11 called a "Who cares aperture"?

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Jul 23, 2013 at 4:18	comment	added	jrista		@BlueRaja-DannyPflughoeft: Actually, f-number takes focal length into account. Diffraction at the sensor plane is solely dependent upon f-number. A short lens with an f/4 aperture means light travels a shorter distance to the sensor than an f/4 aperture on a long lens. The greater distance traveled means light spreads out more, so the effect of diffraction with a long f/4 lens is the same as with a short f/4 lens.
Jul 23, 2013 at 4:16	comment	added	jrista		@Shizam: The best aperture, and what aperture is really diffraction-limited, really depends on the lens. The concept of DLA, or Diffraction-limited Aperture, as it pertains to a sensor has to do with the aperture of a theoretically perfect lens at which diffraction would begin to affect IQ. If the lens is aberration limited at f/5.6, then it doesn't matter if the sensor DLA is f/5.6. If the lens doesn't become diffraction limited until f/8, then it is quite likely, even with a high resolution sensor, that you could get better results by stopping down.
Jul 22, 2013 at 22:47	comment	added	BlueRaja - Danny Pflughoeft		"if you look at lens tests of any really good lenses and the peak sharpness tends to be at f/4" - The amount of diffraction primarily depends on the physical size of the aperture, but the physical size of "f/4" depends on the focal length. f/4 at 15mm will actually be physically smaller than f/5.6 at 100mm, and thus should exhibit more diffraction, despite being a smaller f-number. So I think the aperture at which "diffraction has a measurable effect with a high megapixel DSLR" must be dependent on both the f-number and the focal length.
Jul 22, 2013 at 20:48	comment	added	Matt Grum		@Shizam I should have said "detect" rather than "see" but if you look at lens tests of any really good lenses and the peak sharpness tends to be at f/4, meaning diffraction is causing the f/5.6 image to be softer.
Jul 22, 2013 at 20:46	history	edited	Matt Grum	CC BY-SA 3.0	added 22 characters in body
Jul 22, 2013 at 19:34	vote	accept	Andy M
Jul 22, 2013 at 19:22	comment	added	Shizam		Could you provide references for diffraction being visible as low as f5.6 on high MP DSLRs? I ask because the commonly held theory 'when in doubt shoot at f8 for maximum sharpness' wouldn't be the case anymore and I'd wanna know! :)
Jul 22, 2013 at 11:46	history	edited	Matt Grum	CC BY-SA 3.0	added 29 characters in body
Jul 22, 2013 at 9:30	history	answered	Matt Grum	CC BY-SA 3.0