Digital photo manipulation basics
by John Orth and Ursula Hoffmann

Acquire an image from a digital camera or by scanning -- or copy it from the Web or from a CD -- provided it is not copyrighted.

When using digital images for any type of work, regardless of whether or not the image will be reproduced on the screen or in print, it should be first priority to be certain that the image you use is the absolute best it can be. Keep in mind what the image is intended for. This will not only make things easier when making adjustments to the image but will also help you to avoid making unnecessary alterations.

Know the image's intended purpose -- for printing? for display on the computer / a web page? -- regardless of whether you are using a scanned image or one from a digital camera. If using a digital camera, adjust the resolution before taking the picture if possible.

Cameras take pictures in pixels = dots. File size might be several megabytes, too big for use on the screen.
The key measure is resolution, expressed in dots per inch, or dpi.
The higher the dpi, the greater the quality, the larger the computer file size.
Know the resolution of the printer to be used. Do not print at a larger size than that of the original image. Aim at a minimum of 400 dpi. The higher resolution, the larger the image may be printed, still sharp. Examples: 100 dpi, 3 x 5" print, 586 KB. For twice that, a 6 x 10 printed image, you need to scan at 200 dpi and its file size will be four times larger.
A 1 MB picture will print a decent 5 x 7" photo.
A 2 MB picture will print a decent 8 x 10" photo.
On the whole, it makes more sense to have a copy made at a film store.
Display on the computer or Web: 72 dpi, file size about 20 to 40 K. That is VGA (640 x 480). On a monitor set to SVGA, the image will simply be smaller than the screen. A higher resolution is overkill: the file is larger but the extra detail will not show.
For magazines and printing at home: use at least 400 dpi.
For computer screens, use just 72 dpi.

File sizes that are too large will slow down a web site and any image that is too small will lack the detail many images require. Images posted on the web work well for VGA 640 x 480. They may fill a part of the screen, the whole screen, or you may need to scroll, if the image is very large, to see all of it. For large images with a large file size which will download slowly, we recommend the use of small so-called thumbnail images which the user may choose to download and view at will. These have to be created separately. Never reduce or enlarge an image more than, say, 5%, for quality will be drastically corrupted.

After the appropriate size is attained you may wish to adjust or manipulate the colors within the image. Color adjustment can be handled in any photo editor. To avoid possibly losing image quality only change colors that absolutely need it. Brightening or darkening an image will also seriously ruin image quality if care is not taken. Always make adjustments to a copy and not to the original image.

It is possible to adjust the color balance in small portions of the image without affecting the rest. This is done by choosing the selection tool on the editor and outlining the part of the image you would like to adjust. This is much easier if done after zooming in on the specific area. Also make sure that when selecting a portion of the image that you completely close the loop of the selection or any alterations may affect the whole picture.

The image may contain a blemish that you would like to remove. It does not matter what the undesirable blemish is because you can paint over it. Use the color dropper on the tool bar to choose the color, either from the image or from the color selection chart, that you would like to paint with. Then select a tool for applying that color, a paint brush or a spray paint can for example. It is much easier to make corrections like these after you have zoomed in on the working area. Also, you will want to use small tools. Small corrections, small mistakes. You can change the size of almost any tool on the tool bar. And remember to save often.

When the image has reached the quality desired you might then want to crop out undesired portions of the image. For example, you may wish to reduce the amount of background or even remove the ex-boyfriend or ex-girlfriend from the photo. The area you select with the cropping tool is the area that remains. If an error is made when cropping, or when making any other adjustment for that matter, remember to use Edit Undo immediately.

The cropping tool can also be used to reduce the file size of images. If the image has a file size that is clearly too large for the web (more than 50k) and can be rephotographed or rescanned, shoot or scan a larger image and then crop out the desired portion. The new image will have a smaller file size. This technique is extremely effective when done properly so do not be afraid to experiment and practice.

Do not edit and re-edit images that are in compressed (potentially lossy) format, such as .gif or .jpg. Their quality may disintegrate whenedited and saved several times.

Image requirements for submission to or any other website:
(Re)name every image -- do not use the number assigned by the digital camera.
Format: .jpg/.jpeg or .gif -- resolution between 72 and 96 dpi.
(Re)size every image, in pixels: maximum 400 width x 300 height but preferably smaller.
Image filesize: maximum 100.000 bytes or 100 kb. Anything larger will load too slowly or may even cause the DownloadWizard to time out. Viewers will not like it.
This also applies to images used in a Powerpoint presentation on the web.

Prof. Ursula Hoffmann, last rev. March 2005