Getting it the Right Size
Although there are now photo library programs that will adjust the size of your photo automatically when you select print, a little knowledge will put you in control of the process, enabling you to be much more precise.
Photos for the Screen
When publishing photos on the web, it is best to make the photo the exact size, in pixels, that you need. This way the download time will be minimized. There is no advantage in having a greater number of pixels in the picture as they will not show on the screen and often the picture will have a strange 'squashed in' look about it. Minimizing the size of the file is not so important if you are just going to make a slide show for your computer or a CD but you will find the whole operation will be slicker and work faster if you do.
The first and most important thing to realize is that reducing the number of pixels in a photo to make it smaller is a destructive process, the pixels are discarded and cannot be reclaimed. So always make a new file by choosing 'save as' and giving the photo a new name. This way you will still have the original file as well.
In Photoshop, once you have your new file open, select 'image' then 'image size' from the menus which will open the window below. Other programs will have a similar window.
If we are dealing with a picture destined for the web, we are interested in the top section headed 'Pixel Dimensions'. The section below deals with sizes for printing which we will come to later.
At the bottom of the window we have two tick boxes, the top one 'constrain proportions' keeps the horizontal and vertical proportions of your picture in ratio and, unless you are looking for a special effect, it is important to always keep this box ticked.
The second box 'resample image' just means that we are changing the actual file size as opposed to just the dimensions of the photo for printing. Keep this box ticked when working with photos destined for the web because, as I mentioned above, we want to make the picture the right size in pixels, which means we do want to change the file size.
Resizing pictures should only ever be done downwards. If you made the photo too small and now it needs to be larger then go back to the original and start again. (see 're-sampling' for an explanation)
The drop down box next to the 'Resample' tick box allows you to choose a re-sampling method. These different methods govern how the new pixels are created when you upsize your picture or how they are selected for deletion if you are downsizing. Bicubic is the default setting and this setting gives you the best quality result, it is also the slowest method but that shouldn't be a problem on modern computers.
When designing web pages, like this one, the unit of measure we use is the pixel, we are not really interested in how many pixels there are to an inch or a centimeter because that differs depending on the screen resolution set by the viewer on his monitor. Most screens are capable of showing either 72 pixels per inch or 96. So that gives you a guide to how big your photo will look on most screens.
The JPEG Format
The JPEG format is the most popular file format for photos by far. It has a variable system of file compression which can make files up to ten times smaller than the full TIFF or RAW files or more. All digital cameras have an option to store data in the JPG format and it is a very good compromise between file size and quality. The method of compression used is fairly complex, it basically squeezes the colors a little to give you a slightly smaller palette but one that contains 'almost' all the colors in your original photo. The important thing to know is that the compression system is 'lossy'. This means that when you decompress the file, some of the original information has been lost and if you compress the file again, after making some changes in Photoshop, some more information will be lost and so on.
Most of the time the effect of this 'lossyness' is not too apparent on a first or second generation file but after that you begin to notice the deterioration especially when you resize the picture. Resizing a JPEG can often result in a softening of the sharpness of the image. Sometimes you can fix this easily enough with the unsharp mask in Photoshop but other times this quick fix will not look right.
Always keep the original file safe and do your editing on a copy. The best thing to do, especially with your best pictures, is to save the file straight from the camera to TIFF or PSD format. Even though it may have been saved in the camera as a JPEG it is still only undergoing one level of compression/decompression and will look fine. Then you can work on the TIFF or PSD version, resizing it or correcting the color, and then save the finished photo as a JPEG if it is for use on the web or keep it as a TIFF for printing.
When you save a JPEG file for the first time an extra window will appear with a quality select slider. I have found that the best setting for the web is 8 and for print I use 12.
Photos for Printing
Photos for printing need to have a much higher resolution. 300 pixels per inch is considered the minimum for printing in a magazine and seems to be a good optimum for printing on a color inkjet printer. When sizing a photo for printing refer to the bottom of the two areas in the image size window headed 'document size'. Here you can select the unit of measure inches, centimeters etc. that you are happiest with and choose your print size. If you untick the box at the bottom marked 'resample image' then the file size won't change. If you reduce the print size, the resolution (number of pixels per inch) will increase. As the resolution for printing is largely a matter of the more the merrier there is no point in reducing the file size unless you need the space on your hard drive.
Do not be tempted to increase the resolution by re-sampling. It is possible to make your file size bigger by re-sampling to a higher resolution. The way the program does this is to fill in the new pixels based on the color values of the pixels around it. This normally makes for a fuzzier picture, it will never get better. If you have a photo that you want to make a large print of and the resolution is lower than 300 pixels per inch you will be better off printing at whatever resolution you have rather than trying to increase the resolution. I have printed pictures with a resolution as low as 100 dots per inch and they don't look too bad.
Printing lower resolution will save your printer ink, which can get expensive, try Cartridge Discount for low cost high quality ink and toner cartridges.