I tend to ignore the whole issue. I know what the sharpest f stop is for each lens I use, usually ƒ5.6, so unless I'm shooting for a narrow DoF effect, I shoot the lens at it's sharpest point. Then I change that based on circumstances. You can shoot a landscape at ƒ5.6 if you're shooting off a balcony and it's 50 feet until there is anything in the foreground. The closer the desired to be in focus foreground is the more you have to stop down, until from 6 feet away with a 50mm lens you need to be at ƒ16 or ƒ22 and diffraction is seriously limiting your image quality. But I find it pretty intuitive, if you careful look at each of your results you get the hang of it pretty quick, for each lens that you use. Couple that with the shorter the focal length the wider the DoF, and you can probably handle just about everything.
Quote: As the predominant print size was 8" X 10" acceptably sharp was determined to be when a point or line measured less than 1/100 of an inch, when a full frame exposure was blown up to 8" X 10". Nothing more, nothing less.
If you want 1/100 inch or less on digital a K-3 produces about 2700 distinct lines per inch. A print 27 inches high would fulfill that requirement. A K-1 would be about 36 inches high. My K-1 and K-3 images both look really sharp printed 16x20 on glossy paper, to the point where you can't really tell the difference. Despite what folks say, once you get over 100 lines per inch, at least in my opinion, it's overkill. It's nice if you have it, but, you aren't getting much for it.
In actual usage though, the algorithms used in upscaling images are much better than what you can do with an analogue reproduction system, where the grain just gets larger and larger. As long as you have relatively noiseless starting image, enlargement can go well beyond that and still be acceptable if the image depends on colour or shadow and light. Of the 5 elements of design, 6 principles of organizing basic compositional elements and the five favourite rules of pictorialist composition, in all 16 general guidelines, only the reproduction of texture needs to be razor sharp. The others require varying degrees of sharpness. If you aren't trying to reproduce fine detail you can get away with a lot less, and it may improve the image quality if your image if it is less. Sometimes to much detail just looks messy. And with enlargement algorithms cleaning up the jaggies and minor imperfections in the subject the enlarged image may look better than the one taken at higher resolution.
Originally posted by Lowell Goudge just so long as you remember, even hyper focal distance is based upon the definition of acceptably sharp, and the dreaded circle of confusion, which means it too, only really exists in the mind of a 8 x 10 inch print.
On film, but a 27 inch to 36 inch print in digital for resolution. As for circles of confusion, there's no digital advantage, but you have the ability to decide from the file how big it can go and still look good. I'm usually a couple feet from my monitor so the larger circles of confusion are in part negated by the further viewing distance.
In the end, the bottom line is ƒ5.6 or 8 is preferable for DoF with ƒ11 being acceptable when really wide is needed, and sometimes ƒ16, even with the diffraction looks better than the others. Of my bracketed exposures maybe a couple ƒ22 images have been the best choice, after between 200 and 300 thousand images over the last 10 years.
When shooting for narrow DoF, printing bigger is going to enlarge those circles of confusion, and the blurry background should look smoother as you enlarge the image (as long as you were shooting with the DFA*50 1.4 and not a lens that produces messy bokeh) , but you do have to pay attention to your 100 lw/ph minimum in the subject if there is texture, although I suspect you can get away with 70 lw/ph if there isn't, which means there will be many images where the upper limit of a K-1 file will be 50 inches, and I have a Monet like fall colours reflections K-3 image that looks great printed to 42 inches. It doesn't depend on resolution for it's effectiveness and I could probably print it at any size I wanted.
The reason DoF is such a muddle is different styles of image create different DoF requirements. It's not one size fits all.