Lens Basics Part 2 (Depth of Field)

Last week we talked about the 2 main properties of lenses, aperture and focal length. This week we’ll talk about other properties and characteristics of lenses. Since this post is about basics and I am not an engineer or physicist, I won’t get into the science behind all this. Partly because I don’t want to lose my reader, but also because I don’t understand all the deep science behind optics. 🙂

Last time, we talked about how the lens’s aperture controlled how much light hits the camera’s sensor. It achieves this much like the human eye. When you look into bright light, your pupil contracts and limits the amount that enters the eye. When you are in dark room, your pupil dilates and lets in more light. Your lens does the same thing. There is diaphragm inside the lens that opens and contracts in increments called f/stops.

While the aperture controls the amount of light that hits the sensor, it does more than that. It also controls depth of field (DOF). So what is depth of field? It is the area in a photo that is in focus. For example, look at the two photos below. The first photo was shot at f/1.2 with a 50mm lens. Notice that the pot is mostly in focus, but as you move away from it, things quickly go out of focus. (Now, shooting at f/1.2 is pretty extreme, most lenses don’t open that wide but it helps illustrate the point). Now look at the photo below it. It was shot with the same lens, but was shot at f/8. You’ll notice that you can now see the background and also that the handle is now in focus. The depth of field is much wider at f/8 than it was at f/1.2.

The top image at f/1.2. The bottom image at f/8.

Depth of field is not only a function of aperture, but also a function of focal length. For example, if you shoot a photo with a 200mm lens at f/2.8, then you shoot that same photo with a 50mm lens at f/2.8, the photo shot with the 200mm lens will have a shallower DOF than the photo shot with 50mm lens.

The final component of DOF is the distance between the lens and the subject. The closer you are to the subject the shallower the depth of field. The greater the distance, the wider the depth of field. Simple, right?

OK, now that we have this information, how do we use it? Let’s discuss this scenario, you are shooting a photo of a person in an alley. The alley is really cool looking, but when you get just the right angle there is a garbage dumpster that is just visible in the background. So what do you do? You select an exposure that uses a wider aperture that allows the background to go out of focus. The dumpster no longer becomes an issue.

Here’s another scenario. You are shooting snapshots at a family function and must shoot at f/2.8 to get a good photo. If you want to have a wider DOF, select a wider angle lens. Your DOF will increase.

There is a particular characteristic of lenses called bokeh. This term describes the quality of the out of focus area and it varies between lenses. There are lots of factors that goes into creating the bokeh, including the number petals on the diaphragm, the types of glass, and the lens construction. Generally a smoother, creamier bokeh is more favorable. If you look at the photo above at f/1.2, you notice how the background is very smooth and the colors blend together in a pleasing way. Some lens’s bokeh as less pleasing that the out of focus area can be blocky or you’ll have unpleasing artifacts.

Now that you understand depth of field, go out and take some photos and play with the depth of field. It’s a great tool and can be used to create powerful images.

Thanks for reading!!!

-Ken.