# Making a perfectly sharp lens corner to corner idea



## kphoto99 (Dec 30, 2013)

I was thinking that a FF lens used on crop camera is sharper in the corners because it only sees the centre of the lens. 
The lens designers could make a lens that has a much larger image circle (maybe 1.5x the size of FF), this would make the lens' corners sharp at full aperture on a FF camera.

The drawbacks that I see would be the size (thickness and weight) of the lens, but ignoring that why is this approach not taken?


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## mackguyver (Dec 30, 2013)

kphoto99 said:


> I was thinking that a FF lens used on crop camera is sharper in the corners because it only sees the centre of the lens.
> The lens designers could make a lens that has a much larger image circle (maybe 1.5x the size of FF), this would make the lens' corners sharp at full aperture on a FF camera.
> 
> The drawbacks that I see would be the size (thickness and weight) of the lens, but ignoring that why is this approach not taken?


See the new Zeiss Otus 55 f/1.4 lens. Most people believe that's exactly what they did with this FF design.


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## Marsu42 (Dec 30, 2013)

kphoto99 said:


> The drawbacks that I see would be the size (thickness and weight) of the lens, but ignoring that why is this approach not taken?



Probably the ef mount doesn't allow a larger circle to be projected into the camera body, so you cannot repeat the same "crop" effect, at least not to this extend? If you look at the open ff camera, the sensor edges go right to the mount edges, and the lens goes *into* the mount so it cannot be larger than that.


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## kphoto99 (Dec 30, 2013)

Marsu42 said:


> kphoto99 said:
> 
> 
> > The drawbacks that I see would be the size (thickness and weight) of the lens, but ignoring that why is this approach not taken?
> ...



What if the circle "projects" outside of the mount onto the inside walls of the lens, I hope I'm explaining this correctly.


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## kphoto99 (Dec 30, 2013)

mackguyver said:


> kphoto99 said:
> 
> 
> > I was thinking that a FF lens used on crop camera is sharper in the corners because it only sees the centre of the lens.
> ...


So if Zeiss did this, why would they be the first ones to do it?
I realize that the glass elements would be larger, but they would not need to be ground/polished to the edges, just the flatter part.

This brings up an other idea: How do medium format lenses perform when attached to a FF camera with an adapter, are they then sharp corner to corner?


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## eli452 (Dec 30, 2013)

Marsu42 said:


> Probably the ef mount doesn't allow a larger circle to be projected into the camera body, so you cannot repeat the same "crop" effect, at least not to this extend? If you look at the open ff camera, the sensor edges go right to the mount edges, and the lens goes *into* the mount so it cannot be larger than that.



Isn't that what an TS lens does?


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## rs (Dec 30, 2013)

That is exactly what's already done with the TS-E lenses when they're set to zero shift.

However, this concept is simply a way of using the sweet spot of an acceptable lens. If the calibrated image circle is good, such as with some of the best lenses like the 70-200/2.8 IS II, 100 L Macro, 300/2.8 II, 600/4 II then this issues is a moot point. Also, if a lens is designed for a large imaging circle, making a fast version becomes very expensive. For instance, the 85/1.2 lens on a FF body acts much like a 132/1.9 lens would with a Phase One back. I'm not an expert on medium format, but I couldn't find a single MF lens faster than f1.9 in my look just now. The potential price of an 85/1.2 medium format lens would make the current FF offering seem like a disposable lens.


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## mackguyver (Dec 30, 2013)

Marsu42 said:


> kphoto99 said:
> 
> 
> > The drawbacks that I see would be the size (thickness and weight) of the lens, but ignoring that why is this approach not taken?
> ...


True, but all you have to do is add a mask at the rear of the lens to blackout the additional light. I used to work in motion pictures and film projectors had (or maybe had ) something called an "aperture plate" that would do just that depending on the film aspect ratio (1.85:1 "Flat" or 2.35:1 "Scope").

And I see rs mention TS-E lenses, which is another good example.


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## Sporgon (Dec 30, 2013)

kphoto99 said:


> This brings up an other idea: How do medium format lenses perform when attached to a FF camera with an adapter, are they then sharp corner to corner?



To give you an idea; in the eighties I used the wonderful Pentax 6x7 system. The system was highly regarded in it's time. You could get an adapter to mount the 6x7 lenses into the Pentax 'K' mount for 35mm cameras. 

I tried this with my Pentax MX, and the results were - disappointing !

Larger formats are much more forgiving of the lens yet it is a common belief among small format users that larger format lenses must be better. 

The poster about the TS-E lenses is correct; they have a larger image circle when in the straight position.


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## mackguyver (Dec 30, 2013)

One other related thought I've had - what if they made Metabones Speed Boosters for the EF mount - i.e. Hasselblad, Leaf, etc. to EF. If it works as well as the EF to m4/3, that would be pretty nuts.


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## KBStudio (Dec 30, 2013)

I have mounted three of my old Hasselblad lenses to my Mark II and found that the chromatic aberrations were horrible. The resolution was so so compared to all my Canon lenses, in the center. But because of the "circle of coverage", for 6x6, the image was even from corner to corner. Just not very good. So for expensive medium format Hassie lenses, the adapter was a waste of money. A secondary concern was the weight of the lenses on the lens mount. As there are not tripod mounts on Hassie lenses, the lens mount had to hold all the weight. And these are some heavy lenses. 

Also tested a Nikon adapter for the Canon. Using my old manual focus AI Nikkor lenses, I again discovered that they were not as sharp in the center as the Canons, but they were the same sharpness from corner to corner. No drop off or chromatic aberrations. All the Nikkor lenses were manual focus AI and f2.8 or slower. Not necessarily, Nikkors best. 

But back to the original question, yes. If a lens is designed to throw a "circle of coverage" greater than the diagonal of the format the edges will be sharper. But this is a costly process and may not be profitable enough for Canon.


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## docsmith (Dec 30, 2013)

Look at canon's "CN-E" lenses. Larger front elements. The reason that this isn't done for all lenses is, of course, cost. The larger the front element the more it costs. But this is done, to a lesser extent on lenses available to us. Look at the size of the front element of "L" primes versus their non-L counter point. Grant lot of that is aperture related.


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## candc (Dec 31, 2013)

It's really the optical formula and quality of the optics that make a lens sharp, not the physical size of it. There is no benefit to a lens that projects an image larger than what the sensor see's. You could build a lens that duplicates the center portion of a larger lens and use it on a smaller sensor camera but the outer portion isn't needed.


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## Mt Spokane Photography (Dec 31, 2013)

Actually, some lenses work better on FF, while others are better on crop bodies. I've used MF Hasselblad lenses on my camera, less vignetting, but not sharper.


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## mb66energy (Dec 31, 2013)

mackguyver said:


> kphoto99 said:
> 
> 
> > I was thinking that a FF lens used on crop camera is sharper in the corners because it only sees the centre of the lens.
> ...



IMO your comment about the OTUS is partially right. I am shure that Zeiss did NOT have calculad a MF lens but they just used the Distagon formula which applies a negative front lens(or group) to create a retrofocus design. This allows a longer light path through the lens and gives additional opportunities to correct aberrations.

Usually the Distagon "fprmula" is a retrofocus design used to keep the mirror area clear from lenses to avoid any "conflict". A MF wide angle for a (D)SLR needs the same construction but for different reasons.

The main idea behind the OTUS construction is (again IMHO):
* use a retrofocus design to get a longer light path which can be influenced and
* use different special glasses (i read sth. about 6 different special glass types!)
which can be placed in the longer light path due to the retrofocus design.

To the OP:
A good idea might be to use a MACRO LENS for MF with an adaptor on a FF camera - macro lenses are usually much better in terms of correction than other lenses and give higher resolutions than their "standard" counterparts. As others said: MF lenses have lower spatial resolutions for a larger image circle - the magnificaion in postprocessing is smaller to gain the same "final product size" so there is (was?) an advantage of MF.


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## surapon (Dec 31, 2013)

rs said:


> That is exactly what's already done with the TS-E lenses when they're set to zero shift.
> 
> However, this concept is simply a way of using the sweet spot of an acceptable lens. If the calibrated image circle is good, such as with some of the best lenses like the 70-200/2.8 IS II, 100 L Macro, 300/2.8 II, 600/4 II then this issues is a moot point. Also, if a lens is designed for a large imaging circle, making a fast version becomes very expensive. For instance, the 85/1.2 lens on a FF body acts much like a 132/1.9 lens would with a Phase One back. I'm not an expert on medium format, but I couldn't find a single MF lens faster than f1.9 in my look just now. The potential price of an 85/1.2 medium format lens would make the current FF offering seem like a disposable lens.



+ 100 for me too, Dear Rs.

http://www.oopoomoo.com/2012/03/seven-advantages-of-using-tilt-shift-lenses/

Surapon


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## AprilForever (Dec 31, 2013)

mackguyver said:


> One other related thought I've had - what if they made Metabones Speed Boosters for the EF mount - i.e. Hasselblad, Leaf, etc. to EF. If it works as well as the EF to m4/3, that would be pretty nuts.



Here's to that!!! Excellent point!!!


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