is that combined with the AF-point spread of the R, makes for an incredible freedom when composing.



Sometime you can't have both, at least not in the same measure as one can have either one or the other with different designs.

That's why Zeiss makes several series of prime lenses in some of the same focal lengths and maximum apertures. Otus? Planar? Batis? Milvus? 

They all have different characteristics for different purposes.

50mm isn't UWA unless you're shooting LF, in which case you need an image circle between 3.75X and 8X larger than that cast by the EF 50mm f/1.2L.

If you use the center AF point, it will focus the center of the lens' FoV at the center. Even with field curvature, if you use the far left AF point, it will focus the portion of the lens' FoV under the far left AF point on whatever is visible at that point in the lens' FoV and the lens will be focused slightly further than that at the center of the FoV.


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## Michael Clark (Oct 10, 2019)

Maximilian said:


> As I stated in my post you've quoted I compare a new lens design to its predecessor or similar lenses (EF vs. RF).
> And I want a NEW one to be BETTER or at least AS GOOD as those. (or to be cheaper)
> And the IQ improvement of the RF primes (compared to the EF counterparts) made me (and other people) hope that the zooms would continue that.
> So somewhat disappointing - but still not bad.



You compared them only in terms of how well they image a flat test chart. Not everyone wants to make their mark by being the best flat test chart photographer around.


Some lenses that are "soft" in the corners are actually "sharp" in the corners when the focus is adjusted to account for field curvature.

This actually comes in handy in a few applications, particularly portrait photography when fabrics have a strong weave pattern.

There's nothing like using a flat field macro lens for a head + half torso shot of a model in a "shoulder forward pose" with her face perfectly in focus and the spot on her shoulder down in the corner of the frame that is the exact same distance from the camera as her left eye screaming "LOOK AT ME!" because the weave of the fabric at that spot is sharper than the fabric on the other parts of her garment that are further away than the focus point. Use a lens like the EF 50mm f/1.2L and the field curvature pulls the field of focus just enough in front of that forward shoulder near the edge of the frame that it doesn't distract from her face.


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## SteveC (Oct 10, 2019)

Well, let's say you _are_ taking a picture of something flat, e.g., artwork in a museum. I would think then you'd want to (after assuring you're perpendicular with the subject) focus halfway to the side, halfway between the center and the edge. That way the center is slightly out of focus and so are the edges, but nothing is more than slightly out of focus (hopefully less so than you'd need pixel peeping to see). (Obviously you'll benefit greatly from a telephoto at higher distance.) If you were to instead focus on the center, it will be sharp, but the edges will be _at least_ twice as much out of focus as they were in the first case. Likewise focusing on an edge will make the edge clear, but the center will be well out of focus. If your camera is centered on the picture however, at least the opposite edge should be in focus again. (I assume as I write this that the focus "plane" is actually part of a sphere.)


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## LesC (Oct 10, 2019)

RF15-35 Review from Brendan van Son:


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## Maximilian (Oct 10, 2019)

Michael Clark said:


> You compared them only in terms of how well they image a flat test chart.


You're right  I didn't think about vigneting ot flare or anything else - not. It's the summary.



> Not everyone wants to make their mark by being the best flat test chart photographer around.
> ...


You're right 
Everyone wants to put subject into the center and then reframe in post by croping.
Nobody want's to use corner sharpness to compose according to the golden rule or else.
Everything has to be centralized

/sarc mode


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## Michael Clark (Oct 11, 2019)

Maximilian said:


> You're right  I didn't think about vigneting ot flare or anything else - not. It's the summary.
> 
> 
> You're right
> ...



You still fail to understand. Many of those lenses *are* sharper in the corners and on the edges than an ignorant perusal of the image of a flat test chart made when the lens is focused at the center of the test chart would lead an uninformed viewer to believe.

If one is concerned with focusing a subject on the edge of the field of view, why would one focus the center of the lens? Focus the lens so that the subject at the edge of the FoV is in sharpest focus. See how easy that was?

Further, if one is forced to use focus and recompose, a lens with field curvature will introduce *less* error than a lens highly corrected for flat field will.


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## Maximilian (Oct 11, 2019)

Michael Clark said:


> You still fail to understand. Many of those lenses *are* sharper in the corners and on the edges ...


Now I understand your argumentation! Thank you. I hope that you are not only good in explaining but also right when it comes to your assumption on how the lens could be optimized.


> ...
> If one is concerned with focusing a subject on the edge of the field of view, why would one focus the center of the lens?
> 
> Further, if one is forced to use focus and recompose...


Right on the first one but
I was never talking about "focus and recompose".


But that leads to one question:
If you are right here (R&D on field curvature), why do the RF primes look so well on flat field?
I would assume that Canon R&D would take the same primary IQ goals (field curvature sharpness) for lens design, even though primes and zooms are designed differently.


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## SteveC (Oct 11, 2019)

Maximilian said:


> Now I understand your argumentation! Thank you. I hope that you are not only good in explaining but also right when it comes to your assumption on how the lens could be optimized.
> 
> Right on the first one but
> I was never talking about "focus and recompose".
> ...



As I understand it--and someone correct me if I am wrong--a lot of high end lenses actually play games with the optics to make the focal plane wavy rather than spherical. It reduces the error towards the edges but is in no way flat.


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## Michael Clark (Oct 11, 2019)

SteveC said:


> Well, let's say you _are_ taking a picture of something flat, e.g., artwork in a museum. I would think then you'd want to (after assuring you're perpendicular with the subject) focus halfway to the side, halfway between the center and the edge. That way the center is slightly out of focus and so are the edges, but nothing is more than slightly out of focus (hopefully less so than you'd need pixel peeping to see). (Obviously you'll benefit greatly from a telephoto at higher distance.) If you were to instead focus on the center, it will be sharp, but the edges will be _at least_ twice as much out of focus as they were in the first case. Likewise focusing on an edge will make the edge clear, but the center will be well out of focus. If your camera is centered on the picture however, at least the opposite edge should be in focus again. (I assume as I write this that the focus "plane" is actually part of a sphere.)



That's why you use a macro lens highly corrected for field curvature and optimized for close focus distances to do 2D repro work. Or a T/S lens with a much larger image circle (so the "edges" are well out of the frame) as well as having a high degree of flat field correction and the ability to correct for shooting position when perfectly centered/perfectly perpendicular with respect to the target is not an option. As a 2D repro lens, the EF 85mm f/1.8 wipes the floor with the EF 85mm f/1.2L because the latter is not designed to do that job. But that does not mean the latter can't be sharp near the edge of the frame when properly focused to make a subject near the edge of the frame sharp.

The idea that a lens "sharper" on the edges and in the corners when imaging flat test charts is always "sharper" on the edges and in the corners than other lenses that do not have as flat a field of focus is often an incorrect assumption many make.

Here, for instance, is why you sometimes _don't_ want to use a macro lens for 3D, wide aperture portrait work:







Different tools for different jobs.


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## Michael Clark (Oct 11, 2019)

Maximilian said:


> Now I understand your argumentation! Thank you. I hope that you are not only good in explaining but also right when it comes to your assumption on how the lens could be optimized.
> 
> Right on the first one but
> I was never talking about "focus and recompose".
> ...



Every lens is not intended to be used to do 2D reproduction work. Sometimes the way out of focus areas are rendered is considered more important than flat field performance. Highly correcting for field curvature creates a field of focus that looks more like a lasagna noodle than a flat baking sheet. It also has an effect on the other optical characteristics of a lens (such as the way out of focus areas are blurred).


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## Michael Clark (Oct 11, 2019)

SteveC said:


> As I understand it--and someone correct me if I am wrong--a lot of high end lenses actually play games with the optics to make the focal plane wavy rather than spherical. It reduces the error towards the edges but is in no way flat.



Yep. That's what Uncle roger discusses here and here (and more than a few other places).


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## Michael Clark (Oct 11, 2019)

Maximilian said:


> But that leads to one question:
> If you are right here (R&D on field curvature), why do the RF primes look so well on flat field?
> 
> I would assume that Canon R&D would take the same primary IQ goals (field curvature sharpness) for lens design, even though primes and zooms are designed differently.



A well-recognized authority far more knowledgeable than I goes into fairly great detail to explain field of focus at the link in my above comment.

As to why RF primes look so well on flat field:

Because the makers of practically every new lens for the last half decade or so have *sold their souls to the gods of the flat test chart*. Why? Because most of the masses buying the majority of lenses on the market these days are too ignorant to realize that "sharpest" shooting a flat test chart does not always mean "best" for real world usage.


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## Larsskv (Oct 13, 2019)

Michael Clark said:


> A well-recognized authority far more knowledgeable than I goes into fairly great detail to explain field of focus at the link in my above comment.
> 
> As to why RF primes look so well on flat field:
> 
> Because the makers of practically every new lens for the last half decade or so have *sold their souls to the gods of the flat test chart*. Why? Because most of the masses buying the majority of lenses on the market these days are too ignorant to realize that "sharpest" shooting a flat test chart does not always mean "best" for real world usage.


I agree! I for one love the images I get from the EF 50L and the EF 85LII. They make subjects look good in a way few other lenses can.

I sold my EF50L after getting the RF50L, but bought another one after a few months. I absolutely love the RF50L, but it hasn’t made the EF50L obsolete in my camera bag.


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## LesC (Oct 15, 2019)

TDF.com review of the RF15-35. In short, he likes it a lot!









Canon RF 15-35mm F2.8 L IS USM Lens Review


Is the Canon RF 15-35mm F2.8 L IS USM Lens right for you? Learn all you need to know in The-Digital-Picture.com's review!




www.the-digital-picture.com


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## LesC (Nov 20, 2019)

RF24-70 Review:


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## Bundu (Nov 20, 2019)

Not much of a review, but not shooting flat test charts it looks very sharp.


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## Joules (Nov 21, 2019)

Michael Clark said:


> Buse most of the masses buying the majority of lenses on the market these days are too ignorant to realize that "sharpest" shooting a flat test chart does not always mean "best" for real world usage.


To be fair though, it is not just the people who have a hand in this. Reviewers could go through the trouble of finding the focus distance for points away from the center and measure the edge sharpness with subjects at these distances. Than they could publish edge sharpness and curvature measurements that are comparable... But at much higher effort. And is that effort worth it?

If I have subjects that stretch across the frame, like a tree or a group of people, there tends to be a single plane of focus that I would like to be sharp. I wouldn't want the tell the people that they have to align in some half circle, making some of the appear bigger and more important, just so that they don't get blurry.

Do we have an idea of how big the compromise between blur quality and focus flatness is?


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## Michael Clark (Nov 21, 2019)

Joules said:


> To be fair though, it is not just the people who have a hand in this. Reviewers could go through the trouble of finding the focus distance for points away from the center and measure the edge sharpness with subjects at these distances. Than they could publish edge sharpness and curvature measurements that are comparable... But at much higher effort. And is that effort worth it?
> 
> If I have subjects that stretch across the frame, like a tree or a group of people, there tends to be a single plane of focus that I would like to be sharp. I wouldn't want the tell the people that they have to align in some half circle, making some of the appear bigger and more important, just so that they don't get blurry.
> 
> Do we have an idea of how big the compromise between blur quality and focus flatness is?



A well-recognized authority far more knowledgeable than I goes into fairly great detail to explain field of focus at the link in my above comment.

He has also published modified MTF charts for lenses in which the sharpest position (on his lab bench) is used for each position, rather than the position of the lens when the center is sharpest. In fact, he has included such tests and placed them side by side for the same lenses (he rarely tests fewer than ten copies of a particular lens model in order to reduce the influence of random lens-to-lens variation, and he tells us when he has used test results from less than ten copies of a lens model) in the article linked above.


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## Michael Clark (Nov 21, 2019)

Joules said:


> To be fair though, it is not just the people who have a hand in this. Reviewers could go through the trouble of finding the focus distance for points away from the center and measure the edge sharpness with subjects at these distances. Than they could publish edge sharpness and curvature measurements that are comparable... But at much higher effort. And is that effort worth it?
> 
> If I have subjects that stretch across the frame, like a tree or a group of people, there tends to be a single plane of focus that I would like to be sharp. I wouldn't want the tell the people that they have to align in some half circle, making some of the appear bigger and more important, just so that they don't get blurry.
> 
> Do we have an idea of how big the compromise between blur quality and focus flatness is?



To be fair, though, if you "have subjects that stretch across the frame..." you may not want the same characteristics in a lens as folks who do not have subjects that stretch across the frame, but rather occupy one specific part of the frame and are more concerned with how the out of focus areas of the rest of the frame look than how sharp the lens is at the corners/edges when there is nothing but empty air in the corners/edges of the frame at the distance from the camera to which the lens is focused.


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## navastronia (Nov 21, 2019)

Joules said:


> To be fair though, it is not just the people who have a hand in this. Reviewers could go through the trouble of finding the focus distance for points away from the center and measure the edge sharpness with subjects at these distances. Than they could publish edge sharpness and curvature measurements that are comparable... But at much higher effort. And is that effort worth it?
> 
> If I have subjects that stretch across the frame, like a tree or a group of people, there tends to be a single plane of focus that I would like to be sharp. I wouldn't want the tell the people that they have to align in some half circle, making some of the appear bigger and more important, just so that they don't get blurry.
> 
> Do we have an idea of how big the compromise between blur quality and focus flatness is?



Great points, all of them.


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