Depth of Field

Years ago a customer picked up a roll of film from a lab I ran. After looking at the pictures, she complained parts of the pictures were out of focus. I examined the pictures and saw they were of teenagers in a field with a horse. Some of the kids were further back while others were in front of the area of focus which caused some of them to be out of focus. I tried to explain to the woman in simple terms that this problem was because of the photographer. She said the pictures were taken by her brother-in-law who ‘was a great photographer’. She proceeded to tell me that I must have done something to cause the pictures to be blurry. Again I tried to explain in non- technical terms that the problem happened at the time of exposure. She didn’t want to believe me so I said something like this “Mam the problem with the images is called depth of field. The photographer was focused on one spot but because the aperture on his lens was too wide open the plane of focus was too shallow causing everything in front of and behind the focus plane to be out of focus.” She gave me a blank stare…

Have you ever seen a photograph and noticed everything in front of and behind the subject were out of focus? Did you wonder how it happened? This technique is called Depth of Field. It is something good photographers do on purpose. If you aren’t aware of this concept in photography it is easy to get a partially blurry or out of focus image. Hold your index finger out in front of you at arms length.  Now focus on your finger, notice how everything behind your finger is out of focus.  Now focus out past your finger and your finger will be out of focus. Depth of field works like that but has an added element, the aperture in the lens of your camera. The aperture controls what is sharp in front of and behind the plane of focus.

Exposure in your camera is made up of three different things. The ISO at which your camera is set, the shutter speed, and the aperture opening size in the lens.

ISO is a setting on your camera that will increase or decrease the CCD’s sensitivity to light. If you grew up with film, ISO and ASA are the same thing. ASA is the speed of the film 100, 200, 400, 1000 and so forth. The lower the number the less sensitive the film or CCD is to light. Having the ISO set on your camera for the conditions you are going to shoot in is mandatory for getting a good exposure.

The shutter is a mechanism in front of the CCD that opens for a set duration of time depending on your camera settings. The shutter literally is like your eyelids. Close your eyes and now open them and close them again quickly like a blink in reverse. That is how the shutter works in the camera. The longer your eyes stay open the more you can see. With a camera, the longer the shutter stays open the longer the exposure will be and the more light can enter.

The aperture in the lens is just like the iris in your eye. Look in the mirror and notice how your iris is small if the lights are bright and larger if the lights are dim. Your eyes are compensating for the light by opening and closing your iris. The aperture in the lens of your camera does the same with one added feature. The smaller the aperture hole is, say f22, the more in focus everything in the image will be. The larger the aperture hole is, say f2.8, the more out of focus everything in front of and behind your focal plane or area of focus will be. Controlling depth of field is done by how large or small the aperture opening of the lens is.

Proper exposure is metered by the ISO sensitivity, and then a combination of shutter speed and aperture.

Let’s look at a real world scenario of taking a picture and controlling the depth of field. In my image “Whoo are Youu” notice how sharp the young owl is, but how out of focus the background is. The ISO was at 400 because of the dim lighting in the forest. The shutter speed was 1/180th because I was hand holding the camera and I wanted to minimize blur. The aperture was at f6.7 to make sure the background would be out of focus. In this shot I was zoomed out to 200mm.  I always manually meter my subjects so I have complete control over the process. With the ISO at 400 using the spot meter on my camera the owl metered at 1/180th of a second at f11. I purposely over exposed by about 1.5 stops to get the exposure I wanted and to decrease depth of field to make the background blurry. Had I shot at f11 the background would have been sharp and the exposure would not have been as good.

Controlling depth of field in this manner helps make the subject jump out of the image. In this case the young owl really stands out against the blurry back ground.

So after the blank stare the lady gave me, I gave her a short lesson on depth of field and how camera settings besides focus can cause parts of the image to be out of focus. She thanked me for the help and walked off still looking dazed. I had to chuckle to myself as I wondered what her brother-in-law was going to say about his blurry photographs.

Remember good photography isn’t about luck, it is about skill and knowledge of how to capture your subject the way you want it to be recorded.

Happy Trails!!

Tomas W. Mitchell