Crop Factors

Started Sep 20, 2008 | Discussions thread
Ron Parr
Forum ProPosts: 13,758
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Re: No such thing...
In reply to hbx2004, Sep 21, 2008

hbx2004 wrote:

This isn't the case when we talk about APS-C sensor cameras. Because
APS-C sensor covers smaller (cropped) portion of projected image,
angle of view is smaller. Thus, optically, 200mm lens on APS-C
(1.6-crop) camera delivers the same result as 320mm lens on FF
camera.

The point is:
AP-S cameras do crop (by factor), thus we can apply "focal length
multiplier".
P&S cameras don't crop -we use focal length (35mm) equivalence for
easier understanding only.

As a user of a lens we never need to know about or care about the size of the image circle a lens was designed to project. All we need care about is that the image is large enough to cover our sensor

Let's say we're viewing an images taken with several 50mm lenses, each designed to project different image circles. One is designed for a 1/1.8 P&S sensor, another is designed for a 1.6 crop sensor, another is designed for a standard 24x36mm sensor, and the last is designed for medium format. If all of these are captured with a 1/1.8 P&S sensor, then then they all will see exactly the same thing (excluding aberrations, of course).

In reality, we almost never know about or care about the exact size of the image circle projected by our lenses. We usually just know that it's large enough and it's purely a supposition based upon an assumption of economy that the image circle is no larger than required the typical use of the lens.

For these reasons, there is absolutely no reason to attach the notion of a crop factor to some guess about what the lens maker had in mind when he designed the lens. Even if we guess correctly, it doesn't change anything. Therefore, the notion of a crop factor cannot be fundamentally about about lenses; its about the size of the image captured relative to a 24x36mm frame.

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