Steve West will be hosting the
meeting for this week. It is vacation time for Wes and Alice – and also
for John Watkins and John Skanderup. But we will see you on August 16th.
I don’t know what Steve’s
topic will be, but I thought I would share this article with you. It has
good information about trying to understand what resolution is.
If you're working with photos
in the computer, you're dealing with different kinds of resolution every
step of the way. And since that one term, "resolution," refers to
a few different things, we're here to help you understand this sometimes
complicated issue.
How is resolution measured?
Resolution is two things: 1) the number of pixels per area (as in dpi--dots
per inch); and 2) pixels measuring a fixed area (such as 1600 x 1200).
These two types of resolution particularly affect three different working
needs: input, image and output.
Output resolution is often
most important to photographers. It's a measure of the highest resolution
your printer can handle, and you want your print to be good, right? Most
printers are optimized for image resolutions of 240 to 300 dpi at
the size printed. That's not the same as the number of ink dots a printer
puts out, which is the printer resolution. If you have a 300 dpi,
4x6-inch image file, but you enlarge it to an 8x10 for printing, the computer
has to interpolate (or make up) image information to fill in the
gaps, and it will lose overall quality. It's the printer's resolution (usually
measured in larger numbers, such as 720 dpi) that dictates how many physical
ink dots into which the printer converts your image file. But dpi isn't
the only measure of a printer's ability. The size of each dot, as well
as how the dots are laid down, can dramatically affect how a printed image
looks to the human eye, which is why some printers that output at 720 dpi
with specialized ink patterns and dots look as good or better than other
printers at 1400 dpi.
The other output resolution
is the monitor. Its resolution affects your work in two ways. First, it
dictates what you see on the screen. Most monitors display 72 to 100 dpi,
which is why a full-sized, 72 dpi image looks the same onscreen as a full-sized,
300 dpi image (unless you zoom in to the picture until the image pixels
are bigger than the screen pixels).
Second, if your images are
intended only to be viewed on other computer monitors (as with e-mail or
the Web), they don't need to be bigger than 72 to 100 dpi because you won't
get a better-looking image. That's why some online pictures from people
who don't understand monitor resolution seem to take forever to load--they're
way too big, even though they look just like smaller images.
What's the difference
between a printer optimized for 300 dpi and a print resolution of 1200
dpi? These are two different measurements--1200 dpi is a measure of
the number of dots of ink the printer lays down, while 300 dpi is the picture
resolution that the printer needs for a good print. Simply, printer ink
dots are interpreted versions of the image pixels. Even with image resolution
set below 300 dpi, at its highest quality, the printer will still lay down
a line of 1200 dots of ink in an inch.
What's interpolation?
Imagine a 300 dpi, 4x6-inch image file as a grassy field with 300 different
golf balls equally spaced over two square yards of turf. There would be
space between each of the balls. When you spread out the 300 golf balls
to cover four square yards of the field, there's more space between each
one.
That's what would happen
if you enlarged the 300 dpi, 4x6 image to 8x10. Now, if you didn't have
enough golf balls (dpi) to fill in the gaps at 8x10, you'd have to guess
at which balls to throw in. The computer also "guesses" what would be in
between those spaces in a photo and fills them in with other pixels accordingly.
That's interpolation. We recommend that you don't use interpolation with
scanners at all, and sparingly for anything else.
How should the input
resolution be set? If you're making prints from a digital camera, keep
it at its maximum. But for a scanner, you need to consider how it's going
to be output. If you're going to make a printout on a desktop inkjet, most
of those printers are optimized for 300 dpi resolution, so anything more
than that for the size being printed is overkill. It just uses up disk
space and processing power.
What's a megapixel?
The term "megapixel" refers to one million pixels. A megapixel digital
camera has an image sensor that's made up of a million pixels (1024 x 1280
= 1.3 M). Typically, each sensor pixel determines each resulting image
pixel. More pixels on the sensor means the camera has a better ability
to resolve fine detail.
Why is the resolution of
my film scanner so much higher than my flatbed scanner? The two resolution
measurements define the maximum number of pixels into which the scanner
can convert the image. And while it would seem that a scanner of 2400 dpi
(like your film scanner) would make an image of much higher quality than
one of only 300 dpi (like your flatbed), remember that they're each looking
at drastically different image sizes. A flatbed scanner is meant for images
usually up to about 8x10, but a film scanner is examining an area of only
about 1x11/2-inches. Think about it--a one-inch, 2400 dpi scan gives you
an image made up of 2400 pixels, but an eight-inch scan at 300 dpi also
gives you a 2400-pixel image (8 x 300 = 2400), so they end up with the
same image resolution.
Does file format alter
resolution? No. However, the compression can affect your image. Compression
means that the computer disregards some image information so that it can
shrink the file size. There are two types of compression: lossless and
lossy. Lossless (such as LZW compression in the .tif format) means
there's no degradation of image quality. But lossy compression (such as
.jpg
compression) means that you may notice a loss of quality between the original
and compressed images, but resolution doesn't change.