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DOF and scale

Discussion in 'General Technical Discussion' started by Beezle, Oct 2, 2005.

  1. I had a brain cell fizzle on me tonight, and a question popped into my head.

    Say I have a subject that is a given size. Say, a loaf of bread.

    And I want to fill the frame with this loaf of bread, regardless of the focal length of the lens I use. Assume I have a really big kitchen and could use anything from 17mm through 500mm to photograph this subject. So, I would set the tripod down at different distances from the subject to accomplish this. At 17mm, I would be right there. At 500, way over at the breakfast nook in the far corner.

    Now let's say I want to sell this loaf of bread on eBay, and I want a shot of it where the entire loaf is within the DOF so that bidders can read the label, check for mold, etc.

    Does it matter what focal length I shoot the loaf at, in order to achieve the DOF I want, or is the aperture the only thing that matters?
  2. nfoto

    nfoto Guest

    Depth-of-field (DOF) depends mostly on the image magnification, and the aperture setting. Focal length has in practice very little influence.

    So, what does the above mean? If you make an oversized loaf of bread, you would need low magnification and thus any lens could give the DOF you want (provided you have a really large kitchen if using a 500 mm lens). However, with an ordinarily sized bread, you might find yourself in a position where DOF could be limiting, because magnification had to be greater since the subject itself is smaller. A tilt/hift lens would give the DOF you need, however. If you make a very small bread, then magnification would need to be even higher and now DOF could be limiting for the image, even with a tilt/shift lens (if you want to shoot in other angles than the usual oblique-from-above).

    None of the options above says anything about how your subject will appear in the images, since this is governed by perspective, which itself is controlled by distance between subject and camera. So if you achieve a given magnification by going close with a wide-angle lens, or step back and get the same magnification with a telephoto lens, DOF would be virtually identical, but perspective very different. If you keep the distance and swap lenses, the short lens would give a small subject and more DOF, while the longer lens will give a bigger subject and less DOF; in both pictures, the perspective will be identical.

    Hope the above answers your question.
  3. Thanks for answering.

    I think I get what you mean, but I guess magnification is something I don't have a very good understanding of. That is, how does one determine magnifcation given the distance to the subject and focal length of the lens?

    I will go search for an explanation of that.
  4. MontyDog


    Jan 30, 2005
    #1064 - You have an error in your SQL syntax;
  5. Thanks. I'll be absorbing that for a while.

    The funny thing is I do have a good understanding image geometry from the processing point of view. I have worked on image stitching and projection software off and on since the early 90s.

    Trouble is, what I am good at is programming. Learning to be a good photographer is actually going to be harder for me than that was.
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