Reviews
Glossary
On a camera, the aperture is literally the hole that light comes in through.
It is, of course, a lot more complicated than that. The larger the aperture, the more light comes in, but the narrower the depth-of-field. The smaller the aperture, the larger the DOF, but the less light comes in. Less light means you need to lengthen the exposure or increase your sensor's sensitivity. It's all about what you want to achieve in your photograph.