The fallacy of 35mm "equivalent focal lengths"

Started 3 months ago | Discussions thread
FingerPainter Senior Member • Posts: 7,623
So two lenses of same focal length on same camera aren't equivalent

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.

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