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Thread: Situations where Depth of Field *Does* Change with Focal Length

  1. #1

    Situations where Depth of Field *Does* Change with Focal Length

    Regarding the depth of field tutorial, I have one question... I was surprised to read that the focal length didn't effect the DOF if the magnification was the same. To explore this issue, I used this site's DOF calculator, and I found what I feel is a significant increase of DOF for short focal lenghts, even at the same magnification levels as you get closer to the hyperfocal distance then your example showed. My results:

    At an f/stop of 5.6:

    with a 25mm lens at 3 meters focus distance, DOF = 4.7 m
    with a 50mm lens at 6 meters focus distance, DOF = 3.6 m
    with a 100mm lens at 12 meters focus distance, DOF = 3.4 m
    with a 25mm lens at 4 meters focus distance, DOF = 12.7 m
    with a 50mm lens at 8 meters focus distance, DOF = 6.8 m
    with a 100mm lens at 12 meters focus distance, DOF = 6.1 m

    ...and the differences at these distances get larger at smaller f/stops. It seems valuable to discuss this in the context of trying to get any significant DOF in a landscape photo. for instance, taking a full body portrait of a hiker at Yosemite, wishing half dome to be in focus too. According to this site, it shouldn't matter much if I use a 100mm portrait lens if I can stand back far enough, but I think that at the same magnification, a wider angle lens (say 35mm) would give a sharper rendition of the distant mountains.

    At an f/stop of 11:
    with a 35mm lens at 3.5 meters focus distance, DOF from 1.7m to inf
    with a 100mm lens at 10 meters focus distance, DOF from 7.36m to 15.6

    I would love to here people's thoughts on this

  2. #2
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    There are (at least) two situations where the DoF can be different at equal magnification:

    First is at situations of very high magnification (close focus distance), where the calculator predicts a larger DoF for the wide angle lens. The problem with comparing lenses at high magnification is that another (unaccounted for) factor begins to influence the DoF significantly: pupil magnification. This actually offsets some of the DoF advantage for the wide angle lens; DoF comparisons therefore become inaccurate and misleading for high magnification. The problem with trying to account for the pupil magnification factor is that this varies significantly depending on lens (even at the same focal length), and on whether the lens was designed to also take macro photographs.

    The other cause for a difference between the DoF of a wide and telephoto lens is for focal distances near the hyperfocal distance. Since a wider angle lens typically has more of its DoF behind the focal plane (as shown in the chart), this can cause the DoF to reach "infinity" more easily. In fact, the DoF of a lens becomes exponentially sensitive to the focal distance as one focuses close to the hyperfocal distance (for example, a change in focal distance of just 0.1m can increase the DoF by 100X). These scenarios can also give misleading differences in DoF between a wide and telephoto lens because of this sensitivity-- even if in a real-world print the DoF looks very similar.

    You could verify this by using the depth of field calculator which allows changing the arbitrary circle of confusion value. For focal distances near the hyperfocal distance, one can change the definition of CoC such that the ratio between the DoF of a wide angle and telephoto lens become nearly equal, or in a ratio of 10:1. Such results should help show that for these scenarios, real-world prints examined by a person will show negligible difference in DoF, whereas the strictly numerical calculation shows a huge difference. This is really a limitation of the basic concept of depth of field and its use in these calculations.
    Last edited by McQ; 16th April 2008 at 01:34 AM.

  3. #3

    Re: Situations where Depth of Field *Does* Change with Focal Length

    After some more thought I have come up with a bullet list summary. For the same magnification, it can be shown mathematically that:

    - Wide lens have larger rear depth.
    - Telephoto have larger front depth.
    - Wide lens have an overall larger depth of field.
    - Around the wide angle range, the difference is more significant.
    - As the aperture becomes smaller, the difference increases.
    - As focus distance increases, the difference increases.

  4. #4
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    Re: Situations where Depth of Field *Does* Change with Focal Length

    I also agree that there is a slight change in the total depth of field for *very* wide angle lenses, but this is negligible relative to aperture and magnification. The depth of field tutorial therefore carefully uses the words "virtually constant", and shows the changes in the table under that section. To emphasize this effect, I later added a 10 mm (widest regular lens available) to the line-up for a camera with a 1.6X crop factor. There are of course much larger variations for even wider lenses, however this is a pure mathematical quirk as these focal lengths are not practical (or available) in real-world use.

    Regardless of the above details, the key with that tutorial is to emphasize that focal length impacts DoF much less than is commonly thought.

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