Understanding the function of F-stops, or the aperture of your lens, is undeniably the most confusing of your DSLR’s features. However, once you master it, you will make a huge leap forward in the quality of your photos.
As a photo student in high school and college, I stuck a slip of paper onto the back of my camera (yes, in film days SLRs had a bookplate slot on the back of the camera where you slipped in a flap from your film box so that you could remember what film you had loaded. Can you imagine?!). On this slip I wrote, “open aperture=blurry” and “small aperture=sharp.”
An open aperture lets in a lot of light. On your lens it is a setting like F/2.8, 3.5, 5.6, or 6.3. Think of the aperture as your eye’s pupil. If there is not much light, your pupil gets bigger, or “opens” to let in more light. But a big result of using an open aperture is the effect of a blurry background that really makes your subject pop off the page (or screen). You can easily control the aperture setting on your camera by putting the camera on “A” and dialing in one of the aforementioned numbers. Your camera will automatically pick a shutter speed to make a proper exposure.
If you have a distracting background, dialing in a wider (or open aperture) will make the background blurry and eliminate the distractions. However, maybe you want the background really clear. Like if you’re standing in front of a sign that you want to be able to read in the photo. Then you would dial in a small aperture.
In this first photo, her eye is in focus, but her earring is not, and neither is the tip of her nose. I was just a few feet away and had the camera set to “A” (Aperture Priority) and dialed in F/6.3. Now conversely, if you choose a small aperture, like F/22 or F/32, your background will appear more sharp. Again, imagine a bright sunny day and how your pupils react: they get smaller and let in less light. But also, think about when you squint and how things get clearer. This is sort of how a smaller aperture works. In the second photo, I dialed in F/32 and her earring is now in more focus.