Crop Factors

Started Sep 20, 2008 | Discussions thread
PixelMinded Senior Member • Posts: 1,032
Re: No such thing...

Ron Parr wrote:

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

The point is:
AP-S cameras do crop (by factor), thus we can apply "focal length
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.

The smaller the format, the sharper the lens needs to be. Because the smaller frame have to be magnified many more times when viewed or printed. So while a medium format lens can be RELATIVELY softer than a 1/1.8" P&S lens and still produce perfectly sharp images on a medium format sensor or film, it won't suit a tiny 1/1.8" sensor. Even using full frame lenses on APS-C DSLR's result in softer performance than they do on full frame, but not a big deal because the two formats are relatively close to each other. Ignoring this sharpness issue however, if multiple 50mm lenses intended for different formats are used on one particular format then as you said they will all give the same focal length equivalent on that particular format.

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