Defining a "normal focal length"/"normal AoV" and "absolute zoom"

Started Apr 17, 2012 | Discussions thread
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dark goob Contributing Member • Posts: 980
Defining a "normal focal length"/"normal AoV" and "absolute zoom"

Awhile back an interesting debate arose at a photo industry convention.

I said that I was annoyed by the way camera makers always say "4x zoom" or "10x zoom" but it doesn't really mean anything, because it's relative.

I suggested that we should use an "absolute zoom" factor and base it on something real. That's what we already do with telescopes and binoculars, where we say their range is "20x-60x", relative to normal human vision.

But what should constitute a "normal" angle of view for cameras, and why?

My suggestion was that a "normal" angle ought to be when the distance-to-subject is the same as the horizontal distance across the focal plane. So if I am using a 1x zoom, then if I am 10 feet away from a car that is 10 feet long, it will occupy the entire frame horizontally (53.1° horizontal AoV).

((To support my suggestion: a 36mm lens does this on 135-format. For the past 20 years, a point-and-shoot camera has not been considered to have a "wide zoom" unless it started with at least 60° horizontal AoV -- 31mm on 135. Most consumers seem think of 65.5° horizontal AoV as where wide truly starts -- 28mm on 135.))

Surely the AoV of the lens is the thing that should be the defining factor in what constitutes a "normal focal length" on any given format (not the focal length in mm).

But several people argued that I was wrong.

Some said that 50mm on 135 was "the same as human vision." I take it to mean that looking through the optical viewfinder with one eye, while keeping the other eye open, then the images can line up and stuff looks to be the same size out of either eye, with no geometric distortion etc. On my old OM-2n and Nikon F2, this indeed holds true.

However human vision has, over-all, a much wider field of view compared to the AoV of a 50mm lens on 135. (Are all human eyes the same? I have read somewhere online that 40mm or 43mm is the true "normal focal length" for 135 format, so there seems to be some debate on this.)

Other people said that 50mm gives the same "geometry" of human vision (that the lines of perspective are the same as we see). And that the lower the mm of the lens, then the more "distorted" the geometry becomes -- independent of the sensor size used! (This just seems totally wrong, since with my point-and-shoot Elph, which has a maximum focal length of 20mm, I am able to achieve pictures at 10mm that do not look "distorted" at all, like they would on a 10mm lens on a 135-format camera.)

So I ask you, the dpreview community, what do you think? What should be considered the "normal" AoV if we were to start using an "absolute zoom" system like binoculars and telescopes use (but for cameras)? (My vote is for 53.1° horizontal.)


((SIDE NOTE: I would also like to see cameras simply display the AoV on the screen, in degrees, so people can stop the arcane practice of using 135-format focal lengths as a basis for thinking of angles. For the time being I'm using an iPhone app called "Angle of View" but it would be nice to see the angle display actually built into the darn cameras.))

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