The fallacy of 35mm "equivalent focal lengths"

Started 2 months ago | Discussions thread
OP Tony Beach Forum Pro • Posts: 11,909
Re: The fallacy of 35mm "equivalent focal lengths"

Gerry Winterbourne wrote:

Tony Beach wrote:

Gerry Winterbourne wrote:

Tony Beach wrote:

Many wrongly assume that if you double or halve the focal length the AOV (or FOV for a given distance) changes accordingly. It's easy to imagine the photographer saying she wants to zoom twice as "close" to the subject and having the lens designating degrees instead of putative millimeters facilitates rather than hinders that.

Which is precisely why using focal length in millimetres is the best method.

If I halve or double the degrees I will change the AOV (FOV of the subject at a given distance) by that much,

Read the numbers of your graph - what you say is clearly untrue. 12mm to 24mm (your factor of two) gives angles of 122 and 84 - a ration of about 1.5.

I'm terrible at math, but nonetheless here's what I see:

96mm to 48mm widens the AOV by 90%

48mm to 24mm widens the AOV by 73%

24mm to 12mm widens the AOV by 45%

which won't happen when just changing the millimeters by that much.

No, I'm afraid it does exactly that.

At longer focal lengths it does.

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