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02-03-2014, 01:48 AM   #1
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Is there a way to get full field of view with APS-C camera?

Just wondering. Is there an adapter out there that "compresses" the image circle of a full frame lens down to APS-C size, so that the wide angle still is wide angle? I recently bought a lovely Pentax-F 28mm f2.8 on ebay, but on my Pentax K-30 it is not really a wide angle lens (but still very useful even indoors).

02-03-2014, 01:53 AM   #2
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You are talking about a focal reducer. There are technical trouble with designing one for dSLRs, because mirror gets in the way of lens elements. Apparently it needs to sit pretty close to the sensor. I am sure it's possible to design one*, but probably impractical (at least that's why Brian Caldwell, designer of Speed Booster said). Your best bet would be to use a Sony Nex with a speed booster (better, more expensive) or lens turbo (cheaper, probably worse).

*(for example have a huge element that projects fully telecentric image from far away. You might need to enlarge the lens mount though, which is infeasible).
02-03-2014, 02:32 AM   #3
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Focal reducers (FR) are well known in astrophotography with telecopes. Here, the FR is basically a positive lens with a focal lenght typically around 200-300 mm, reducing the effective focal length of the telescope by some 0.5X to 0.7X.

See for example here, Focal Reducers: Useful Formulae

The reason that this works with telescopes is that one usually has an ample amount of back-focus (a long draw tube) that allows the sensor to be brought closer to the telescope lens. This is not the case with a DSLR with its fixed registration distance.

IN THEORY one could do the trick by means of a couple of lens groups that would 1) reduce the size of the image formed by the camera lens while projecting the image further away from the lens mount and 2) reversing the image and thus projecting the image even further away.

See for example here, Eyepiece Projection: Useful Formulae

But, as already said by rrstuff, that would most likely be impractical.
02-03-2014, 02:33 AM   #4
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Well, I hadn't thought about the mirror! That must make things more complicated ...
Oh, I'll just have to wait for a while, no budget for mirrorless camera now.

02-03-2014, 02:44 AM   #5
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QuoteOriginally posted by Stone G. Quote
But, as already said by rrstuff, that would most likely be impractical.

It is :/. I talked to Brian Caldwell about making one for K-mount but he said they don't know how to go about it.
02-03-2014, 06:29 AM - 1 Like   #6
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Yeah, 28mm is sort of close to the 50mm view on a FF camera. A nice view, IMHO but really very wide. Think of it as "pre-cropped."
You'll need to get down past the 20mm barrier for that.

The good news is a DA16-45 is an inexpensive way to get this wider end.
02-03-2014, 07:36 AM   #7
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QuoteOriginally posted by kristaps Quote
I recently bought a lovely Pentax-F 28mm f2.8 on ebay, but on my Pentax K-30 it is not really a wide angle lens (but still very useful even indoors).
The bright side of it is that 28mm is the "perfect normal" (equivalent to 43mm) on APS-C, just like FA43 is on FF. It would be great if Pentax had a compact 28mm lens in their current lineup.
02-03-2014, 08:28 AM   #8
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QuoteOriginally posted by gazonk Quote
The bright side of it is that 28mm is the "perfect normal" (equivalent to 43mm) on APS-C, just like FA43 is on FF. It would be great if Pentax had a compact 28mm lens in their current lineup.


Yes, I've been wondering about that, too. The F and FA 28mm's are not exactly pancakes, but still quite compact, and very useful for APS-C format. Why don't they at least maintain the FA 28mm in the line-up like they have the 35mm and 50mm FA's?

02-03-2014, 08:39 AM   #9
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QuoteOriginally posted by kristaps Quote
Why don't they at least maintain the FA 28mm in the line-up like they have the 35mm and 50mm FA's?
I'm pretty sure the 35/2 is discontinued. Any that come up for sale are old stock, like the FA 20-35 or DA 16-45.
02-03-2014, 08:47 AM - 1 Like   #10
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QuoteOriginally posted by kristaps Quote
Just wondering. Is there an adapter out there that "compresses" the image circle of a full frame lens down to APS-C size, so that the wide angle still is wide angle? I recently bought a lovely Pentax-F 28mm f2.8 on ebay, but on my Pentax K-30 it is not really a wide angle lens (but still very useful even indoors).
I guess you didn't notice, many FF cameras come with 24 to70 type lenses, the Pentax comes with an 18-55. What you are asking for is done in lens selection. Sounds like you might need a DA 10-17 or a Sigma 8-16 if you're looking for ultra wide.

No adapters needed, just a reprogramming of your brain that will come as you use the camera and learn to understand each lens. Something you have to do with every camera and every lens anyway. SO start shooting and quit thinking.

The advantage to this approach when you are shooting with a lens like the 28, is, you're always shooting with the strongest part of the lens and cropping off the weak edges and vignetting, and can see the results right in your viewfinder.
02-03-2014, 09:01 AM   #11
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QuoteOriginally posted by kristaps Quote
Yes, I've been wondering about that, too. The F and FA 28mm's are not exactly pancakes, but still quite compact, and very useful for APS-C format. Why don't they at least maintain the FA 28mm in the line-up like they have the 35mm and 50mm FA's?
They have a very crowded lineup between the FA31 and the two 40s, but it would indeed be nice to have a compact, lightweight 28mm lens, although sometimes I wonder if a 24mm would fit me better. Then we're getting close to the DA21 again, but so far I've not been so impressed by that lens…

Of course now they suddenly have something else in that range, the DA20-40, which is very tempting too, but I'm not so sure if I want that instead of a compact prime. I think a really compact 24mm lens would serve me well. It could even be f/2.8 as my old Vivitar 24mm is (a quite nice lens, but the focussing is worn out, almost impossible to use).
02-03-2014, 09:31 AM   #12
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The Tamron 17-50 2.8 is probably what you're looking for.
02-03-2014, 11:19 AM   #13
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They do make such devices for mirrorless cameras with a shorter flange distance. The Speedbooster and Lens Turbo will allow you to use your full frame lenses at approximately their native field of view on an APS-C camera like the Sony NEX series. I have the cheaper Lens Turbo. There is some loss of sharpness, but they mostly do what they advertise.
02-03-2014, 12:43 PM   #14
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Does metabones plan to bring their speedbooster to other mounts or is there something about it that limits the usefulness to only mirrorless cameras?
02-03-2014, 01:14 PM   #15
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QuoteOriginally posted by mikemike Quote
Does metabones plan to bring their speedbooster to other mounts or is there something about it that limits the usefulness to only mirrorless cameras?
Focal Flange Distance.

In order to achieve infinity focus on a DSLR you would have to have optical correction adding more complexity and reducing IQ even more.
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