Please correct me if I'm wrong but...

... I think this information is pretty inaccurate:


"In principal the Canon EOS 450D provides everything needed for portrait
photography. In combination with one of the excellent fixed focal
length lenses and flash units it is suitable for high-quality
portraits. However, as an APS-C DSLR is "looses" about one stop in
terms of depth-of-field so e.g. a 85mm f/1.8 degenerates to a "136mm
f/2.4" which is less than ideal in this specific context."


I found it while reading a review of the Canon XSi (which I own) in Photozone (http://www.photozone.de/dslr_reviews/395_canon_eos_450d?start=3).

Quote:

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Originally Posted by Carlos Lindado

Please correct me if I'm wrong but...I think this information is pretty inaccurate:

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Sorry, but consider yourself corrected. The 'crop factor' applies to both focal length and aperture, as it pertains to depth of field. The reason still derives from angle of view. To get the same subject framing with a crop sensor's reduced angle of view, you need to be further from your subject. Since the 'crop factor' does not change the actual focal length of the lens, moving further away from your subject to increases the depth of field relative to the same subject framing on a FF camera.


DoFMaster has a pretty detailed article on this:http://www.dofmaster.com/dof_dslr.html.

John if the crop factor applies to both focal length and aperture would this not make the 85mm f/1.8 a 136mm f/2.88 equivalent, not f/2.4???

Thanks for the replies!


I was thinking of the situation where you simply move the lens from a FF to a crop-sensor camera, without changing distance to subject. I mean, optically (regarding to the lens only), the DOF only depends on aperture diameter (given a subject's distance), so one can't always make the assumption that the photographer will change the distance to subject (maybe he/she changes from a FF to a crop-sensor to frame a subject tightly without changing the perspective)

Quote:

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Originally Posted by btaylor

John if the crop factor applies to both focal length and aperture

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Focal length and f number, but not aperture. Aperture is the great invariant. Ie, crop an 85mm f/1.2 and it acts like a 135mm f/2 (roughly), but the aperture is about 70mm in either case.


This means that if you take a picture with a 100mm @ f/2, no matter what size sensor you use and no matter how much or little you crop, it will look like a picture taken with a 50mm aperture lens.


As the man said, aperture rules!

Quote:

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Originally Posted by The Man

Aperture Rules!

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See what I mean?

Quote:

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Originally Posted by btaylor

would this not make the 85mm f/1.8 a 136mm f/2.88 equivalent, not f/2.4???

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That's what I get, too. But I'm not so good at arithmetic.

Quote:

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Originally Posted by Carlos Lindado

I was thinking of the situation where you simply move the lens from a FF to a crop-sensor camera, without changing distance to subject.

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But that's a silly comparison, because you're getting a totally different picture. The point of "effective" f number and "effective" focal length is that you get the same picture on crop with an effective fl and f ratio as you would on full frame with a real fl and f ratio. Same subject distance, same perspective, same diffraction, same everything. (Well, not really, but that's a different story)

Quote:

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Originally Posted by Carlos Lindado

the DOF only depends on aperture diameter (given a subject's distance)

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True enough.

Quote:

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Originally Posted by Carlos Lindado

so one can't always make the assumption that the photographer will change the distance to subject

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I agree. Photogrpahers can be crazy. I wouldn't assume anything about what they might do.