Image Resolution versus File Size

Q  I like to make cards for my family, but when I put graphics into my cards, the program slows down so much that it’s almost unusable. Help!   – M.D.

A   Many people find that their computers become horribly slow when they work with graphic files. Fortunately, there are ways to
solve some of the problems without expensive
hardware upgrades (although a little more RAM always helps).

The most important thing you can do in working with images is to make them the proper size.
For most applications, you don’t need a
high-resolution image. The higher the resolution, the larger the
image file, and your software works much more efficiently with small images than large ones.

Let’s talk about the difference between size
and resolution. Resolution is a measure of detail: An
image scanned at 400 dpi (dots-per-inch) has twice
as much detail as an image scanned at 200 dpi,
because it has twice as many dots (also known as pixels)
to store information about the image. Size is simply
a measure of the image dimensions (width and
height) in pixels. Your goal is to produce the most
efficient image for the use you have in mind for it. To
show you how to do that, let’s walk through an example.

Let’s say you want to put a photograph onto a greeting card. Your color printer prints at 300
dpi, and the area for the picture is 4.25 inches wide
by 3.25 inches tall. Your ideal image in this situation
is 1,275 (300 x 4.25) pixels wide and 975 (300 x
3.25) pixels tall. Anything larger is a waste of disk space
because the extra pixels are discarded when the photo
is rendered for the printer. Anything smaller will
lose detail because extra pixels will be inserted into
the photo, and the software has to guess what color
those pixels should be by looking at adjacent pixels
(this process is called interpolation).

Now that you know your ideal image size, you
can make intelligent decisions when scanning your
photograph. Suppose the photo you want to use is a
standard 6 by 4 snapshot of your family. Perhaps
your scanner is capable of scanning at 1200 dpi, but is
that the resolution you should use? Probably not.
That resolution would produce an image that is 7,200
pixels wide and 4,800 pixels tall! That’s way more
than you need. Assuming you don’t need to crop the photo, figuring the scanning resolution is easy:
Divide the image width (1,275 pixels) by the photo width (6 inches), and you get 212.5 dpi. Do the
same with the height (975 / 4) to get 243.75 dpi. Take
the larger figure and use the scanner setting that is
closest, but not below it (probably 300 dpi in this case).

Is it any accident that the scanning resolution is
the same as the printer resolution? No, because the
dimensions of the printed image are close to the
dimensions of the original photo. If there is a
bigger difference, then the resolutions will be different.

You can use this same procedure for images you want to put on your web page. You start with the
size of the image on the web page, let’s say 300 by
225 pixels. Using the same photograph and
calculations, you get a scanning resolution of 56.25 dpi (300/6
= 50 and 225/4 = 56.25), but your scanner software probably only goes to 75 dpi, which is close enough.

What if your photo came from your digital
camera, and you already have an image that is 1024 by
768? If you use that image as is, your Web page will
load far slower than it needs to. Even if you tell the
Web page to display the image in a 300 by 225 box,
visitors still have to download the full-size image.
Instead, use your image editing software to crop
and/or resize the image to the actual size you need.

If you use these techniques to optimize your images, your Web pages will load faster and your
card and imaging software will be more nimble.