The fallacy of 35mm "equivalent focal lengths"

Started 3 months ago | Discussions thread
Lee Jay Forum Pro • Posts: 54,144
Re: Diffraction

Mark Scott Abeln wrote:

Lee Jay wrote:

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.

Same as in same, actual, physical, width, in real life, on the sensor.

Right, but that's not a particularly interesting thing because it defines basically nothing about the final image.  It doesn't define either total resolution or angular resolving power.

That's a basic principle and it is easy to remember.

Once you realize that, then you can simply figure it out for various sensor sizes. Double the sensor width and you have to double the f/stop to get the same relative amounts of diffraction.

I just don't think this is an easy way to understand the impact of diffraction on the final image.  In fact, I don't think there is an "easy" way because it depends on what you're trying to achieve (total image resolution or angular resolving power).

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