# 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:

hesbehindyou wrote:

thinkinginimages wrote:

I think "equivalence" created more confusion than it was worth.

I've often wondered why we don't just go with field of view? I'm assuming that it's because it's not a property that's inherent to the lens but will also involve the sensor...

It's not just that although it is, of course, a factor. What do we mean by field of view? If we mean width there's the problem that it gets wider the further away the scene; so the FOV for portraits at a few metres away would be tiny compared to the FOV with the same lens used for a landscape. So we'd really have to mean angle of view.

But I've never come across anyone who thinks of scenes in terms of how many degrees (or radians) they cover. And angle of view isn't linear with focal length - especially at wide angles - so there's another complication to have to deal with.

It's not a complication at all, but rather an opportunity:

An opportunity to complicate things by changing something linear and easy to understand with something that isn't.

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.

There have always been different formats, going up to full plate. While it's silly to contemplate putting a full plate lens on a 35mm SLR there has always been some ability to swap lenses between formats. Now, it's easy to print the focal length of a lens on a body and it is a single number regardless of the body used. But printing the angle of view is tricky because you'd have to do it for every format the lens might be used on, as you say ...

For ILC lenses, and even on the front of compact cameras, the lens has its actual focal lengths printed on it. Now when I turn on my S95 the camera will display the "equivalent focal length" which I find marginally useful inasmuch as I know I'm fully or partially zoomed -- but seeing a number followed by a degree symbol rather than a misleading "mm" symbol would be more useful for me, and I am arguing here that others would find that more useful too.

Really? Which angle of view do you want? The horizontal or the diagonal? Do you want it to distinguish between different aspect ratios (which many cameras now offer)?

Do you really believe that giving a lens's AOV as 8.25 degrees (or 8 degrees 15 seconds) will mean anything to anyone? Compared, say, to 46.8 degrees (46 degrees 48 seconds)?

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

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