digital basics
When you take a digital photo, or digitize film (or a photographic print) by scanning, you are simply dividing the image into a grid of squares called pixels. A pixel is a solid color and has no size, it's just a bunch of "1"s and "0"s saved to your camera's memory card or computer hard drive. A digital image file doesn't have a "resolution" per se, it's only a grid x pixels in width and y pixels in height. The digital image here is 400 pixels by 400 pixels.
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| Press the buttons to simulate different pixel grids |
Click the buttons to see what it would look like with different pixel grids. More pixels = more detail.
A digital image file doesn't have a "resolution" and isn't even an image until it's output.
Output devices such as a display monitor, photoprinter (like the drugstore minilab), film recorder, or desktop inkjet printer DO have a fixed output resolution. Sometimes this output resolution is adjustable (such as the display settings on your monitor or the print quality settings on your desktop printer), and sometimes it isn't, as is the case with that minilab that prints your digital files at 300 pixels per inch (ppi).
This picture is 400 pixels wide, so it looks fine on your screen, which is probably 800 or 1024 pixels wide. Photoprinter output resolution is 300 pixels per inch, so those 400 pixels get you a print only 1.33" inches wide (400 pixels divided by 300 pixels). A file 1200 px X 1800 px that's perfect for a 4" x 6" print (300 pixels per inch) is too big to view on your screen.
A digital image file has only two important specifications: the file size (the total number of pixels), and the aspect ratio -- the proportion of width to height of the pixel grid. There are several other characteristics such as pixel bit depth, file type, etc., but the two that really matter are file size and aspect ratio.
Common digital image file aspect ratios mirror popular photo film aspect ratios:
- 2:3 (same as 35mm film)
- 1:1 (same as 2 1/4" square film)
- 4:5 (sheet film size)
- 3:4 "digital", which is the aspect ratio of your monitor or TV
As you can see, your capture "resolution" (really your camera or scanner quality setting) must be appropriate for your intended output. Small file size for screen, medium for drug store size prints, and large for larger prints. Figure 300-450 pixels per inch of your desired print size.