Image Resolution, dpi, and quality
The Rule of Thumb
As a general rule of thumb you should work on these resolution values:
Any graphics that you are producing for screen use (ie website graphics or on-screen displays)
should generally be 72dpi.
Any graphics that you intend to print yourself on your desktop printer
should generally be 150dpi.
Any graphics that you are having professionally produced by a commercial
press or digital printing provider should be 300dpi.
There are exceptions to these rules of thumb but if you keep these
basic rules in mind whenever you create images you won't go far wrong.
Some advice on the exceptions will follow later.
What is Image Resolution?
Image resolution or "dpi" ("dots per inch", also called "ppi" or "pixels
per inch") determines the number of pixels that a digital image can
contain and as such the depth of quality and size of the file. The
more pixels an image can contain, the better quality it can be but
also the larger the file size. It is important to understand how resolution
affects image sizes - in print, the resolution does not affect the
image size, but on screen, resolution can have dramatic effects on
the image size. See the example images below. All three of these images
have been originated at 3 inches square but at three different resolutions.
If we were to print these images then they would all be identical
size but would appear in degrading quality as the resolution gets lower. However as you can see, when viewed on a screen they are wildly
different in size. This is because on a screen you can only view images
at the screen resolution of 72dpi (96dpi on some systems). This is
why the larger resolution images appear to be so big when viewed on
a screen.
Quality
As you can see from the simple example below, the more pixels an image
contains the more detail and quality it can contain. If you look at
the smaller size images below, you will see that when they are expanded
they become very "blocky" or "pixellated". There are a number of adopted
standards in resolution which you should adhere to as often as possible
to ensure best results. The commercial print world uses 300dpi for
most of its output, so you should try and originate artwork to this
resolution where possible.
Q and A
Q: Do images always have to be 300dpi
for commercial printing?
A: No, there are exceptions to the rule.
If you are reproducing a fairly large low-resolution image at a small
print size then you can very often get away with this. A classic example
these days is the use of a digital camera to take photos for printed
brochures. The camera may be a 6-megapixel camera capable of taking
photos at 72dpi or 180dpi (below the commercial 300dpi standard) at fairly
large print sizes (quite often bigger than A4 in size). If this image
is to be reproduced faily small on the document then the resolution
becomes irrelevant as the physical size of the image usually
compensates for the lower resolution.
Q: What should I do to create images
across several media?
A: Always create your images at 300dpi
for print usage, and then down-size them for use on desktop printers
and websites. Never work at a low resolution and attempt to up-size
them as this results in blocky and untidy images, whereas reducing
images tends to work fine.
Q: If I increase an image to 300dpi
will it improve the quality?
A: No, this is a common misconception.
If your original image is under 300dpi then resizing it up to 300dpi
simple expands the pixels, it does not improve the quality. In some
cases it can actually degrade the quality. It is always best to down-size
an image rather than attempt to up-size an image. If your original
file is very small and low-resolution then you may have to accept
that it is simply unusable in a commercial application.
Q: Should I ever originate images above
300dpi?
A: There are times when you may wish
to create a very high resolution image at 600dpi, 900dpi, 1200dpi
or even higher to preserve the quality. You may submit images of this
extreme resolution for commercial printing - the quality may be just
that bit better - but the downside is that production costs might
be higher or leadtimes may be longer as the digital processing equipment
will require more time to process the huge volumes of data. In general
there is no need to submit images to a commercial printer above 300dpi
but there may be special jobs when it is desirable.
Q: What size should I originate images
at?
A: There is no simple answer to this
question as it depends on the final output. As a general rule you
should, where possible, originate your image either the same size
or slightly bigger than the final output. However, if your final output
is to be very large such as the side of a van or a giant poster then
this will be unfeasible as it will generate such a huge file. We have
found that any images which have to be output at very large print
sizes are usually fine at about 30% - 60% of the final output size.
Q: Does the colour mode (RGB or CMYK)
affect the resolution?
A: No. The colour mode is a totally separate
issue and not related to resolution. |
