by Rich Seiling
are confused by resolution, and the way it affects image quality. This
is particularly true when digital cameras are involved, because large
prints have to be made at lower dots per inch than the "standard"
of 300 dpi.
question we hear at West Coast Imaging helps illustrate the confusion: “I
have a 6 megapixel camera. How big of a print can I make from a 3008
by 2000 pixel file at 300 dpi?”
If you have 3008 by 2000 pixels and choose to lay them down at a resolution
of 300 pixels per inch, it’s simply a matter of dividing the number
of pixels on each side by 300. In this case, dividing 3008 by 300 gives
you the width of 10.027", and dividing 2000 by 300 delivers a height
of 6.667", rendering a final file that is 10.027" by 6.667".
is not the answer most photographers are seeking. In virtually every
case, what the photographer wants to know is, “How large can
I print my digital camera file, and still have it look good?"
is to learn the difference between absolute resolution and relative resolution.
Until you learn the difference between the two, you won't have the answer
you are looking for.
you want to know absolute resolution, or relative resolution?
So what’s the difference between absolute resolution and
relative resolution, and how does it apply when you're trying to decide
how big to print your digital camera files?
We can get to the heart of the matter by opening a digital camera file
in Photoshop, then bringing up the image size dialog. Here we
see the “size” of the file expressed in two different ways.
The first way is to look at the Pixel Dimensions. In this box,
we see that the file is 3008 pixels wide by 2000 pixels tall. These are
absolute pixels. Each and every pixel on this 3008x2000 pixel grid corresponds
to a pixel generated by the digital camera from the CCD or CMOS sensor,
then stored as digital data in the file. In the case of a digital camera
file, the number of pixels we have is limited by the size of the sensor.
if we only have a 6-megapixel sensor and set it to capture at full resolution,
we only have six million pixels in our file. This gives us the absolute
resolution--the absolute number of pixels contained in the file.
Now that we know that the number of pixels we have is fixed, we can look
at how tightly they are packed into a print, which is the relative
resolution. This is displayed within the Document Size
box inside the Image Size dialog.
begin by un-checking the Resample Image checkbox. This tells
Photoshop that we don’t want to make up any new pixels, but instead
want to either pack the pixels closer together, or spread them further
Size box gives us options for Width, Height, and Resolution. These
are relative to our pixel dimensions. The easiest way to understand this
is by typing in different resolutions and watching what happens to the
width and height in the Document Size box.
If you type
in 300 dpi, you’ll see it creates a 10.027" by 6.667"
you type in 200 dpi, it creates a 15.04" by 10" image (see
If you type
in 400 dpi, it makes a 7.52" by 5" image (see illustration
Now, as you
type in different resolutions, watch the Pixel Dimensions and
notice that these do not change. This is the key to understanding
absolute versus relative resolution.
We only have
so many pixels in our digital camera file or scan. The larger we print
an image, the further we have to spread out these pixels. It’s like having
a room to paint and trying to use a pint of paint when a gallon would
be better. We may be able to get some color on the wall from the pint-size
can, but it will be a thin coat, and probably won’t look very good.
Instead, if we choose the gallon size, we can get good coverage and the
color will look thick and strong.
the question the photographer really wanted to know: How far can I spread
the pixels out, and still make a good looking print?
no simple answer to this question because it depends on what device you
are printing on, the subject matter, and what you think is acceptable
quality. However, there are some benchmarks that will help you recognize
when you'll be degrading the quality of the image. Let’s look
at the benchmarks for two common printers:
and LightJet printers:
These printers make excellent quality prints from 200 dpi files. Even
though their native resolution is 300 dpi, through sophisticated built-in
interpolation and the use of photo paper, 200 dpi is more-than-enough
to make prints 16x20 and larger. Whenever you drop below 200 dpi, you
will start to see a degredation in quality. How much depends on the subject
matter and the size you are printing.
made some 24x32 prints from the Canon 1Ds MK II 16-megapixel camera,
and the original 1Ds 11-megapixel camera. The prints were made at 128
dpi and 105 dpi, respectively, and they looked very good. When compared
to prints from film, there was a noticable loss of resolution, yet
they still held up surprisingly well. This means prints made at 100
dpi can still look good--but the quality isn’t the same as printing
from 200 dpi files. When making prints at less than 200dpi, you'll
need to test the different resolutions yourself to see what meets your
My experience is that any time you give the Epson 9600 less than 360 dpi,
you will see noticeable jaggies in the image. This is particularly evident
in text. 360 dpi is a serious amount of resolution. Fortunately, there
is a trick to help us use smaller-resolution files.
shows that if you have a file smaller than 360 dpi, you can make a better
print by interpolating the file up to 360 dpi using Photoshop. So, let's
say you have a 240 dpi file. If you printed it as-is, you would see some
jaggies in the print. But if you take that same file and use Photoshop
to size it up to 360 dpi, it will not have jaggies, and the print will
look sharper than the original 240 dpi file.
trick, I have found that all I need for the Epson 9600 is a 240-dpi file.
Having more resolution doesn’t increase quality, but if you use
less than 240 dpi, you will start see a decrease in quality. For critical
applications, you'll need to test the resolutions yourself to see how
it meets your expectations.
that your final answer?
Now that we have all of this knowledge, let’s ask the question
again: How far can I spread the pixels out, and still make a good looking
A 3008 by
2000 pixel file will create prints in the following sizes:
is that most photographers who haven’t used medium format film or
larger would find the 20" by 30" acceptable. For photographers
who have used medium format, anything larger than 10" by 15"
will begin to fall short of your expectations.
If you need
more of an answer than what I've spelled out here, your best option
is to make test prints of your files, using different resolutions.
Use our print lab to test your prepared files on both the
Chromira and the Epson 9600. Seeing your own images printed at different
resolutions is the best way to answer your resolution questions for
and photos ©2005 Rich Seiling, All Rights Reserved. This page may
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