Your camera, whatever type or brand it is, is designed to capture an image at the highest resolution your camera sensor delivers. Some cameras allow you to adjust the capture resolution in its menu, using terms like “fine” or “normal”. Most photographers agree that you should capture the image at the highest resolution you and can that storage space allows, typically anywhere from 10 to 60 megapixels (that is, 10 million to 60 million pixels along the longest edge of the image), depending on the camera. This MP (megapixel) number represents just the raw capacity of the sensor to capture information. That pixel capacity of the sensor doesn’t mean much until you size the image. Typically, that means how large a print you want to make. The goal here is that you have at least enough resolution so that you don’t see any individual pixels.
A quick diversion: the visibility of pixels varies by the way the image is viewed. A typical monitor often has a resolution of 72 pixels per inch, ppi. You simply cannot see any higher resolution on the screen, no matter how much resolution there is. Printer resolution gets a bit complicated, as the essential measurement of an inkjet is the number of droplets of ink per inch that the printer is capable of laying down on the paper, typically 300 to 360 droplets per inch, or dpi. Fortunately, your printer driver reconciles the two, so dpi is generally not something to worry about. We point this out only because dpi vs ppi is a frequent source of confusion. What matters to us, and to you, is ppi — pixels per inch. For our purposes, Photoshop and Lightroom and most other editing software care only about ppi.
Think of pixels as a raw commodity at your disposal. You can decrease resolution by throwing some away, or increase resolution by adding pixels, although when you add pixels you are asking your software to guess what information should be there that isn’t. So always start with as many pixels as you can in the original capture of the image — it’s far better to throw away pixels than add them. This process of changing size/resolution, whether reducing or adding pixels, is termed resampling.
The key takeaway is this: print size and resolution are completely interdependent. Increase size and you reduce resolution. Reduce size, and the resolution increases. When you ask the software to resample during the process, you are telling it to add or discard pixels as needed in order to maintain a specific size and resolution. For prints, excess resolution is perfectly OK. We can print a 30 x 45 file at 10 x 15 with no problem. The printer simply won’t use all the excess pixels. However, at less than 300 ppi your print will start to lose detail.
Calculating the image size is easy. An image that is 10” wide at 300 pixels per inch will 3000 pixels wide. To make an image 7” wide at 300 ppi, you know ahead of time that it will be 2100 ppi.
To resize to that spec, put into your image sizing box 300 pixels inch resolution and a width of 7”. You must also select “resample” and “scale proportionately”. The resulting resampled image will be 7” (2100 pixels) at 300 ppi. That is the correct size and resolution for submitting to our exhibits. We ask for that spec so that, if your image is selected by the juror, we have an image suitable for printing in the exhibit catalog. That uniform spec also allows the juror to see all of the images at roughly the same size and with plenty of resolution.
A WORD OF WARNING: Always rename any file before you resize it, and keep the original file unchanged. Otherwise, you will no longer have a full-resolution file, and you will not be able to make a print larger than 7” wide!
Compression
After you’ve gotten the size and resolution adjusted to 7” (2100 px) at 300 ppi adjusted, you may need to compress your file to meet our 5 megabyte maximum file size. A jpeg file of that size is commonly less than our 5MB max file size when the file is CLOSED that is, sitting unopened on your desktop. (If you work with layers in the editing process, be sure the file is flattened so that all layers are combined into one.) Your file will be substantially smaller when it is closed, rather than when it is open in your editing software. The important number is the file size when CLOSED. If it’s larger than 5MB, it will need to be compressed. When you save a file as a JPG, you will be asked how much compression you want to apply. The scales used by different software are not all the same, but try to choose a number of 75-80% compression (8 in Photoshop/Lightroom). If all else fails, experiment with JPG compression setting in your software to find a setting that keeps the closed file at less than 5MB.