The fallacy of 35mm "equivalent focal lengths"

Started 3 months ago | Discussions thread
OP Tony Beach Forum Pro • Posts: 11,934
Re: So two lenses of same focal length on same camera aren't equivalent

SirPeepsalot wrote:

Tony Beach wrote:

FingerPainter wrote:

The logical extension of what you are saying is that the 28-70mm 28.D @ 70mm on your D800 isn't equivalent to the 24-70mm 2.8E @ 70mm on the same camera because the latter out-resolves the former.

I have to wonder how useful a definition of focal length equivalence that is.

That is not to say that it isn't useful to know that one lens / sensor combination will out-resolve another,

But let's go further. You also take into account the effects of the sensor in your assessment of "equivalence". So by your approach, the 24-70mm 2.8E @ 70mm on a D5 at 1/125 f/4 ISO 100 isn't equivalent to the same lens on a D850 at the same settings.

When a lens isn't equivalent to itself when used at the same settings, I think we've gone too far off into the weeds.

Focal length equivalence of a lens is a property of the lens, not of a lens/body combination. The very real effect you are describing does not negate the concept of focal length equivalence. Rather it points out that one shouldn't expect same quality outputs from equivalent focal length, even at identical pixel counts.

What I'm actually saying is that there is no such thing as equivalent focal length. It should instead be referred to equivalent AOV. The very fact that effective equivalence can be changed by changing cameras or lenses even when using the same format demonstrates the fallacy of the term.

AOV is a pure function of focal length and format diagonal length, so if there can be an equivalent AOV there can be an equivalent FL.

No, my point has been you can't conflate those things without ultimately suffering the consequences of such misinformation. For the focal length to be equivalent the aperture also needs to be the same. While diffraction is the same relative to the f-number as it relates to the format, if you want to print larger or crop for more reach then you will discover that the visible diffraction of the shorter "equivalent" focal length is just as visible as it would be on the longer focal length if it also used the equivalent  aperture.

When a camera is marketed as having an equivalent focal length to a larger format and it's marketed as having an f/2.8 lens many assume they are getting something that is equivalent when they are in fact getting less light and more diffraction. In the example I showed in the OP, using f/22 on a 90mm lens resulted in the same end result as using f/5.6 on a 22.5mm lens, which when you consider the role of diffraction shouldn't be surprising because both were using the same aperture size.

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