The word "resolution" has several different meanings in photography and while they are interconnected they can be downright confusing. There is lens resolution in lines per millimeter (lm), printer resolution in dots per inch (DPI), and pixel per inch (PPI) in digital photography. It is not hard to understand these "resolutions" and by understanding them you will be able to use them to your best advantage.
When I shot film, it was easy to drop exposed film at a lab and a few hours later have slides. For jury submission or publication, I'd just select the slide I liked best and send it off via snail-mail to the show or magazine. Now, I spend hours editing digital images. I have different uses for my images and that often means editing and filing the same image at several different file resolutions. The issue for craftspeople is that publishers and juries may ask for several different file resolutions. You need to know what that is all about.
Image File Resolution simply tells a device how to distribute the file for a particular use. For example, most computer screens have a 72 PPI resolution. If your file resolution is 72 PPI it tells your computer to take the 4,000 pixel columns and 3,000 pixel rows (from a 12 MP sensor image) and put 72 in each inch of the screen. But, 72 PPI divided into those 4,000 pixels gives you an image that is about 55.5" long--clearly way too big for the screen. What happened is that your computer sliced and diced the file to fit reasonably well on a screen. Take a magnifying glass and look at a PC, iPhone, or Tablet screen and you will see red, green, and blue phosphor dots. If you could count them, there would be 72 per inch. This is the screen's resolution, and no matter how hard you try, you cannot squeeze 180 pixels into the 72 PPI screen. The device has to edit down your image to fit.
Now take that magnifying glass and look at the pictures printed here in The Crafts Report or any other magazine. You will see dots of ink--lots of dots, some 300 per inch. This is the resolution used in printing. Again, an image file has to be resized for this use. If you printed a photo at 72 PPI it would look very soft and you'd see little pixel boxes like the image at XXXXX of a pixelated ring.
Generally, when you upload an image to the web you want to send a file at 72 PPI. This way it will directly fit most PC and PDA screens without the intervention of the device or the recipient. Sending a higher resolution file (like 300 PPI) won't make the image sharper, but it will increase the upload and download time significantly.
When you take a digital photograph, your camera assigns the image file a resolution. Depending upon the brand, the type of camera, and what the manufacturer thinks most consumers want cameras use different file resolutions. Typically, digital Single Lens Reflexes (dSLRs) save image files at a resolution of 300 PPI. "Super zoom" or "bridge" cameras and many Compact System Cameras use 180 PPI, while high-end compact and inexpensive, pocket, point-and-shoot cameras are set to 72 PPI.
No matter what resolution the camera uses, the total file size and number of pixels remains the same. All cameras with a 12 MP sensor, for example, produce image files that are the same size--roughly 4,000 x 3,000 pixels.
One of the confusing things about image file resolution is that the sharpness of an image relates to the device we view it on, rather than the file resolution itself. For example, the screens on iPads and camera monitors are small, but contain as many as 960,000 dots (phosphors) which make for much sharper images than on a PC. However, this can be deceiving. These images look very sharp on a phone or tablet, but when enlarged on a 17.3" monitor screen they can look awful.
The relationship between PPI and DPI is another area of confusion. Color printers have DPI resolutions in the thousands of dots per inch. When you print an image file, each bit of pixel information is turned into many, many dots of ink. Because the printer lays down so many dots of ink, they can make excellent color prints from very low PPI resolutions. But, there are limits. A 72 PPI image file does not print well because the pixels are distributed too far apart. To get around this, when a printer reads directly from a memory card or receives a file by WI-FI, it will automatically alter the file resolution and pixel dimensions to fit the needs of print you are asking it to make.
As a rule, when printing a photograph adjust the image file in Photoshop or other editing software (see below) and try to keep the file resolution as far above 175 PPI as you can to insure the sharpest prints.
Above: These two images illustrate image file resolution. The sharp image has been saved at 300 PPI for printing on this page. The other shows what the same image would look like if it was at a much lower setting of 20 PPI resolution.
Printing for Publication
Resolution for publication is 300 PPI. When I e-mail an image to The Crafts Report for my columns, I resize it to a 300 PPI resolution before I send it. Publications and other print media (ads, flyers, catalogs, etc.) require this because ink presses use 160 lm line screens and need twice that resolution for sharp publication images.
Changing the image file resolution changes the distribution of pixels per inch and nothing else. So when you change the resolution from 72 to 180 or 300 PPI the file really remains the same. But, suppose you want to send a picture to The Crafts Report for its cover competition. The magazine needs a 300 PPI file that is at least 8.5" x 11" in physical size for the cover. How do you figure out the physical size of your image? Simple. Divide each of the pixel dimensions by the resolution. A 12 MP camera produces a full image file that is 4,000 x 3,000 pixels. Divide each of these numbers by 300 PPI and you get 13.4 x 10 inches. That's bigger than the magazine cover.
While the math may seem daunting, changing image file resolution and image size is easy. Photoshop and similar editing programs include tools to do this. Using Photoshop, click on "Image" from the toolbar. From the dropdown box, select "Resize," then "Image Size." This gives you a dropdown box containing the file's pixel dimensions, physical size, and resolution. Uncheck the "Resample Image" box (if it checked) and type in the resolution you want in the "Resolution" space. Click "OK" and the file is at this new size.
When you change resolution you will see the physical dimensions change too. By practicing with the "Image Size" tool you will soon learn to resize both the resolution of a file and its physical dimensions for any jury submission or print.
Ultimately, understanding resolution is about taking control of your pictures and getting the most out of every image you shoot. It may be extra work, but it's worth it.
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