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11-13-2013, 07:37 AM   #1
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Possibly a stupid question...

I recently got excited at the aspect of adapting the strangest lenses on Pentax bodies. Not being a physicist, I don't know what the optical limitations are in all cases, so I apologize if my question seems stupid. Can the image circle of a lense be optically enlarged while retaining the functionality of the lens? For example, if the image circle of a 21mm f/1.4 lens is large enough to cover 12x14mm, can it be enlarged to cover 24x28mm with the lens still functioning as a normal non-macro 21mm f/1.4 (or f/2.8 since the conversion would cause some loss of light) lens? If not, why not?

11-13-2013, 08:32 AM   #2
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In simple terms no. The image circle is not expandable, this does not mean, however that a lens for a smaller format cannot cover a larger one. The Pentax DA line is an example of that. There is a thread somewhere showing the performance of different DA lenses which are only "specified" to be APS-C compatible, shot using a full frame (film) camera.

From personal experience, I have a sigma APO 10-20f4-5.6 EX DC lens. The DC designation means it is APS-C or 16x24mm format, but form 13mm and above, it covers a full 24x36 frame

You would need to test each lens, and may need to accept some vignetting

To expand the image circle would require a teleconverter for example, but then the focal length changes
11-13-2013, 08:36 AM   #3
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QuoteOriginally posted by Lowell Goudge Quote
In simple terms no. The image circle is not expandable, this does not mean, however that a lens for a smaller format cannot cover a larger one. The Pentax DA line is an example of that. There is a thread somewhere showing the performance of different DA lenses which are only "specified" to be APS-C compatible, shot using a full frame (film) camera.

From personal experience, I have a sigma APO 10-20f4-5.6 EX DC lens. The DC designation means it is APS-C or 16x24mm format, but form 13mm and above, it covers a full 24x36 frame

You would need to test each lens, and may need to accept some vignetting

To expand the image circle would require a teleconverter for example, but then the focal length changes
But if the image circle covers a larger format without vignetting, wouldn't that mean that the image on a smaller format is heavily cropped? Otherwise how can the same image circle cover both with the same content? it can't, right? Interesting that above a certain focal length it covers FF. How would one know if the required TC would be 1.2x, 1.4x or 2x? by trying different TCs? the change in the focal length is unfortunate but with the right lens one could actually get a shorter focal length that would end up being the right focal length after the TC... that would still leave barrel distortion probably. complicated!
11-13-2013, 09:13 AM   #4
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QuoteOriginally posted by tripodquest Quote
But if the image circle covers a larger format without vignetting, wouldn't that mean that the image on a smaller format is heavily cropped?
Yes, that's why someone invented the term crop factor. And because of that, a 50mm on an APSc camera gives you the field of view of a 75mm on a FF camera.

What you throw away is 33% in each direction (36mmx24mm sensor/film size for FF/film versus approaximately 24mmx16mm sensor size for APSc). And hence the crop factor of 1.5x.

PS
It does not matter for which system that 50mm was designed; 50mm is 50mm. So if you put a DA50/1.8 on a film body, you will still get the field of view of an old 50mm on that same film body (possibly with severe vignetting).

11-13-2013, 09:32 AM   #5
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QuoteOriginally posted by sterretje Quote
Yes, that's why someone invented the term crop factor. And because of that, a 50mm on an APSc camera gives you the field of view of a 75mm on a FF camera.

What you throw away is 33% in each direction (36mmx24mm sensor/film size for FF/film versus approaximately 24mmx16mm sensor size for APSc). And hence the crop factor of 1.5x.

PS
It does not matter for which system that 50mm was designed; 50mm is 50mm. So if you put a DA50/1.8 on a film body, you will still get the field of view of an old 50mm on that same film body (possibly with severe vignetting).
right, of course... but crop factor can alter the "effective" focal length, I think that's what was meant... crop also means crop of the width, which means crop of the angle of view, larger angle covering a wider view (a wider distance horizontally) than a narrower angle... that's why 50mm on an aps-c gives the same kind of an image as an 80mm on a FF would give, because both an 80mm lens on a FF and the aps-c sensor crop the same amount of information out of the image circle (plus the cropping caused by the rectangular format itself)
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