According to Wikipedia the visual acuity of the human eye is, at best, around 1 minute of arc, or 60 seconds of arc.

The standard depth of field tables assumes a visual acuity of about 3.75 minutes of arc or about 225 seconds of arc.

The hand holding minimum shutter speed rule of thumb of "1/FL" is based on FX film size and about the same if not the identical visual acuity as the standard depth of field tables, or about 225 seconds of arc.

I quote these values because they are easily researched on the web or computed. I've seen many different numbers that range around 180 - 225 seconds of arc.

I find the 1 arc minute number difficult to fathom and I'm an amateur astronomer who has computed all this very carefully, over the course of many years, in order to determine what I can and cannot see, especially in regards to observation of close binaries. And I have matched my abilities with theory and all these numbers.

My "number" is around 180 seconds of arc - on a good day - and that involves a tremendous effort and a lot of eye strain that I would not personally expend looking at a photograph. I think this is a good upper limit number of visual acuity. I've also trained my eye, to some extent, to do that.

I generally wear contacts and have aging eyes so my close focus is not what it was 20 years ago when it started to recede and I ran out of arm's length when reading books . And for anyone over 30-40 this is an important consideration. My comfortable minimum viewing distance, where things are reasonably sharp, is about 17" (by design). I would not use that as a guide for the general public though, especially for younger people. This is just to say that you have to choose your audience and evaluate your own eyesight.

At 100 ppi, and assuming 180 arc seconds of acuity, a print needs to be held at 11.5" or closer in order to see all there is to see.

At 200 ppi that distance would be cut in half, to about 5.75".

At 300 ppi that distance would be just under 4", which I think even at 20 years of age is unrealistically high resolution in terms of what is needed to display a print. I think it assumes optical aid (inspection with a magnifying glass). I'm not sure if I ever had 4" of minimum focus.

Some printers require certain resolutions or certain minimum resolutions in order to be happy, but that should not be confused with what the human can actually see in the final output. While your printer may "demand" 300 or 360 ppi that does not mean you can actually see it.

I've thrown out some numbers to put basic context to this problem, but since our individual acuity, and more importantly, our individual perceptions are different, it needs to be tested and all photographers should test their own acuity and perceptions when viewing prints.

My advice is to make three 8x10 or 8x12 prints from the same image. That image should be shot from a tripod, if possible, or otherwise selected by careful pixel peeping to make sure it delivers all the resolution you think your camera is capable of. Don't crop it except to aspect ratio if you do 8x10's.

The first print should be downsized to 300 ppi or perhaps left alone, the second to 200 ppi and the 3rd to 100 ppi. Make more and split those differences if you want.

The later two prints should then be up-rezed to whatever makes your printer the happiest, just as you would do when printing any print at a rather large size such that the printing resolution is under 300 ppi or so.

Now compare the prints. If you have never done this test I think you will be surprised at how good that 100 ppi print looks. Or maybe not; you be the judge.

But what *I* think doesn't matter, nor what anyone else thinks you "need". Only you and your eyes can make this determination and it is easy and inexpensive to do.

Alternately, rather than downsizing and then up-sizing the lower PPI prints you can slice out equivalent sections. The method I suggested simply makes for exactly comparable images with as little thought and effort as possible. I wouldn't argue the merits of either method; just do what makes you feel most comfortable but make sure whatever you do reflects the real world.

It might also be a good idea to shoot, on a good solid tripod, a calibrated resolution chart at the calibrated distance to see if your lens(es) can actually resolve to the 300 PPI resolution some might suggest you "need". I've done that and I can tell you it isn't easy and it takes a good lens.

You might then try to shoot that chart hand held at real world shutter speeds to see how much resolution you actually get, relative to what your sensor is capable of delivering. You might be very surprised at the results of that test .

You might come to the conclusion that 300 ppi or more is overkill. You might also conclude that it exceeds the resolution of your lens and/or typical shooting technique. You might also come to the conclusion that printing at a native 300 ppi can't hurt. You might also come to the conclusion that it isn't worth the cost of admission . But these are conclusions you need to make yourself from real world tests or controlled tests.

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Neil
Nikonians Team
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