If you’re worried about whether you need more megapixels, I used to be like you; let me help you.
You do not need more megapixels.
I’ll explain in a somewhat Zen-like manner. If you need more megapixels, you already know it, so you aren’t worried about it. If you’re worried rather than knowing, it is most likely because you don’t have a great understanding of how marginal a difference it makes on most of your pictures.
One variable is almost always given short shrift in discussions of “how many megapixels do I need?” That variable is viewing distance.
This isn’t a minor thing. As images are printed larger, our viewing distance tends to grow in a linear fashion. We photographers have a tendency to pixel-peek, inspecting our images magnified beyond belief in our post-processing software.
As the image gets larger, we see the pixels. We always want more – infinite resolution becomes a subconscious goal.
But this is not how photographs are viewed and judged in the real world. Often in photography, a 50mm lens is said to give a normal viewing angle of approximately 40 degrees. We’re used to seeing things this way and our normal response is to back away from a large picture, and move closer to a smaller one.
When we’re further away, we can’t see the individual pixels.
We blow-up photographs large when they deserve to be blown-up.
Are more megapixels bad? Of course not, but often they’re inconsequential.
There’s an important disclaimer/exception to all this. Sometimes when we shoot a picture, we discover that the “real” image we want is only a small piece of the larger photo.
I’ll use an example with easy math. Pretend a tic-tac-toe board is superimposed on your image and you decide the crop you want is the center square of that tic-toe-board.
So there are nine squares and you want one of them. This means your image will contain only 1/9 the pixels of the original image. If you had 12 megapixels to begin with, after this crop you only have a 1.33 megapixel picture.
Few would dispute that this is basically too few for normal photography. But if it is a superb photograph, it’s quite possible that the lost resolution becomes a minor factor. In other words, what we will tolerate is context-dependent.
With a higher-pixel camera, all other things being equal, you get more compositional room. You can shoot “sloppier” in a few ways. You can be farther from your subject than normal. You can shoot wider than you envision your final image.