The fallacy of 35mm "equivalent focal lengths"

Started 3 months ago | Discussions thread
Lee Jay Forum Pro • Posts: 54,144
Re: Even though you're right, you're actually wrong

tony field wrote:

Lee Jay wrote:

35mm-equivalent focal length isn't about "reach", it's about "field of view".

"Reach", is about resolving power - the smallest angle which an object can occupy and be resolved.

You correctly stated the major parameters that control "reach", but "35mm-equivalent focal length" isn't about reach, it's about FOV. When I crop, my 35mm-equivalent FOV changes but the resolving power obviously doesn't.

Think of 35mm-equivalent focal length as just a different way of stating "angle-of-view". In fact, they are directly related by a simple formula:

Angle-of-view = 2*arctan(sensor size/(focal length*2))

If you fix sensor size to 36mm, then horizontal angle-of-view gives 35mm-equivalent focal length using the above formula. You can use a different measurement if you prefer vertical or diagonal angle-of-view.

Of course there is the third angle of view which is based upon the diagonal of the format.

I said that (see above).

This is the standard by which "Equivalence" works.

I disagree, actually.  I always use horizontal and fix aspect ratio because I never frame diagonally.

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Lee Jay

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