In Lesson #4 I described the tradeoffs involved in your choice of ISO. Increase the ISO and you can shoot at higher shutterspeeds or in low-light conditions. Higher ISO will also get you more digital noise or graininess - that's the tradeoff.
This lesson covers the tradeoffs involved with aperture. You know..... f/stops....... f/numbers!
Remember that the aperture is the hole in the lens through which light travels on its way to the film or sensor. The aperture is adjustable from large f-numbers which correspond to small apertures to small f-numbers which correspond to large apertures.
Confusing? Maybe a definition will help; -- MATH ALERT! -- The f-number (or f/stop) is the ratio of the focal length of the lens to the diameter of the opening in the aperture. An f/stop of 8 means the diameter of the aperture is 1/8th of the focal length of the lens. If you wanted to double the area of the aperture to let in twice as much light, the f/stop would go from 8 to 5.6.
Remember that the area of a circle varies with the SQUARE of the diameter. Go through the math and you'll find that the area of the aperture doubles when you divide the f/stop by 1.414, which of course is the square root of two.
Don't worry about it. That last part was for geeks like me that just MUST UNDERSTAND THE MATH!
Here's the important part: Big aperture = small f-number AND small aperture = big f-number.
So when I say "OPEN up the lens," I mean go to a small f/stop or f-number. When I say "stop down the lens" I mean go to a large f/stop or large f-number.
Now for the tradeoffs (finally!)
Stop down the lens to a small aperture (like f/22) and you will increase your depth of field. Open up the lens to a large aperture (like f/2.8) and the depth of field will become shallow.
Depth of field is all about how much of the subject matter in your photograph is in focus. Focus on an object out in front of you and there will be a zone in front of and behind the object that is also in focus. With a small aperture (like f/22) the zone will be large and may extend from a few inches in front of you all the way to infinity. With a wide open aperture (like f/2.8) the zone will be small - perhaps only a few inches in front of and behind your subject. How wide the zone is for a particular lens depends on the size of the aperture you choose.
When is this important?
How about a couple of examples:
In landscape photography it is sometimes interesting to have the foreground and the background in focus. Stop down the lens to a small aperture and compose the shot. Now remember, the small aperture lets in less light than a large aperture so it naturally follows that a good exposure will require a longer shutterspeed. The aperture is small so it will need to be open longer. That could be a problem if you are handholding the camera or if your subject is moving.
The photo above was taken in October of 2007 in Grand Teton National Park. The clouds were obscuring the beautiful mountains in the background so I decided to focus on the leaves in the near foreground and the barn in the background. I shot with my f/stop at f/13 - a fairly small aperture setting. This dictated a shutterspeed of 1/80th of a second. For the lens length I was using, 1/80th was fast enough to handhold so I didn't need my tripod. The shot was done at ISO 400. If I had been concerned about digital noise, I could have shot at ISO 200 or ISO 100 which would have forced the shutterspeed to decrease to 1/40th or 1/20th, respectively. Both of those speeds would have required me to use a tripod to make sure the shot was steady. (See how I slipped in that review of ISO!)
In portrait photography it is sometimes nice to blur the background so that the subject really stands out. Open up the lens to a wide aperture and compose the shot. Again, the wide aperture means the shutter doesn't have to be open very long. The fast shutterspeed could be great if you need to handhold the camera.
In this shot of the young lady, the ISO was 400, the shutterspeed was 1/200th of a second, and the aperture was at f/2.8. I handheld this shot because the shutterspeed was fast enough to compensate for any camera shake that might have occurred and because I didn't want to set up a tripod on the railroad tracks!
The most important thing to understand is that your choice of aperture affects how much of the photo will be in focus - or how much "depth of field" you have.
Small aperture = large f/stop = lots in focus. Large aperture = small f/stop = very small plane of focus.
If you remember back in Lesson #2 I talked about shooting in Aperture Priority. This is how I shoot about 90 percent of the time. I control the aperture and the ISO and I let the camera pick the shutterspeed. I do this because I want creative control over what is in focus and what is blurry. Even though I'm not controlling the shutterspeed directly, I ALWAYS watch the shutterspeed to make sure it is fast enough for my conditions. If it isn't, then I bump my ISO upward to speed up the shutter. It's all about knowing how the camera works!
Go out and try to shoot with different f/stops and see what effect it has on your photographs.
If you have questions you can leave them here as comments or you can e-mail me at firstname.lastname@example.org
Thanks for reading!