Two ways of looking at how large to print
 

In another thread the question came up as to how large can you make a
print from a give size image, in this case from a 3.2 MP camera.

There are two ways to look at how big a print should be made, both are
correct under some circumstances. On one hand you might want the print
to be just as sharp as you can get it, in this case you would want to
print at 300 dpi and some people would say even higher. This would
limit a 3.2 MP image to about a 4 x 6 print. But the other way to look
at it is that the photo has a certain amount of detail in it and you
might want the people looking at the print to be able to see all the
detail that is in the photo. The human eye has a hard time seeing low
contrast details that are small, so printing the photo large will make
more visible to the viewer.

To many photographers the sharpness of the print it critical and they
are reluctant to make prints that would reduce the resolution below 300
dpi. But for any give photo printing it larger will make a print that
almost all people will prefer to look at. I have seen this over and
over again, I used to make small prints from my Nikon 995 (3.2 MP
camera) that where very sharp and I would also make 8 x 10 prints of
these same photos. People overwhelmingly preferred to look at the 8 x
10 prints.

So what resolution is right for a print depends on the circumstances,
if you know you are going to be making 8 x 10 prints, then you are best
off using a camera that has somewhere around 8 MP. But if rather you
have a photo form a given camera and you want to make the best looking
print from it then you would print larger, printing at a DPI of
somewhere between 150 and 200 seems good.

Just to be clear I am not saying that for a given size print 150 dpi
will look better then 300 dpi, far from it. What I am saying is that
for a given digital image printing at it 150 to 200 dpi will produce a
print that most people will enjoy looking at more then a smaller print
made at a higher dpi.

I should also point out that the amount of noise in a photo will
greatly effect the optimum size to print it at, more noise smaller
print. And of course it always help if the photo is in focus.

Scott