Crop Factors

Started Sep 20, 2008 | Discussions thread
Sean Nelson
Veteran MemberPosts: 9,961
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Re: No such thing...
In reply to Guerito, Sep 20, 2008

Sean Nelson wrote:

So my Canon A650IS camera with it's lens set to 7.4mm gives the same
field of view as a 35mm lens would on a 35mm film camera. Therefore
it's "crop factor" is a little less than 5.

Guerito wrote:

Actually, Bogdan has it right, there's nothing to crop from the fixed
lens. You're confusing crop factor with equivalent FOV.

I refer again to the widely accepted definition of the term "crop factor":
From: http://en.wikipedia.org/wiki/Crop_factor :

"a crop factor is the ratio of the dimensions of a camera's imaging area compared to a reference format; most often, this term is applied to digital cameras, relative to 35 mm film format as a standard."

Since my Canon A650IS's sensor has a diagonal measurement that is about 1/5 the size of a full 35mm film frame, it's crop factor is 5.

I believe you're referring of the idea of cropping the image from a full-frame sensor because you're using a lens designed for APS-C coverage. It's true that in that case the lens doesn't cover the full frame of view, and therefore you're effectively using a smaller sensor (actually the central portion of the full-frame sensor). The term "crop factor" applies here too. But the term is used because you're using a smaller effective sensor size, not because the lens doesn't cover the full sensor.

To illustrate the difference, I can attach medium-format Pentax 6x7 lenses to my APS-C Pentax K100D DSLR. These lenses actually cover about twice the image area of even a full-frame sensor, so if your notion of "crop factor" were true then these lenses would have a crop factor of less than 1 (about 0.5, in fact). But that's not the case, all lenses mounted on my K100D have the same crop factor - 1.5 - that's determined by the ratio of the K100D's sensor to the size of a full 35mm frame. Therefore, if I mount a 100mm 6x7 lens on my K100D, the 1.5 crop factor tells me that I'm getting the same field of view as a 150mm lens would give on 35mm film.

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