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Thread: Mirrors and Depth of Field

  1. #1

    Mirrors and Depth of Field

    This is an email I sent the author a few minutes ago:

    Hi,

    I read through the page on depth of field, and their was one thing that couldn't understand was what would happen if you took a picture of someone through a mirror. If I take a picture of a mirror and the focal plane exactly matches the reflective surface of the mirror, will everything reflected in the mirror, regardless of distance, come into equal focus? Logic would dictate that since all the light reflects off the mirror at the same distance from imaging plane of the camera, all objects in the frame are effectively the same distance from the camera. What happens in practice?

    My forums username is Flammel.

    Thanks,
    Any ideas? I though about trying this out myself, but I thought that maybe one of you already had the answer.

  2. #2

    Join Date
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    Re: Mirrors and Depth of Field

    Assuming that you are talking about a flat mirror, the image of a point in a mirror is the same distance behind the mirror as the object is in front of it. The rays of light from a given point of the object reflect off the mirror as if they had come directly from the corresponding image point behind the mirror. You can show this with a simple diagram, using the fact that the angle of incidence equals the angle of reflection. The optical system that is looking at the image can't tell the difference. So you still have to choose something to focus on and depth of field issues still apply. You wouldn't normally focus on the mirror itself.

  3. #3

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    Re: Mirrors and Depth of Field

    Thank goodness for people like you Will -- I can never get my head around this kind of thing!

  4. #4
    Moderator Dave Humphries's Avatar
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    Re: Mirrors and Depth of Field

    I agree.

    In practical terms, you can manually focus on any dust or smears on the mirror, OR the subjects reflected by the mirror, but not both, (unless Depth of Field is huge and/or distances are small).

    Cheers,

  5. #5
    PopsPhotos's Avatar
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    Re: Mirrors and Depth of Field

    This is one of the things which drove me nutzo last Summer. I was shooting the progress of an artist pinstriping a car. Full sun, new, polished paint and the camera insisted on focusing on the reflection rather than the pinstriping.

    Pops

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