# The fallacy of 35mm "equivalent focal lengths"

Started 3 months ago | Discussions thread
Re: The fallacy of 35mm "equivalent focal lengths"

Tony Beach wrote:

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 ratio of about 1.5.

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

96mm to 48mm widens the AOV by 90%

At 96mm it's 26.5 degrees; at 48mm it's 48.5 degrees. 48.5/26.5 =1.83; 83 is less than 90; and if what you said was true the number should be 100%. It isn't that in any of your comparisons.

48mm to 24mm widens the AOV by 73%

24mm to 12mm widens the AOV by 45%

About 1.5 ratio, which is what I said.

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.

As the lens gets longer the angular proportion gets closer to linear but never reaches it. The focal length proportion is always linear whether long or short.

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Gerry
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First camera 1953, first Pentax 1985, first DSLR 2006
http://www.pbase.com/gerrywinterbourne
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