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Thread: Focusing technique

  1. #1

    Join Date
    Dec 1999
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    Focusing technique

    I tried to get the objects at the background go soft,very soft. The distance between the main object and the one I wanted to be soft is about 1-2 Ft. I wasn't able to get the result I have in mind although I applied anti Scheimpflug correction. The F stop I was using was F 22. To get the result I have in mind, do I have to use F 5.6 or larger? and put the objects in the background further away?
    Thanks for the tips.

    Theo

  2. #2

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    Focusing technique

    A larger aperture would help. Regarding the distance between the objects, it depends on how far away your nearest object was. If you were close to the nearest object, a one or two foot separation would put the more distant object well out of focus, but if the objects are further away, the difference in focus would be much less.

  3. #3

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    Focusing technique

    Theo,

    You might also try stopping your lens down to see the exact areas of the image that are either in focus or not.

    The alternative is to look at f-stop and depth-of-field charts.

    Lastly, if it's a critical image, take a Polaroid of the shot and look at it from a technical standpoint to see if the background is soft enough or not.

    Hope that helps....
    Life in the fast lane!

  4. #4
    Tachi Bloke
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    Focusing technique

    Theo,

    We need more information to really be of help. In addition to the distances to the subjects from the lens, can you also tell us the focal length of lens you were using and the format of your film?

    Ernest was on the right track: using a larger aperture is desirable for the shallow depth of field you require. (Depth of field tables aren't terribly useful with a view camera, and we can assume you are "stopping down" in any case, since you are asking your question on the LF forum .....)

    As a rule of thumb, the longer the lens, the shallower the inherent depth of field. This means you probably don't want a wide angle lens for this shot, and should be looking for something around twice the "normal" focal length for the format you are using (shorter lenses will do at a pinch, especially if they have a large image circle allowing you to really twist those bellows!)

    Did you also apply your "anti-Scheimpflug" swing movements? Just using the tilt may not have been enough.

    Graeme
    If I were more creative, I'd write something witty here .....

    www.scenebyhird.com

  5. #5

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    Focusing technique

    What is an "anti-Scheimpflug correction"?

  6. #6

    Focusing technique

    What is an "anti-Scheimpflug correction"?

    That is what photographers do who dislike poor Mr. Scheimpflug :-))

  7. #7

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    Dec 1999
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    Focusing technique

    Graeme Hird: Is the depth of field not determined by the lens to subject distance and aperture used? and the focal lenght of lens don't matter? For instance: the subject covered 2/3 of your finder using 210 mm lens, now you change your lens to 90 mm (you have to move closer) and the same subject filled 2/3 of your finder, you should have the same depth of field. Am I correct?

    Henry C, Ernest Purdum: : The reason why I am using a 4X5 is hoping that even with F22 I will still have the effect I want, otherwise I could use my medium format and fully open the lens.

    Gentelmen, your response is much appreciated.

    - Theo -

  8. #8
    tim atherton's Avatar
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    Focusing technique

    Excellent discussion here

    http://www.cinematography.net/DepthOfFieldImageSize.htm

    for example

    "In his invaluable book, "A Hands-On Manual for Cinematographers", the inestimable David Samuelson states at the conclusion of his section on depth of field : "Depth of field remains the same, regardless of lens focal length, so long as the image size (and f-stop) is the same. There is no point in changing to a shorter focal length lens and moving closer, because if the image size remains the same so will the depth of field.""
    You'd be amazed how small the demand is for pictures of trees... - Fred Astaire to Audrey Hepburn

    www.photo-muse.blogspot.com blog

  9. #9
    Tachi Bloke
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    Focusing technique

    Theo,

    I'm not disputing what you are saying. The fact still remains that we need the details of how far away your subjects are from the lens. Having the focal length of the lens will make the calculations easier for those who would like to help you.

    Though, of course, you can keep them secret if you like. The choice is yours.

    Graeme
    If I were more creative, I'd write something witty here .....

    www.scenebyhird.com

  10. #10

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    Focusing technique

    Depth of field and focal length:

    If you fix the aperture and distance to the plane of exact focus, the depth of field does depend on focal length. To suggest otherwise is nonsense and just results from not paying attention to what the well known formulas for depth of field say, or interpreting them incorrectly. You can find the formulas at www.photo.net/learn/optics/lensTutorial (but see below for a caveat). If you have some skill at high school algebra, you can figure it out yourself without having to depend on experts.

    The problem is that the DOF depends on THREE things (assuming a fixed coc): the distance to the plane of exact focus, the focal length, and the aperture. This can also be stated in terms of the magnification since the distance u to the plane of exact focus, the focal length f, and the magnification M are related by the formula u/f = 1/M + 1. So in some circumstances you can play games by varying two of them while not changing things much.

    For a fixed subject in the plane of exact focus, the image size is proportional to the magnification. Note in particular, that if you fix the distance to the plane of exact focus then the image size of anything in that plane is dependent on focal length. So saying the DOF depends on aperture and image size is just another way of saying it depends on aperture and focal length.

    One source of confusion about this is a remark that Jacobson makes in his Lens Tutorial. He gives the formula for depth of field in terms of magnification, and asserts that to a first approximation, for fixed magnification, the DOF doesn't depend significantly on focal length. But that remark assumes the magnification is relatively large. (Of course, if you change the focal length, and keep the magnification constant, then you also change the distance to the plane of exact focus, so this too is a variant of what I said above.)

    However, Jacobson's remark is obviously wrong in many situations. It is easy to construct examples where, with the same magnification (i.e., image size), the rear DOF will be finite with one focal length and infinite with a shorter focal length. And these situations are commonly encountered in large format photography, particularly landscape photography. For example, if the subject is at the hyperfocal distance for a short focal length lens, the rear DOF will be infinite. But with the same magnification, with a long focal length lens, the rear DOF will be finite. The problem is that there is a denominator in the formula for rear DOF, which depends both on focal length and magnification. Jacobson's remark depends on that denominator being fairly close to one, something which is only correct in certain circumstances.

    In cinemetography, one is often focusing on a relatively nearby subject, and that is the situation Samuelson is referring to. He may have discovered this by experience or he may have looked at the formulas. Needless to say, experience has to agree with the formulas. If they didn't everything we do in photography wouldn't make sense.

    Caveat about Jacobson's formulas: He forgets to mention that in certain circumstances, the rear depth of field is infinite. That is when a term in his denominator is either zero or negative.

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