The fallacy of 35mm "equivalent focal lengths"

Started 2 months ago | Discussions thread
Lee Jay Forum Pro • Posts: 54,071
Re: Diffraction

Mark Scott Abeln wrote:

A Wingnut wrote:

My apologies if this was brought up, some of this is over my head.

At f/22, defraction is going to be an issue anyway, so heavy crops are generally out of the question. It’s difficult to compare IQ with f/5.6 and f/22. Most start at around f/13, give or take. I can get stupid good crops at f/8-f/13, and anything above that starts to deteriorate a bit. At f/22, my crop goes from being decent at even 125% to similar quality at 50%.

I am bringing this up because I wondered if it was even conclusive to have anything at over f/13.

One of the nice things about diffraction is that it is the same for all cameras and lenses at the same f/stop. The width of the diffraction blur—also known as the Airy Disk—is proportional to the f/stop value.

Which means it's not the same for all cameras, because the size of the Airy Disk relative to the size of the sensor (which is what matters for resolution) changes with sensor size.

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Lee Jay

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