Why pixel peeping (usually) makes no sense

There's a great difference between screen resolution (72-96 PPI) and
good printing, where rules of thumb for normal viewing distances are at
least 130 DPI for barely acceptable results, and up to 230 DPI for
excellent results. With current technology, anything more than 300 DPI
is pretty much wasted. This translates to:

4x6 5x7 8x10 11x14 16x20 20x30
------ ------ ------ ------ ------ ------
Acceptable: 0.4MP 0.6MP 1.4MP 2.6MP 5 MP 10 MP
Very good: 0.8MP 1.1MP 2.6MP 5 MP 10 MP 19 MP
Excellent: 1.3MP 1.9MP 4 MP 8 MP 16 MP 32 MP
Best: 2.2MP 3.2MP 7 MP 14 MP 28 MP 54 MP

For my 7.2MP Panasonic FZ8, 11x14 is "excellent" at about 216 DPI.
The same effect on my high-quality 96 PPI display is 96 / 216 = 44%
zoom, more realistically 50% zoom. On a standard 72 PPI display it's
72 / 216 = 33%.

Even these reduced zooms exaggerate the issue, since display pixels are
so much bigger and more distinct than high-resolution print dots. For
this reason display zoom needs to be reduced even further for meaningful
print judging, to about 25-33% (depending on display) for a high-quality
(8MP) 11x14 print.

Because screen pixels are so much larger and coarser than good printing
dots (effectively a magnifier), zooming in more than this doesn't make
sense (unless the ultimate objective is screen display of 100% crop
rather than normal screen display or printing). If you can't see an
issue at this reduced display zoom, then you're not going to see it in
an excellent print either.

Thus my normal practice is to assess images on screen at no more than
33% zoom, zooming in farther only to examine the effectiveness of
sharpening and/or noise reduction.

This relationship does of course change for larger print sizes, but then
print degradation due to pixelation becomes an offsetting issue -- more
pixels are needed to make larger high-quality prints. When I anticipate
printing larger than 11x14 with my 7.2MP Panasonic FZ8, I shoot multiple
overlapping images and then stitch them together, multiple-frame
super-resolution. 4 image stitching is sufficient for excellent 16x20
and very good 20x30 prints, and even larger sizes can be produced by
stitching more images. In which case the same reduced zoom is
appropriate.

--
Best regards,
John Navas
Panasonic DMC-FZ8 (and several others)
.

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