Resolution, DPI and Image Sizes

GREAT questions today from Gretchen!

Q. “I am going to burn a CD for a friend of images I took, and I want the images to be low-resolution. I want the photos to be fine to print up to about 4×6, or for uploading to facebook or a blog, etc.

A. Ok so this is a question about image SIZE. There are 200 pixels per inch of photo. So 4 inches would be 800 px and 6 inches = 1200. To resize an image to be printed no larger than 4×6, set the size to 800 x 1200 pixels!

Q. “In Lightroom, when I export, what should I set the Quality Percentage at? And what should I set the resolution to?”

A. This part of the questions is about RESOLUTION. In Lightroom, leave the quality at 100% and let’s talk about resolution. The resolution of a photo is commonly referred to as DPI (dots per inch). In your photos and on your computer screen, images are made up of teeny little dots. The more dots that are used, the clearer the picture. And more dots also = larger file sizes. Resolution is measured in how many dots are in an inch.

So how many DPI do you want in a really high quality photo? No matter how many dots per inch you tell Lightroom to put into the resolution of your photo, the maximum will really be determined by the size your image. But regardless of that, what you see as the default is usually 300dpi. 300 is an excellent, professional quality, 200 is still acceptable and excellent unless you’re printing for the side of a bus! If you really want to calculate down to the dot, consider this equation:

Let’s go back to the aforementioned 4×6 which will be 800 x 12oo pixels in size. To calculate the max DPI for your photo, you just divide the height or width by the inches in the photo. The width of our 4×6 is going to be 1200 pixels. Divide that by 6 inches and you get 200 dpi. So you can set the DPI at whatever you want (the default in Lightroom is 300) but it will print at 200 because that’s how many dots can be squeezed into a 6 inch wide print. Setting the DPI at 300 makes for a larger file size, taking up more space on your computer which will only matter if you regularly set an excessive DPI.

If all of this confuses you and you’d rather not worry about it, don’t! These things are good to understand, but in the end, your photo editing software won’t mess you around unless you’ve accidentally messed with something and not set it back.