Should you resample photos for printing?

[James McNangle]

In answer to another question, CSM1 referred to Wayne Fulton's interesting
'Scanning tips'. While I was reading this, I was reminded that when I bought a
new digital camera a few years ago, the shop gave me a two-hour introductory
course to digital cameras. During this the instructor stated dogmatically that
photos should be resampled to some fairly low resolution before printing, as
otherwise the voter would take much longer to print, and the quality of the
print would be markedly less.

I have a Canon S800 printer, and when I got home I found a high-quality
photograph, and did some tests. After I had resized the image to A6 size
(without resampling), the nominal resolution of the image was about 650 pixels
per inch. I made copies of this image, resampling to 300, 150 and 75 ppi, and
then I did a test print from each image.

The instructor was right about the printing time -- the original image took
about 65 seconds to print, while the lower resolution versions all took about 30
seconds, but he was certainly not right about losing quality if the resolution
was too high. To the naked eye the 300 and 650 ppi images were virtually
identical, though when I examined them with an eight power magnifier the higher
resolution one looked marginally better, but the 150 ppi version was noticeably
worse and the 75 ppi version was unacceptably worse.

I decided that unless I wanted to print a number of prints from a single image
there was no point in resampling before printing, as the time to resample would
cancel out the saving in printing time, and I would then have to worry about
keeping both the original and the lower resolution copies of the image.

James McNangle

 

[Wayne]

The instructor was right about the printing time -- the original image took
about 65 seconds to print, while the lower resolution versions all took about
30 seconds, but he was certainly not right about losing quality if the resolution
was too high. To the naked eye the 300 and 650 ppi images were virtually
identical, though when I examined them with an eight power magnifier the
higher resolution one looked marginally better, but the 150 ppi version was
noticeably worse and the 75 ppi version was unacceptably worse.

FWIW, I always do resample excessive size to the smaller 300 dpi size for
printing. The "worse" referred to by the instructor probably is because after
you resample to the appropriate size, then you have opportunity to sharpen the
image before printing. But if you let the printer ignore all the excess
pixels, any previous sharpening goes away with it.

 

[Norm Dresner]

First, understand that unles your image happens to be at the exact
resolution of the printer, the driver is going to resample it anyway.

Second, there is a limit to the resolving power of the human eye and
anything beyond it is wasted. Since this resolving power is really an
angular one, the limit depends on the viewing distance: bilboards need
significantly lower resolution than picture postcards.

From personal experience, I've found that the unaided eye is unable to
distinguish -- at close viewing -- anything much beyond 300-400 PPI and I've
standardized on 300 PPI for anything but "gallery" prints; it's a waste of
time and ink.

Norm

 

[SamSez]

What about the old sage of resampling to a multiple of the hardware
resolution of the printer at a size chosen so the driver doesn't resize?

Then any resampling in the printer driver to match a higher hardware
resolution at least shouldn't give any 'surprises', as presumably, any non-
trivial resampling needed to match sizes [fractional resampling] would have
already been done by the hopefully smarter software engine.

 

[babaloo]

No printer prints at much beyond 350 dpi, if that. Printer manufacturers are
not always truthful in this regard.
Unless you send a file to the printer at a dpi the printer can handle and
specify that the file be printed at that dpi the printer driver will
resample the image. The printer driver will also convert colors to the
printer's color space in an arbitrary fashion. The printer driver has to do
these things or printing cannot take place.
This is why serious printing requires both monitor calibrated color
management and use of an imaging program, such as Photoshop, that employs
color managment and allows the user to control the way image/printing dpi is
implemented.
For large prints that will be viewed at a distance you can sometimes get
away with 200 dpi. Otherwise it is a wise policy to standardize on 300dpi
and to make the conversion, if necessary, yourself rather than blindly
depending on a printer driver.

 

[James McNangle]

First, understand that unles your image happens to be at the exact
resolution of the printer, the driver is going to resample it anyway.

Second, there is a limit to the resolving power of the human eye and
anything beyond it is wasted. Since this resolving power is really an
angular one, the limit depends on the viewing distance: bilboards need
significantly lower resolution than picture postcards.

Agreed. It is very nice to have an A4 print which is dead sharp all over, but
in the real world you will probably hang it on the wall anyway, so you won't
appreciate the sharpness. I suspect that if the resolution is enough to make a
really sharp A6 print that will be adequate in 99.7% of applications.

From personal experience, I've found that the unaided eye is unable to
distinguish -- at close viewing -- anything much beyond 300-400 PPI and I've
standardized on 300 PPI for anything but "gallery" prints; it's a waste of
time and ink.

It will only save time if the time taken to resize is less than the time saved
in printing (though I suspect you probably have a batch process for doing this).
There is also the point that in resizing you introduce another layer of JPEG
artefacts.

On the other hand I think it is most unlikely that the resolution will have any
effect on the amount of ink used. I strongly suspect that, as somebody else has
pointed out, the printer will always resample the image anyway to decide where
to place the dots of ink, so that the original resolution will effectively be
ignored.

The basis for the instructors advice may well have been that some printers use a
relatively unsophisticated resampling algorithm.


James McNangle