# DXO Mark review the 28mm f/2.8 IS...



## traveller (Jan 3, 2013)

FWIW:

http://www.dxomark.com/index.php/Publications/DxOMark-Reviews/Canon-EF-28mm-f2.8-IS-USM

I'm puzzled as to how an f/2.8 lens manages to achieve a measured T-stop of 2.7; doesn't that violate the laws of physics? I'm guessing that either Canon are being very conservative with their f/2.8 rating, or DXO Mark has made a cock-up... I wonder which


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## rs (Jan 3, 2013)

Yeah, it does defy the laws of physics. However, DxO are _never_ wrong, so this has to be canon rating their lens incorrectly at f2.8


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## kubelik (Jan 3, 2013)

well, not necessarily a violation of the laws of physics. it could be that Canon actually built a t/2.7 lens and is marketing it as an f/2.8. which would violate the law of "canon shafting us on price with every opportunity that they have", which seems to be a law many abide by here.

it's a question of who do you trust/distrust more, Canon's marketing department, or DxOmark?


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## Zlatko (Jan 3, 2013)

It's a very good lens, as the DxO report shows. High quality in a small package. There is nothing wrong with the lens or its marketing. I don't understand why every thread needs some cynical remarks about marketing.


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## paul13walnut5 (Jan 3, 2013)

Or perhaps it was a typo? F and T are quite close together on the keyboard.

OR PERHAPS T STOP MEANS SOMETHING ENTIRELY DISTINCT FROM F STOP.

EVER THINK OF THAT?

U-S-E G-O-O-G-L-E.

I know how much we love to dxo bash (I prefer to treat them like being at a busy party where an ex from an acrimonious split is also at - don't want to talk, don't want to engage, don't want to know what they are up to) and how much we all think canon is shafting us.

But F stops (mathematical expression of diaphragm entrance pupil) and T stops (actual measured transmittance from realworld sample, this takes into account coatings etc) the f stop and t stop are different things and will most likely change depending on the focus position of the lens.


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## Axilrod (Jan 3, 2013)

paul13walnut5 said:


> Or perhaps it was a typo? F and T are quite close together on the keyboard.
> 
> OR PERHAPS T STOP MEANS SOMETHING ENTIRELY DISTINCT FROM F STOP.
> 
> ...



+1


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## neuroanatomist (Jan 3, 2013)

@ paul13walnut5, I think the point of contention is that the DxOMark measured T-stop is actually 'faster' (letting in more light) than the rated f/stop of the lens. As you point out, the T-stop takes real-world things like coatings into account, but such things cannot add extra light transmission, only reduce it. 

I can't find the 28/2.8 IS patent, but a recent patent for a 24mm f/2.8 lists the actual calculated Fno as 2.86. Almost all of the Canon lens patents have an f/number that is rounded _down_ to the value printed on the lens (but there are exceptions), so a T-stop that's faster than the stated f/number is pretty surprising.


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## paul13walnut5 (Jan 3, 2013)

@neuroanatomist

At what focus setting do DXO measure the T setting?

Do you accept that as a lens focuses the mathematical corelation between physical focal length and aperture diaphragm pupil diameter changes?

If at any point in the focus range the aperture ratio is f2.8 then thats what they have to go with, although even if it were truely a constant f2.7, the f2.8 might better suit marketing, as it's the kind of term spoon fed idiots can relate to in marketing land.

That said Canon do use things like f3.5 and I remember the Tokina / Angenieux 28-70 with the f2.6-f2.8 aperture.

HOWEVER, I think the actual point of contention is that people are too keen to tear apart DXO's musings and any Canon new product, when it's demonstrable that they don't have a clue about the difference between F & T stops, which are distinct, and what the review is referring to.

I know you do, and thanks to my work, I do, but for the most part, folk shooting stills with consumer (or at least non industrial) gear won't have came accross specialist terms like T stop before.

That is my point.


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## neuroanatomist (Jan 3, 2013)

paul13walnut5 said:


> @neuroanatomist
> 
> At what focus setting do DXO measure the T setting?
> 
> Do you accept that as a lens focuses the mathematical corelation between physical focal length and aperture diaphragm pupil diameter changes?



No, I don't know where they focus wneh they measure. Yes, it's true that focusing changes the apparent focal length of the lens (well, not all lenses, but a lack of focus breathing is one reason cine lenses are so expensive). For example, the 100L IS Macro, when focused at 1:1, is giving the equivelent FoV of a 65mm lens. But convention is to state the focal length when focused at infinity, and I would guess that's where DxO tests the T-stop. 

Regardless, the vast majority of DxOMark's lens T-stop measurements come in at or lower than (in terms of light transmission, so a higher number) the rated f/stop of the tested lens.



paul13walnut5 said:


> HOWEVER, I think the actual point of contention is that people are too keen to tear apart DXO's musings and any Canon new product, when it's demonstrable that they don't have a clue about the difference between F & T stops, which are distinct, and what the review is referring to.



While I agree with you, I will point out that DxOMark tested the 28mm f/2.8 IS on 17 bodies - 15 of them show a T-stop of 2.8, one (the 1D X) shows T2.9, and one (the 550D) shows T2.7. The 40mm f/2.8 also shows a T-stop of 2.7 on the 550D, and likewise, it's T2.8 on the other tested bodies. With those very few exceptions, every lens on every body that I looked at shows a T-stop that is equal to or less than (again, in terms of light transmission) the lens' specified f/number (or for zooms, the average f/number across the zoom range). 

So...how would you explain DxOMark's apparent outliers for a couple of f/2.8 lenses tested on one type of body? A DxO mistake? Defiance of the laws of physics? Or maybe they have a magic 550D body that just sees the brighter side of life?


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## paul13walnut5 (Jan 3, 2013)

Could use a sup from that 550ds cup!

Don't know enough about dxos methods to comment, there is however the issue that electronic aperture lenses often have diaphtagms that rest wide open when the lens is unpowered, but stop down very slightly when powered. This means that although the powered aperture creates a max aperture of f2.8 the lens entrance pupil may actually be slightly wider, although the lens is not intended to be used without the blades being closed ever so slightly.

Off the top of my head the old 28-70 f3.5 II did this, as did some of the early push pull tele's.

The key thing to note is that although there is a causal relationship between f and t, they are distinct.

If Dxo measured through the viewfinder then different results could be expected from pentaprism and pentamirror cameras, from different focus screen types and designs, if it's taken from the sensor it would be different than if taken from image data or during live view.

.1 of an f number or t stop? Folk aren't caring about that, its just dxo and canon bashing, but neither is fair, as they are distinct things, one theoretical, one measured.


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## kubelik (Jan 3, 2013)

paul13walnut5 said:


> Or perhaps it was a typo? F and T are quite close together on the keyboard.
> 
> OR PERHAPS T STOP MEANS SOMETHING ENTIRELY DISTINCT FROM F STOP.
> 
> ...



I don't know, it sounded like the OP has a decent basic understanding of the difference between a T-stop and an F-stop, which is why he made the post. generally the rule of thumb is that the T-stop will not exceed the F-stop, since the F-stop is a mathematical calculation and the T-stop measurement involves all sorts of real-world losses.

I'm not sure what set you off exactly, but it seems odd to say that you take the high road when it comes to DxOmark bashing, and then go off bashing a forum poster. I think that neuro is steering this in a more positive direction of: what do DxOmark's T-stop measurements consist of? I'd love to know the precise methodology (I did try to look it up on their page and it wasn't clear to me) and I'd love to know what technical experts on the forum here have to say about their methodology. maybe DxOmark have revealed that Canon made an awesome f/2.8 lens that actually transmits at T-2.7, that would be awesome. maybe that's not the case, which is what we would normally assume.


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## paul13walnut5 (Jan 3, 2013)

More food for thought, aperture maths depends on it being the diameter of the iris.
But the iris usually isn't a circle, save for waterhouse stop lenses or the likes of cooke lenses.

So the iris octogon, hexagon whatever, the area is going to be different from a circle of the same radius.

Depending on whether you measure the inside or outside edge of the iris shape there is going to be a slight difference, like say f2.7 from an "f2.8" lens.

If lenses didn't breathe, if apertures were always perfectly circular then such INSIGNIFICANT difference would not occur. But that ain't consumer mass produced lenses.

And it isn't a hook on which to hang canon or dxo.


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