Proper Image Cropping: Staple knowledge for photo labs, confusing topic for new photographers, and absolutely ignored by people whose photos only live on social media.
What do we mean by 'Proper Image Cropping'? Constraining your cropping aspect ratio to standard sizes to produce appropriately sized prints. Total gibberish, right? Here's what we mean in plain English: Different print sizes are different predetermined shapes. Some are more elongated rectangles while some are more square-ish and some are just square. We can make our shapes bigger or smaller, but we have to do it proportionally - if we make one side longer, we have to make the other longer too.
What happens if you ignore these preset aspect ratios? Well, if you are posting to social media, nothing really. If you are sending your images to the lab to be printed or delivering them to clients, then a lot can happen. If you have cropped your images tight at the get-go, then an 8x10 might crop out some people in a horizontal group shot or cut off heads or hands/feet in a vertical. Or, you might have cropped an image into a shape that doesn't fit any standard size print and the final print won't look anything like the original file.
How do we prevent this? A few ways. One is to not shoot your subjects super tight. Leave a little room for cropping and straightening - cameras these days have plenty of resolution so a little loss isn't a big deal. It is much easier to crop in than to rebuild some background or have a weird composition. The other way we prevent this is to make sure that our editing software is set up to correctly crop. In Adobe Lightroom, there is a little lock in the cropping module you can click and/or select a standard aspect ratio from the drop down, in Photoshop you can select a standard ratio from the cropping toolbar or enter the one you need, etc. Basically, if you can move the vertical and horizontal edges of your crop independently, it's not going to end up well. Make sure you either lock those ratios in or select a standard aspect ratio when cropping!