Well, I've devised a little test. Create an image file--doesn't have
to be big at all because we're just experimenting around and the
output can be rather tiny. Create this image file of, say, 300x300
pixels. Now create a custom fill or brush with alternating black and
white lines. These lines are one pixel in width. Follow so far? While
you are at it, you might as well create a few other squares like this
with other colors.
What you now have is one or more squares made up of white/black
one-pixel lines. At 100% on screen they should be perfecty
sharp-edged.
Now print this at 300 ppi. Examine the print and tell me if you can
make out the lines. Change resolution to 240 ppi. See the lines? Keep
going down until you can make out the lines. Now tell me again that
you NEED 300 ppi resolution from camera to printer. Most
printers/inks/papers won't even SUPPORT 300 PPI resolution as there is
too much bleed.
In the film days you could figure a reasonable enlargement of 8x
before the image resolution started dropping to the point where
golden-eyes could see the difference. So, if you figure that a 300ppi
print is equivalent to a "contact negative", you should be able to use
a quality scaling tool to upres the photo to the equivalent to 8x
before the image is hashed up enough to be problematic. But other than
Genuine Fractiles, most scaling tools aren't anywhere near as good as
optical enlargement, so chop that 8x in half. 4X is a nice round
number. And amazingly enough, in my own tests, I've found that 4X
enlargement from digital files (considering that 300ppi is equivalent
to a contact negative) seems to be the point where my own eyes detect
the resolution loss.
Saying the above paragraph another way... Take your image file and
uprez it 4x. Now when you print this at 300ppi, you're at the
threshold where print quality, according to those with perfect vision
will suffer. 8x uprezing is still fine for most general-purpose
applications.
To bring this full circle, the 5D, when using the 4X factor, allows
for a print 38.8 inches in height. Inotherwords, you can make a 40x60"
print that is essentially perfect to everybody except those with
magnifying glasses.
The 5D's native image height is 2912 which at 300ppi is the equivalent
to a contact negative print 10x15" in size. This is why digital has
passed film in the EFFECTIVE resolution department years ago. Because
it isn't the capture side that is the limiting factor of output
resolution, but it's the output side that is the limiting factor of
output resolution.
Back in the film days there were two reasons to shoot larger formats:
enlargement and tonal smoothness. With the 5D, the native non-enlarged
size is already almost 10x15" and tonalities is a non-issue as it
doesn't change regardless of how big or small you make the print.
AG (do the test, I dare you) Schnozz
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