File Size and Resolution
The reason that resolution is so important is that the higher the resolution, the larger the file size of each individual picture. A large file size has a number of consequences. For example, large files can take a lot of storage space.
First, large pictures will fill up your camera's memory or memory card quite fast. For example, a 512-megabyte card can hold 3,995 640×480 photographs or 147 3072×2304 photographs. In other words, the camera can hold more than twenty times the photographs at the lowest resolution as at the highest.
But the news gets worse. If you save your photographs and modify them in a digital darkroom program, you will need to make copies; you should always keep an unchanged copy of the original. After processing in your digital darkroom, you should back up all of your images (the originals and modified ones) on a backup drive.
There are other considerations as well. You may not be able to work with more than a couple of large images at a time in your computer as they may fill up the RAM when you are working on them. When posted on the Internet, large images often take a long time to download so that online viewers may not be happy campers.
Printing has its own language for resolution. Generally, a printer prints out the same dots that make up the image, but the printer can place dots very close together. The closer these dots are together, the sharper the picture will look on paper. Printer resolution is usually expressed as dots per inch or dpi. And quality printer resolution can be 600 dpi and go as high as 2880 dpi. At 600 dpi, for example, 600 very tiny dots will be in one inch. An image of this quality can rival the quality of film photography.
Unfortunately, there are two terms that are often confused — dpi (dots per inch) and ppi (pixels per inch). While closely related, they are not the same. Dpi refers to the number of dots a printer can put on a piece of paper and ppi the number of pixels in a digital photograph. Read more about this in Chapter 20, but suffice it to say, the ppi of an image must be in synch with the printer requirements or the print won't look good.
Printing can do odd things when the pixels in your image do not match the dimensions of your printout. Printers are adjusted to work with images set to a particular resolution. When you try to print an image file that's at a higher resolution than the printer is geared for, some printers will eliminate the extra pixels, causing the printed image to be of lower quality. The reverse is even more true — a low resolution image printed at a higher resolution will often produce a terrible blocky image. Yet there are solutions to both of these problems.
Adjust the dimensions of your image to fit the printer's output requirements. This will give you much more control over the resampling. If you do not make this adjustment, the printer software will do it instead, which often has disastrous results. In other words, using your own image software, resize your image exactly for the printer's dpi requirements.
Adjusting the resolution can be tricky. In order to match the size of your photograph to the proper output size, you may need to change the size of the image, while always saving an untouched copy of the original. If you do this incorrectly, your picture can look terrible. If you forget to give your new modified image a new file name, it will overwrite the original, which will be lost forever.
You can enlarge or reduce the pixel count by resampling (often called resizing). This is often done by specifying new pixel dimensions — that is, a new height and a new width for the image. You can upsample, which adds pixels to an image, or downsample, which subtracts pixels from an image. Generally, you can get a good quality image by downsampling, since the photograph already contains detail that simply needs to be reduced.
Upsampling is a different issue. In this case, the software is adding pixels that do not exist. Because this can cause a lot of problems, the software performs what's called interpolation, which means that sophisticated calculations are performed to make the enlarged image as close to the original as possible. Bad interpolation can lead to what are called jaggies or aliasing, in which edges in particular no longer look smooth but are jagged and coarse.
Some people have been very successful at enlarging an image using the following technique: they upsample only 10 percent at a time. They do this over and over until they reach the desired size. A number of people have reported that the resulting larger image printed splendidly and that they were quite surprised at the quality.
When you resample you will often be asked if you want to keep the same aspect ratio, which is the ratio of the height to the width. In almost all cases you will want to keep the aspect ratio the same, since changing this will distort the image.