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When judging sufficient DoF or, on the contrary diffraction issues at high F-numbers, the typical print size sharpness is what really matters. A few tutorials on the Internet as well as some answers here say so.

But what if my target audience are people with 30" (or even 40") 4K screens/monitors at the 3840x2160 resolution that are already available for affordable prices. Will those people with eyesight 20/20 find them acceptably sharp if those pictures were perfectly sharp only in the typical print? (If not, how do I ensure sufficient sharpness for that target group?)

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Yes, the standard viewing situation which the DOF calculation applies to, is an 8x10 inch print, viewed at 10 inches. Circle of Confusion (CoC, see Wikipedia) is an arbitrary factor in that calculation (about how well the eye sees the magnified blurriness in the enlargement).

If you print it 3 or 4 times that 8x10 size, then the allowable Circle of Confusion becomes 1/3 or 1/4 size too, because it will be enlarged 3 or 4 times. CoC is not a "hard fact", it is just someones considered judgement about what the eye can see at the 8x10 inch degree of enlargement. CoC has become smaller today, due to better film, and better digital. But enlarged CoC is key for the DOF calculation.

So as far as calculating DOF, then in the DOF calculator (in the 3x or 4x case), you could choose another camera (smaller sensor size) that gives a CoC 1/3 or 1/4 the original CoC size given for your camera. Camera model does not matter, it is about sensor size, specifically only about its necessary enlargement to 8x10 size. 1/3 CoC size, enlarged 3x more, gives the same numbers.

However... a big deal - viewing distance of large prints is yet another factor of DOF (for the eye). Unless you still view it at the standard 10 inches (held close under your nose), then 3 or 4 times viewing distance increases the allowable CoC that much, the opposite effect, which in actuality, probably cancels out the enlargement factor. Like billboards, large prints do look good when viewed from across the room.

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You are correct that the standard viewing criteria used by most DoF calculators can be inaccurate when considering large prints or high resolution displays.

Your best bet is to bracket your focus/aperture or check your image at the maximum possible enlargement using the rear LCD of your camera.

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Just a thought: If you want to see what a given image will contain in the way of detail at 3840x2160, resize the image to fit that bounding rectangle, then display it at 1:1 on your screen, making sure that an in-focus area of interest is within the "crop" of the screen. You can then see exactly what actual detail will exist at that pixel dimension, and you can decide at what viewing distance this is or isn't acceptable to you.

Of course, knowing what "acceptably sharp" might mean to any other given casual or critical observer is another problem altogether...

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