Back To Basics - Exposure: The Great Balancing Act - 3 of 5

Aperture

In the last article, we discussed shutter speed and learned that while lengthening the duration that shutter was open allowed more light into the camera, it also introduced motion blur into the photograph.

Today we'll talk about another fundamental control that the camera provides us - aperture. Hopefully the water faucet and bucket analogy from the shutter speed article made sense because you're going to read a variant of it right now...

 

Imagine a water faucet filling a bucket. Once we turn the valve on, it makes sense that the amount of water filling the bucket would be determined by the diameter of the pipe. The larger the diameter of the pipe, the more water that flows when the faucet is open, and vice versa, the smaller the diameter of the pipe, the less water fills the bucket. For job security, photographers refer to the diameter of the "light pipe" that allows light to flow into the camera as "aperture".

Think of the implications of aperture for a second. We can now vary the amount of light coming into the camera by adjusting the diameter of the shutter opening, while keeping the shutter speed constant. We said yesterday that the end goal was capturing the right amount of light to get your desired photographic result - we've just added another tool to the toolbox for doing so. However, like the motion blur introduced by varying shutter speed, aperture comes with it's own by-product which requires consideration as well.

Trade-Off: Depth of Field

People who design lenses are very smart. I can assure you that I don't understand all the mathematics that govern optics and I won't bore you with the few equations I do understand. What you need to know is that aperture results in a phenomenon called "depth of field" and I'll try to explain it here. Depth of field relates to the distance in front of and behind the object you're focusing on that will be in focus. There are lots of depth of field calculators all over the internet if you'd like to calculate the exact numbers for a particular situation. We'll leave that for another discussion, but here's the general rule: the greater your aperture, the more shallow your depth of field. The smaller your aperture, the greater your depth of field. Memorize that right now.

Tangent: Another thing photographers like to do to protect their job security is refer to aperture values using the "f-scale." When using the f-scale, the smaller the number the greater your aperture diameter. That's the opposite of what you'd probably expect. Don't you just love photography!

To illustrate the depth of field, with the help of my three-year-old, I covered a table with 5 feet of miscellaneous stuff. I then lit the scene with overhead lights, set the camera up on a tripod, and started shooting away. The focal point selected by the camera (for all of the shots below) is on the "2FT" mark on the ruler. FYI: There is the a watch right behind the Nikon D90, and if you zoom in on it, you might think it's a Rolex... don't be too impressed... it's a knock-off - I don't have the funds for a real one!!)

Using a depth of field calculator, I'm also including the estimated depth of field. I've included larger zoom images than usual so you can really look at what is in focus and what isn't. It's worth clicking on each of these to see the subtle difference in focus.

Aperture: f/1.8, DOF: 0.32 feet

Aperture: f/2.0, DOF: 0.35 feet

Aperture: f/2.8, DOF: 0.5 feet

Aperture: f/4.0, DOF: 0.71 feet

Aperture: f/5.6, DOF: 1.01 feet

Aperture: f/8.0, DOF: 1.44 feet

Aperture: f/11, DOF: 2.08 feet

Aperture: f/16, DOF: 3.08 feet

Aperture: f/22, DOF: 4.77 feet - At this point, the entire scene is in focus.

I know I'm probably starting to sound like a parrot, but again let me say - like shutter speed, there's no right and wrong aperture. Depth of field is a powerful tool for composition of your photos. It can be used to isolate a subject from a distracting background, or it can be used to ensure that an almost infinite distance to the horizon all remains in focus. You as the photographer have to decide what you're going for in any particular photograph.

Now you've probably started realizing why I refer to exposure as a balancing act. You can capture the same amount of light in two different ways, a fixed diameter pipe which is left open for varying amounts of time (shutter speed), or varying diameters of pipe left open for a fixed amount of time (aperture). Each option carrying its own advantages and disadvantages. At this point, I'll ask for you to stop thinking about balancing the trades between adjusting aperture and shutter speed. Why you might ask? Cause that's what we'll discuss in part five of this series... and I don't want you spoiling anything.

Tomorrow, we'll discuss our third and final option, Sensitivity...