Many of us amateur photographers don't bother perfecting the composition of a portrait while we're shooting because we can just crop the photo after the fact. But as photographer and educator Joe Edelman points out, if you aren't filling the frame with your subject, you're just wasting your camera's precious pixels. Instead, you should fill the frame with your subject as much as you can while you're shooting — and not in post. It really comes down to two factors, as Joe explains: You're either lazy or afraid (and in my case, both!). It's simply easier to have your subject in focus and in the centre of the frame so that you can edit it later. That also means you're wasting the capabilities of your camera's sensor and, ultimately, compromising the quality of the image. You might not be able to crop it later if you do fill the frame, and yes, you might even make a mistake that you can't correct, but it's better to take advantage of your camera's capabilities than to hope you can fix it in Photoshop.
Most Important Composition Rule for Photographers [Joe Edelman]