Each lens has an aperture that opens to let light into the camera. Aperture is the size of the lens opening for a given shot, measured in f-stops. The higher the f-stop, the smaller the aperture; the lower the f-stop, the larger the aperture. So, f/22 is a very small aperture, and f/1.4 is very large.
The most important characteristic of an image that depends on aperture is the depth of field – the distance in a scene between the closest and farthest objects that appears acceptably sharp in an image. The smaller the aperture, the greater the depth of field; the larger the aperture, the shallower the depth of field. So, an image taken at f/22 will most likely be sharp overall, even if there is a noticeable distance between the foreground and background; on the other hand, an image taken at f/1.4 will have very little depth of field – probably only a portion of the image will be sharp.
Images with shallow depth of field
Shallow depth of field can give an image a really nice softness, especially for weddings. It creates a nice separation between the subject and the background (as you can see in the featured image of this article) and helps you guide the viewer exactly where you want it. Shooting at a wedding with a shallow depth of field can also be really practical, because it helps you reduce, or completely eliminate, distracting background elements over which you have no control.
I usually use a very open aperture during the wedding
For most of the day, I rarely shoot at an aperture smaller than f/5.6; I shoot more often at f/2.8 or f/4. Be careful when shooting at super-wide apertures like f/1.4. Aside from aperture, there are other factors that affect depth of field. A longer lens (like a 200mm zoom) will produce a shallower depth of field than a wide-angle lens (like a 35mm). Also, the distance to the subject makes a difference – the closer you are, the shallower the depth of field.
Always be careful to use the appropriate depth of field
Once, I was photographing the bride during the toast with an 85mm lens, and since the ceremony took place in the evening, it was very dark; so my aperture was wide open at f/1.8. As I stood next to the bride, I noticed that, with her face angled three-quarters away from me, I couldn't focus on both eyes – the depth of field was too shallow. That wasn't the look I wanted, so I switched to a 50mm lens and increased the aperture slightly to f/2.2; the depth of field increased enough to achieve sharp focus of both eyes.