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03-24-2011, 12:58 PM   #16
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QuoteOriginally posted by jboyde Quote
The thing that confused me the most when I started switching to digital was if only the FOV changed. I too wondered how far the "equivalent" went. What I mean by that is if you'll get the same wide angle distortion on let's say, a Sigma 10-20, close up on a subject on an FF camera as with an APS-C. I figured you would at the same distance BUT since the crop factor will require you to back up to get your whole subject in frame then it will affect it, at least by a small amount.

Same thing goes for telephoto compression. You pop a 100 mm lens on an APS-C and you'll get the compression and DOF of a 100mm, not 260mm. Once I got that cleared up in my head it became easier to choose lenses.

not quite DOF is not the same with a 100 2.8 wide open on a digital as it is on 35mm (35mm is a fair bit narrower, it's still affected by how far you are from the subject and what size the print is as well)
there are a number of threads arguing this i've been on usually related to lens speed
where did 260mm come from? it would be 150 fov equivalent on the apsc sensor

03-24-2011, 02:28 PM   #17
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QuoteOriginally posted by jboyde Quote
The thing that confused me the most when I started switching to digital was if only the FOV changed. I too wondered how far the "equivalent" went.
There's actually a pretty elegant way of extending the notion of "crop factor" to also calculate the differences in DOF for a given aperture as well as in the amount of noise you might expect to see for a given sensor technology (or conversely, the shutter speed you would need to keep the noise level the same). But if you think talking about "crop factor" leads to controversy, you don't even even want to think about mentioning "equivalence".

QuoteQuote:
What I mean by that is if you'll get the same wide angle distortion on let's say, a Sigma 10-20, close up on a subject on an FF camera as with an APS-C.
This is actually a relatively easy question to address. Perspective distortion is a function of one thing and one thing only: angle of view. A lens on one system that has the same angle of view of as a different lens on another system will yield the same perspective distortion.

QuoteQuote:
I figured you would at the same distance BUT since the crop factor will require you to back up to get your whole subject in frame then it will affect it, at least by a small amount.
If you choose to keep the same focal length between the two cameras, then the angle of view will be different (angle of view is what "crop factor" is trying to address). So whether you move or not, the perspective distortion will differ. On the other hand, if you keep your feet firmly planted but instead simply twist the zoom ring to yield the same angle of view on both systems, then perspective distortion will be the same.

QuoteQuote:
Same thing goes for telephoto compression. You pop a 100 mm lens on an APS-C and you'll get the compression and DOF of a 100mm, not 260mm.
Not sure where you got 260mm from, but you're conclusion isn't true here. A 100mm lens has a smaller angle of view on APS-C than on FF, so it will absolutely positively without the slightest shred of doubt have the same perspective distortion (the "compression" effect) as a 150mm lens would on FF. The images will be completely indistinguishable in this respect. The only differences would be in the DOF at a given aperture and/or noise levels you got at a given shutter speed and/or or shutter speed required to achieve a given noise level at a given DOF.
03-24-2011, 09:16 PM   #18
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My mistake. It would have been 160mm. I added back the original 100mm on my calculation.
03-25-2011, 08:13 AM   #19
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FWIW, the "crop factor" is on Pentax DSLR's is 1.5, not 1.6 (Canon uses 1.6). Actually, those are just approximations, but pretty good ones.

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