"Crop-ability" of images?

Started 3 months ago | Discussions thread
john isaacs Veteran Member • Posts: 5,885
Re: Exposure vs Exposure

bobn2 wrote:

john isaacs wrote:

Iliah Borg wrote:

john isaacs wrote:

"f/2.8 is f/2.8" is true.

"50/2.8 is 200/2.8" isn't always true.

It is never true. What is true is 50/17.86 is 200/71.44.

But you actually knew that, right?

The statement was 'f/2.8 = f/2.8'. If you're talking about the f-number always being the same, then the statement would have been '2.8=2.8' and is of course always true. However, the value of 'f/2.8' depends on what is the value of 'f', and in the example that Iliah gave, the 'f' was different.

It's why it's a silly statement. '2.8=2.8' is a tautology, as is 'f/2.8=f/2.8' if the value of 'f' is the same. However, the people that recite the little mantra do so in situations when the 'f' on the left hand side is different from the 'f' on the right hand side. Using the same signifier for two different things is almost guaranteed to cause confusion, but we've been there before, haven't we?

Well, apparenly someone didn't know it. f/N is not an equation, it is a nomenclature to express the f-number of an aperture. The f-number of an aperture is the ratio of the focal length and diameter of the entrance pupil. Also written as N = f/D where in this case f is the focal length. This can cause confusion because the nomenclature looks like an equation.

And so f/2.8 = 50/17.86 = 200/71.44 (for the 50mm and 200mm lenses in question).

So to speak of the "f" of "f/2.8" as having any value which could be different or the same is erroneous; it is a nomenclature.

But the "f" of focal length can be different (for different lenses), and yet the f-number can be the same.

So, the mantra is correct. Where the error occurs is assuming that depth of field is only dependent upon f-number, and not on actual focal length as well. And thus the fallacy of equivalence; you cannot speak of equivalent focal lengths for field of view without also speaking of equivalent aperture for depth of field.

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