dpreview 90d

Started 2 months ago | Discussions thread
John Sheehy Forum Pro • Posts: 22,306
Re: dpreview 90d

Chris Mak wrote:

macwintux wrote:

John Sheehy wrote:

Try hand-holding 800mm of glass with APS-C and looking at the LCD;

How many people use "800mm of glass" with such a camera?

How many people use "800mm of glass" at all?

For birders and wildlife shooters, 800mm is no luxury, but often a must, although it will take a big lens on the 90D to avoid serious diffraction. I am not sure what good a F8 lens will do on this 32,5mp sensor, where diffraction starts to take a toll from f5.2. I guess it will have to show in real world use, but I am inclined to believe that you will need a lens like the 400/2.8 to take advantage at 800mm.

Don't forget, we don't generally step back further from the subject to use a TC. We use a TC because we can't (or don't want) to get closer to the subject. I don't know why people obsess over "IQ" in contexts where "SQ" or subject quality is what matters. The size of the diffraction blur relative to the size of a given subject depends only on two things: how far you are from the subject, and how large your entrance pupil is (the TCs do not change the entrance pupil). In that paradigm, f-numbers are just a mathematical meandering with no direct relevance to subject diffraction.

A higher pixel density gets more detail of everything, and never results in "TMI" (too much information), other than larger files. Simulating a higher pixel density with a TC only has one extra possible flaw; aberrations created by the TC itself, but those are fairly low with the 2xIII.

The so-called "Airy disk" is actually quite a misnomer; most of the photons from a point light source actually concentrate in the inner part of the cone, and are not all that far from the center, compared to the rings. With very low noise (ISO 100 with ETTR) on my 7D, I could shoot at f/22 and sharpen to significant pixel-level sharpness, and that is like f/13 or so with 90D pixel spacing. f/5.2's diffraction is a cakewalk for sharpening, even at medium ISOs, and f/8's not too bad at all.

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