I have been wanting something like this for ages (wide angle converter that's behind the lens)

Started Jan 14, 2013 | Discussions thread
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Joseph S Wisniewski
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Making it retrofocus is illegal, LOL
In reply to Gerry Winterbourne, Jan 17, 2013

Gerry Winterbourne wrote:

"I tcan't be done for FF lenses on the K-01". ET has said several times in this thread that it's impossible but repetition doesn't turn wrong into right. Every SLR lens shorter that about 40-50mm naturally focuses in front of the image (film or sensor) plane and needs a retrofocus group to pull it back. Different makes have different flange distances but all manage to get 35mm (say) FL lenses to focus. Optical adapters are already available for Nikon (say) bodies to take lenses from shorter-flange mounts.

It's clear, therefore, that it would be feasible to design a version of this adapter that would include some retrofocus capability to allow for the small projection it would need in front of the body.

There are four problems with doing that.

  1. Kodak has a patent on retrofocus wide converters that they have been loath to license.
  2. Fast retrofocus optics are expensive. Price a 24mm f1.4 or a 35mm f1.4.
  3. Fast retrofocus optics are big. A non-retrofocus 24mm f1.4 can be packed into a cylinder about 30mm long, 25mm wide. The retrofocus Nikon is a good 25x the volume.
  4. Retrofocus stuff is hard past f1.4. Oly incorporated wide converter rear sections in their 14-35mm f2, and even optimizing it for the permanently attached 28-70mm f2.8, they couldn't get it past f2.0. 

"Improved IQ part 1". Steeve says that every piece of glass in the optical train degrades IQ; a few others have said that it's possible for extra glass to improve IQ. There isn't actually a contradiction between these two statements. All of the lenses we use have multiple elements because there's no single piece of glass that can give sharp images across a frame with no aberrations or distortions.

Bullseye! Can't add anything, your logic is flawless.

It takes several pieces of glass with different characteristics, each counteracting the adverse effects of the others, to get decent IQ. Different lenses have different characters because different compromises are made in their designs. Usually the design considers all of the elements working together but it's conceivable that once the optimum IQ (by whatever standards - resolution, flat field or whatever - the designer chose) a single extra element might improve every aspect of IQ across the board.

That is what Brian Caldwell claims. The sensor cover glass, because it has a different index of refraction than air, increases spherical aberration of fast lenses. His design compensates for this.

On the whole, though, I think it's more likely that any improvement in one aspect of the design will be offset by a reduction in one or more others. I think Steeve's idea that in practice there will be loss of IQ is most likely true, with the possibility that there might be a few exceptions where IQ improves.

"Improved IQ part 2". The link claims improved IQ but doesn't go into detail. As I read it, things like resolution in l/mm improve. This seems quite logical, because the lens's natural resolution is being compressed into a smaller space; ditto aberrations. But then we hit the fact that the smaller sensor needs more magnification for the same output picture size.

Improve l/mm by 1.5X and then enlarge by an extra 1.5X and you're back where you started - except that you've go the likely overall degradation that I discussed in part 1.

And the SA reduction.

"DOF". I think the same argument applies here. The way a lenses gives tighter or wider DOF is determined by its optics and what it projects onto. Compress that by 1.5X and then enlarge it by 1.5X and you are back where you started.

Yup. This puts the APS mirrorless systems at parity with FF, nothing more.

This is actually just the same as the normal situation with different print sizes: DOF calculations are based on defined "typical" viewing conditions. Equalise the conditions and shooting parameters and you get the same DOF.

"Faster speed". Yes, if you concentrate a given amount of light - the amount entering the front of the lens - onto a smaller area it looks, and is, brighter. So you can use lower ISO. So far, so good, because lower ISO means less noise; but then magnify the image by 1.5X to get the same picture size and you magnify noise. Hmmm ...

Conclusion. The device obviously works. For ordinary print or screen seizes it gives good results. But I think some of the claims made for it are illusory.

I think the claims stand. I have a great deal of respect for the person making them, and they agree with my own optical design experience.

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Rahon Klavanian 1912-2008. Armenian genocide survivor, amazing cook, scrabble master, and loving grandmother. You will be missed. Ciao! Joseph www.swissarmyfork.com

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