Dragonfly Coming at You

What makes the background (and part of the subject) so fuzzy in this picture?

In this, the second in my sporadic photography how-to series, I’m discussing depth of field. This is useful for those using point-and-shoot cameras as well as SLR (single lens reflex) models. Almost every digital camera sold allows some degree of control over settings, and getting a handle on depth of field is a quick, easy way to make your photos stand out on purpose instead of accidentally. (If you don’t care a thing about this, then don’t worry, there are several pictures to see.)

First, a simple definition. Depth of field refers to how much of the picture is in focus. The amount is determined by how wide open the lens is. You know when you go get your eyes checked and they dilate them? Your vision is out of focus until your pupils close back down to normal. Your camera lens works the same way as your pupil — the wider open the aperture is, the more light it lets in, but also the fuzzier it sees things. It isn’t a direct comparison, but it helps relate it to something a lot of people understand.

Now, on to examples — the best way I know to explain this concept.

Train Track Sharp
Figure 1
Aperture: f/18
Shutter speed: 1/90 second
Focal length: 105mm*

Train Tracks Fuzzy
Figure 2
Aperture: f/4
Shutter speed: 1/1500 second
Focal length: 105mm

In Figure 1, the lens shutter is closed down fairly tight. In Figure 2, it is as wide open as it can get. It can be confusing because it’s a relative fraction of the focal length of the lens. Just remember how in fractions, 1/18 is a smaller number than 1/4, and you’ll start to get the hang of it.

Notice how much of the image is in focus in Figure 1 versus Figure 2. That’s because of the difference in depth of field. Also notice that at f/18 (smaller opening), the camera had to leave the shutter open longer to gather as much light as it did at f/4. (These are the only two pictures I took specifically for this how-to. The tracks run behind my office’s parking lot.)

The desire for tack sharpness throughout is why you see so many landscape photographers using tripods even on a sunny day. To have everything look sharply in focus even on large prints, they need maximum depth of field. For that, they have to close the shutter as tight as it will go and, as a consequence, leave the shutter open longer. A tripod helps take best advantage of depth of field’s power by eliminating imperceptible camera shake.

Going the other direction — intentionally blurring parts of the photo besides the subject — often is desirable. Of course, we have examples.

Mater More Background
Figure 3
Aperture: f/3.5
Shutter speed: 1/20 second
Focal length: 75mm

Mater Less Background
Figure 4
Aperture: f/1.8
Shutter speed: 1/80 second
Focal length: 75mm

In both, the background is distracting, and it isn’t a particularly well-composed shot. But in certain situations, the photographer cannot get in a position to eliminate clutter. “Blowing out” the background can help. Even the subtle difference between f/3.5 and 3/1.8 is noticeable and makes Tow Mater stand out more in Figure 4 than in Figure 3. Note that in Figure 4 we start to lose detail on the truck, too.

Here’s an example of how depth of field can help make the subject stand out by blurring things in the front and the back, yet still keep those things sharp enough to be recognizable.

Ben and Neighbor Girl
Figure 5
Aperture: f/4.5
Shutter speed: 1/60 second (with flash)
Focal length: 52mm

In this shot, the little girl’s face is in slightly sharper focus than anything else, but we can still tell who everybody is. This is from our early Monday evening visit to the neighbors across the street. Admittedly, I wasn’t thinking about DOF when taking this picture. I merely discovered it as a good visual aid for this project.

Bird on the Patio
Figure 6
Aperture: f/5.6
Shutter speed: 1/250 second
Focal length: 315mm

Finally, to demonstrate a way to make the viewer look through an insurmountable obstacle, or merely one whose avoidance would frighten away the subject, Figure 6 is my recent picture of a bird grabbing scraps from a local Black Eyed Pea’s patio. The wrought iron fence is blurred to the point of serving as a composition device rather than a distraction (I hope).

One major difference between point-and-shoot cameras and SLR’s is that many of the latter provide a depth of field preview button. When pressed, it “stops down” the aperture to show the photographer (in the viewfinder) the finished product in regards to focus. It will appear dark when using smaller apertures (remember, that means bigger numbers).

There is much more to be said on this topic, but that should start anybody completely new to this. I won’t even begin to discuss Bokeh, a Japanese word with no English equivalent that refers to the quality of the blurriness caused by short depth of field. Ken Rockwell posted a great description and examples at KenRockwell.com.

* All focal lengths listed are the result of multiplying the actual lens setting by 1.5, due to the digital camera’s sensor being smaller than a 35mm film frame. Newer, larger sensors do not require such complicated math and focal lengths can be taken at face value.

The first in this how-to series is a flash how-to.