The fallacy of 35mm "equivalent focal lengths"

Started 3 months ago | Discussions thread
OP Tony Beach Forum Pro • Posts: 11,925
Re: Needs definitions

hesbehindyou wrote:

hesbehindyou wrote:

Hi Tony, skimmed through it twice but got hopelessly confused.
I'd recommend re-writing it to make more concise and also to add in the definitions of the various terms you're using.

Hi again,

I read through the replies and it looks like you think that it would be less confusing if equivalent focal length stopped being used to refer to a field of view that's equivalent to full frame camera, and instead changing its meaning to refer to something like:

'the equivalent image quality taking into account the sensor too'.
I think that would confuse everyone

Another poster wanted to do a similar thing with redefining aperture as he objected to an f1.8 lens on a smaller sensor giving a different image quality & depth of field to an f1.8 lens on a larger sensor.

Essentially, we've got a fairly simple and limited concept that tells part of the story and some people want to divorce that figure and definition from its current, limited, logical meaning and confuse it into a single number that defines what, for want of a better word, I'll call 'image quality'.

It's not possible as things like focal length compression and exposure mean there is no equivalence-in-all-respects for you to attach a single number to. Attaching a number to what things you can (or think you can) leaves you back where we are now (with multiple factors to take into account) but with a system that's arbitrary, difficult to measure and impossible to be objective about.
Best we accept the limitations of what is meant by equivalence and be aware of what it doesn't take into account.

People are free to do whatever makes them comfortable. When I want an idea of what a camera can do (i.e., what it is equivalent to) I look at its actual focal lengths and apertures, and then the size and pixel density of its sensor. The only limitation actually described by "equivalent focal length" is how wide a FOV you can capture in the file. Many consumers (and even some participants in the forums here at DPR) falsely think that when the camera is sold to them with some extraordinary "equivalent focal length" that they would need an enormous lens to replicate that, and that's misleading and fallacious.

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