Little DOF Lesson

Just a little lesson about depth of field

This wasn’t meant to be a lesson at all, just a walk in the park with my camera along. Although the morning was cool, if anything below 80 degrees (27C) is defined as cool, the humidity was quite tiresome. Here in Dixie the nighttime temperature in summer falls down to around the dew point making for a nice steam bath. The birds could be heard but they weren’t about to come out of their hiding places. The larger creatures couldn’t be seen either. I settled on “scenics”, plants, and insects to satisfy my camera. As is my habit, I take a picture as soon as something comes in range, then approach my “model” taking more photos until the beastie takes flight. That is normally a couple of shots at best. But on this occasion the dragonflies were reluctant to escape. A got several sequences. I was happy with my “take” and my walk.

When I inspected my photos I noticed that some of the dragonfly photos taken from a larger distance neatly showed more depth of field than the closer shots. Of course, that is how optics works, and it seemed like a pleasant little reminder, or lesson on this topic. Let me tell you a little more.

First, so I won’t lose you, a few of the photos. They are not masterpieces, but will illustrate my story nicely.

The little strip of images shows all seven exposures of this little friend. All were taken with my lens at 200mm focal length setting and at f/11. In the larger photos I show three crops from three exposures. The top one was from the shot I took from 1.6m, then 1.2m and 1.1m. You can clearly see that the far left wing is sharper in the top photo than the bottom one.

We all know that you get more depth of field, the area where things look sharp, at smaller apertures, that is larger f-numbers. Here is an example of that. Two photos, the left one at f/5.3 the right one at f/11. Everything else was the same.

We also know that at the same aperture setting the depth of field is greater at larger distances from the camera. Not something we usually think about, especially when we are shooting at close distances. If you need more depth, step back! That is not one of the typical advise rules. But it is true. My dragonfly reminded me of this consequence of the way optics works.

For you, dear reader, I did a little bit of extra work. Modern cameras, even my eight-year old little Nikon D60, record all sort of information for each picture taken. EXIF data is what we normally call this set of details that comes with our photos. With auto-focus lenses even the focus distance is recorded. So I was able to go back and get the distance for each shot, then calculate the depth of field at that distance. Here are the results.

At 3 meters, about 6 feet, I get close to 100 mm, 4 inches, of depth of field, DOF. At 2.2 m the field is down to about half that. At my closes distance, about 1.1 m, 44 inches, the DOF is down to 12 mm, a bit less than half an inch.

The moral of the story, once again, if you need more depth, step back! Of course, you need sufficient pixel resolution to be able to crop so you can see your subject.

.:.

© 2014 Ludwig Keck