The fallacy of 35mm "equivalent focal lengths"

Started 3 months ago | Discussions thread
Lee Jay Forum Pro • Posts: 54,147
Even though you're right, you're actually wrong
7

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.

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