Last time we covered the three settings — aperture, shutter speed, ISO — that combine to create one ideal exposure for a scene. Today, a closer look at aperture specifically.
Different f-stops let in different amounts of light — f/2.8 lets in a lot more than f/22. The smaller the number, the bigger the opening, the more light. As you may have noticed already, several different combinations of settings can land you on the same exposure — but each one has its own effect on the final image, and understanding that effect is the real key to getting the photo you had in mind.
Depth of Field
The aperture you choose controls depth of field — the portion of the image that reads as acceptably sharp. There’s always a trade-off: f/2.8 lets in a lot of light but might only give you a few inches of sharp focus, while f/22 might get you close to infinite depth of field.
A shallow depth of field (f/2.8, f/1.8) is great for isolating a subject — a single flower in a meadow. A deep one (f/22) is what you want when the whole scene matters, like a mountain and its reflection on a lake. If you’re not trying to do either in particular, the old rule “f/8 and be there” is a fine default.
Try It
Open a book flat on the table and photograph it at f/2.8, then again at f/22. At f/2.8 you’ll only be able to read a couple of lines before the text blurs out. At f/22, far more of the page stays legible. Try changing the angle you shoot from, and see what happens when you keep the camera parallel to the page instead.
The aperture you pick controls depth of field, and depth of field has a bigger effect on the final photo than most people expect starting out.