The fallacy of 35mm "equivalent focal lengths"

Started 3 months ago | Discussions thread
mamallama Forum Pro • Posts: 54,961
Re: The fallacy of 35mm "equivalent focal lengths"

Tony Beach wrote:

TonyGamble wrote:

Then apparently you stopped reading after the first paragraph.

I did Tony.

I should have put the second paragraph in front of the first one.

Have you got time to write a precis if what you are saying is important to us DPR folk.

I suggested in the second paragraph to skip to the picture:

Describing a lens as having an equivalent focal length is simply wrong. I can't make it any more concise than that.

Provide a concise precise technical definition of equivalent as you are using the term. That will save a lot of the round robin discussions that have taken place.

The longer answer is that a lens has its own focal length and no other, and that has a direct bearing on how large its aperture is which in turn determines how much light comes through it, and when the aperture becomes very small it imposes diffraction (see the upper right corner crop).

In my testing this week I found that I can take a 45mm or 50mm lens used on my D800 and use it to more effectively capture the "equivalent" of a 105mm lens than a camera (my S95) that a few years ago was advertised to go to 105mm "equivalent focal length." Thus, when I see that the camera I'm going to replace it with is being marketed both here at DPR and everywhere else to be a 720mm "equivalent" I thought it would be useful to some to point out why that's a fallacy and my expectation is that its reach will be closer to 350mm (for my D800 -- the number changes depending on the camera, so with some of the newer models it might be more like 300mm and with many of the lower pixel density cameras it might be more like 400mm; and of course there is the issue of which 300-400mm lens you are using).

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