Simple DOF Rule-of-Thumb

Discussion in 'Beyond the Basics' started by Soonershooter, Aug 9, 2008.

  1. Soonershooter

    Soonershooter TPF Noob!

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    I want to share a simple method that I use for determining the aperture setting that I need for getting the depth-of-field (DOF) that I want in my photos. It doesn’t require any charts or tables, just a few calculations that are so simple that most anyone can estimate them in their head. For my camera, the equation is:

    F = 22*DOF/(H^2),

    which I will explain in detail. F is the aperture setting. The symbols (H^2) mean H squared, or H times H. This equation works well for portraits and most any photo where the DOF is fairly small but this technique is not suited to landscape photography where the DOF can be very large. The ideas presented are not original, but I haven’t seen this kind of equation explained anywhere else. Please allow me a few paragraphs to explain.

    Most of my photos are portraits. My vision is not good enough to use DOF preview effectively. I don’t even trust myself to focus manually. I use a Canon 20D, which has a 1.6 crop factor. The equation that I stated above is appropriate for this camera. For cameras with other crop factors, the ideas would be the same, but the number 22 would need to be adjusted.

    Typically, DOF depends on the focal length of the lens, the distance to the subject, and your aperture setting. Accounting for all three of these variables appears to be a difficult task. However, the focal length of the lens and the distance to the subject combine to determine the magnification of the photo. Now we’re down to only two variables, magnification and F-stop, which makes the calculation more reasonable.

    For our purposes, we don’t need to actually quantify the amount of magnification. We merely need a factor that is an indication of the magnification. I have chosen to use the height H at the subject distance, as seen through the viewfinder. That is, the short dimension of the rectangular frame. I could have also used the width or the diagonal, but I have chosen to use the height for a couple of reasons. The height will always be a smaller number than the width or the diagonal, which makes the calculation easier. If the photo is a portrait, it’s pretty easy to accurately estimate the height of the frame. This is especially important because the aperture setting depends on the square of the height H.

    I start by choosing the DOF that I want in my photo. Next, I frame the picture and estimate the height H that I see through the viewfinder at the subject’s distance. Then I quickly compute the F-stop setting I need. Here are two examples.

    My first theoretical photo is an upper body portrait. I plan to focus on the person’s eyes. I decide that I want the DOF to be about 2 feet, knowing that about half of the DOF will be in front of my focus point and half will be behind. I frame the picture and note that, at the subject distance, my frame is about 3 feet tall, so H is equal to 3. (It doesn’t matter whether I am using a 40 mm lens and standing close or using a longer lens and standing further away.) From the equation above, I first multiply the DOF by 22, which gives me 44. The square of the height is 3x3=9. Dividing 44 by 9 gives me approximately 5, so I use F/5 for the photo. It’s that simple. I know that the DOF will be about 2 feet.

    My second theoretical photo is a fairly tight face shot. For this photo, I decide that I want a DOF of 1 foot. While framing the picture, I determine that the height I see through the viewfinder is about 1.5 feet. Multiplying the DOF by 22 gives simply 22. Squaring the height gives me 2.25. Dividing 22 by 2.25 give approximately 10, so I use F/10 for the photo.

    This equation only works correctly if the DOF and the height H are measured in feet. If they were both measured in inches, the number 22 would have to be changed to 264, and the calculations would get harder.

    For those of you who may want to check this formula against the DOF tables, you need to know that, for my camera, the approximate equation that relates the focal length of the lens, the distance to the subject and the height H is:

    H=16* (distance to subject)/(focal length of lens)

    For example, if the distance to the subject is 10 ft and the focal length is 100mm, then the height H is 1.6 ft.

    Although this is admittedly an approximate technique, I think you’ll find that, despite its simplicity, it gives surprisingly good results when DOF is small.

    Sorry about the length of this post!
     
  2. rdompor

    rdompor TPF Noob!

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    Isn't the dof 2/3 behind the focus point and 1/3 in front of it? Not 1/2 behind the focus point and 1/2 in front of it....
     
  3. ksmattfish

    ksmattfish Now 100% DC - not as cool as I once was, but still

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    DOF is 1/3rd in front, 2/3rds behind only when the camera is focused at 1/3 the hyperfocal distance.

    As focusing distance decreases the amount of DOF evens out until close to the camera it's 1/2 in front and 1/2 in back (not macro, just normal close focusing distances like 2' or 3').

    As focusing distance increases so does the proportion of DOF behind the subject, until the hyperfocal distance is reached, and everything behind the subject is within the DOF.
     
  4. Helen B

    Helen B TPF Noob!

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    Just to add to what ksmattfish said, D-o-F rules-of-thumb that don't take focal length into account won't work when the lens is being focused at a point anywhere near the hyperfocal distance - so it's best to have an idea of roughly what the hyperfocal distance is. This simplification of the D-o-F equations only really works at fairly close distances - ie when D-o-F is small, as Soonershooter points out.

    Best,
    Helen
     

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