Depth of Field

A quote from one of the blogs I follow reminded me of the limitations of depth of field in photography, especially wildlife photography.

One final note: The above does not hold water when you are working anywhere near the lens’s minimum focus distance where d-o-f is measured in small fractions of a single inch. The lesson is that d-o-f increases dramatically as the distance to the subject increases. Source Credit: Fast Thinking Rescues Triplets! – A Depth of Field Lesson, by Arthur Morris/Birds as Art.

Depth of field decreases as the distance to a subject decreases and as focal length increases. When photographing odonates, I always try to get as close as possible. I like to work at (or near) the closest focusing distance of my camera lens and at maximum zoom. As a result, depth of field is always shallow and always a concern.

Sample Photo

For example, let’s look at a photograph of a male Blue-faced Meadowhawk dragonfly (Sympetrum ambiguum) taken during a photowalk at Huntley Meadows Park on 06 September 2015.

A Blue-faced Meadowhawk dragonfly (Sympetrum ambiguum) spotted at Huntley Meadows Park, Fairfax County, Virginia USA. This individual is a male.

ISO 100 | 108mm | -1 ev | f/5.2 | 1/800

Look closely at the full-size version of the preceding photo. Notice the head of the dragonfly is in tack-sharp focus, the thorax is acceptably in focus, and the abdomen is out of focus. The following depth of field table — calculated specifically for my Panasonic Lumix DMC-FZ150 superzoom bridge camera set for maximum focal length — tells the story. (Note: The entire range of f-stop values for my camera is highlighted by the red rectangle; the range of f-stop values at maximum focal length is highlighted by the green rectangle.)

The f-stop number for my photograph is f/5.2, according to its metadata. f/5.6 is the closest value shown in the preceding table of f-stops (in increments of one full stop of exposure). At f/5.6 and a distance of six feet (6′) — the approximate closest focusing distance at maximum zoom — the total depth of field is 0.6 inches (0.6″), or 1.525 cm (15.2 mm).

Two features in Apple Aperture that I miss in Abobe Lightroom are the ability to display the autofocus points used to capture a photograph, and the loupe tool that can be used to check the focus of images.

The following screen capture shows a single focus point that was nearly centered on the dragonfly’s face. This is the single plane in which the photo is perfectly focused.

Show-Focus-Points

Apple Aperture | Show Focus Points

Blue-faced Meadowhawks are relatively small dragonflies, averaging 36-38 mm in length. Measuring 7.5 mm from the face of the dragonfly takes us to a point about one-fifth (1/5) of the way along the length of its body, probably near the thorax/abdomen boundary; beyond that point the dragonfly’s abdomen is noticeably out of focus, as shown in the following screen capture of the loupe tool.

Screen-Shot_2015-09-22_at-6.06.59PM

Apple Aperture | Focus Loupe

You may be thinking, “Hey, I thought you said the total depth of field is ~15 mm.” That’s true, but the total depth of field is divided approximately 50/50 on either side of the single plane of perfect focus. The fact of the matter is I wasted half of the available depth of field by focusing on the dragonfly’s eyes. Although it would have been better to focus on the thorax, it was difficult to ignore the number one rule of wildlife photography: If the eyes aren’t in focus, then the photo isn’t a keeper. Making lemonade from lemons, at least the photo doesn’t look flat!

Application

The following photograph of a toy dragonfly purchased from the Visitor Center at Huntley Meadows Park helps to illustrate the limitations of depth of field when photographing odonates. The toy dragonfly is approximately 70 mm in length from head-to-tail — a little smaller than a typical Great Blue Skimmer dragonfly, averaging 56-63 mm in length.

Remember, at f/5.6 and a distance of six feet, the depth of field for my Panasonic camera is 0.6 inches (0.6″), or 1.525 cm (15.2 mm) — that’s less than one-fourth of the length of the toy dragonfly’s body!

Decreasing the aperture to f/8 (the f-stop number gets larger) increases the depth-of-field to 0.8 inches (0.8”), or 2.032 cm (20.3 mm) but that’s still less than a third of the body length.

Dragonfly toy, purchased from Visitor Center, Huntley Meadows Park, Fairfax County, Virginia USA.

~70 mm long (~7 cm), or ~2.76 in

So what’s the take-away from this post? Always be mindful of depth of field and choose camera settings, focus points, and viewing angles that will maximize what appears to be acceptably in focus.

Copyright © 2015 Walter Sanford. All rights reserved.

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4 Responses to “Depth of Field”

  1. The Activist Says:

    Good post, I managed to get some similar shots yesterday but I must sort out and get to know all the settings etc

  2. Mike Powell Says:

    I already understood the basic concepts, but I really appreciate the way that you were able (and willing) to do the math to illustrate the role that distance plays in depth of field calculations. Of course, depth of field is only one of a whole host of factors that come into play when taking a photo, as we busily consider the light, the background, the angle of view, and the shutter speed, knowing full well the subject may fly away at any second.

  3. Victor Rakmil Says:

    Excellent explanation. One point however, at any given aperture the focus is one third in front and two thirds behind. Not much room for error in close up photography.

  4. Charlie@Seattle Trekker Says:

    I really appreciate you sharing your expertise…My photos are improving with this kind of input, slowly improving, but improving.

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