Resolution
I've been using a computer to color and to work with
images in general for years and I've just recently started to
feel like I have a grasp on resolution. The funny thing is I
probably don't, and I just feel like I do ("Oh how sad" funny,
not "Ha ha" funny) What follows is my best explanation of some
basics of resolution.
DPI:
Stands for Dots Per Inch and refers to printer resolution.
PPI: Stands for Pixels Per Inch and refers to screen
resolution
SPI: Stands for Samples Per Inch and refers to scanning
resolution
Sounds straight forward, but software doesn't always keep its D,
P, and S -PI's separate. Scanners will have a DPI setting when
technically what you're adjusting is the SPI which will later
affect the DPI (though mine does say it's adjusting the output
resolution). The same window in Photoshop is used to adjust the
print size and pixel size of an image, depending on whether a
box is checked or not. Confused? Don't worry, I am too. Here's
some stuff about resolution I've found helpful to know.
For scanning images to color: The higher the resolution
you scan at the larger the image created on the computer will be
in terms of pixels, and larger images are easier to color.
For scanning images to work with on the computer: Images
that have been scaled down look better than ones scaled up. You
can always sharpen to get rid of any fuzziness on a scaled down
image. You can't do anything to get rid of the pixilated look of
a scaled up image.
For displaying images on the web: Image size in terms of
pixels is important, not inches. DPI and print size aren't a
concern, just how large the image is on the screen. As an
example of DPI and print size not affecting images on the screen
check out the following 3 images
The one on the left is 20dpi, middle is 72dpi, and right is
300dpi. They all appear the same size, though, because they're
the same size in terms of pixels.
I'll usually set the resolution of the images I'm displaying
online to 100ppi. It's just a number I like. Then I'll adjust
the actual pixel dimension of the image.
PPI more affected the viewer's computer than anything you can
adjust. It's how many pixels the screen creates for every inch
of its actual size. 640x480, 800x600, and 1024x768 are examples
of screen resolution. So on the web you should think about
whether your image might be too large for somebody's screen.
Offer thumbnails when possible and if it's a website you're
creating test how it will look with different screen resolutions
by changing the resolution of your own screen and viewing your
site.
For printing images: The higher the DPI the smoother the
image will be, but it helps to know the max DPI of the printer
you're using. If your printer can only do up to 300 it'd be
silly to try to have it print a 700 DPI image. Ink jet printers
can usually print at 300 DPI and laser printers can usually
print from 600 to 1200 DPI.
How to adjust resolution
in Photoshop:
Go to Images >> Image Size. You'll get a window something
like this.
On the top is information about the size of the image in terms
of pixels, on the bottom is info that helps you know how the
image will print. Now below that there're two checkboxes. One
that maintains the proportions of the image when the size is
changed and another. Uncheck the second box.
The pixel size is locked and you can change the document
dimensions or resolution of the document without affecting the
size of the image on the screen.
Mode/Channels
The mode of an image can be found and changed in Photoshop by
going to Image >> Mode. You can see the separate channels (if
applicable) by clicking the Channels tab next to the Layers tab.
Each channel represents how much of that color is needed to
create the finished image.
For web images: Screens display images using levels of
red, green, and blue (RGB). Generally that means you'll want to
set the mode of any images you're planning to display online to
RGB. There are other modes associated with web graphics, such as
bitmap, grayscale, and indexed color, but knowing that screens
display different colors using red, green, and blue is the
important thing to remember. If you accidentally set the mode to
CMYK the image won't show up online.
For print images: Printers use varying levels of cyan,
magenta, yellow, and black (CMYK) to represent colors. If you're
working with an image that you know is going to be printed
change the mode to CMYK to give you a better idea of how it'll
look printed. Generally CMYK won't be as bright as RGB so
planning the colors in CMYK will help you avoid unwanted dull
surprises. How well you can predict the printed product varies
depending on the program and the screen you're using, but even
if it isn't exact it'll be closer than RGB would be.
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