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Thread: DOF Tutorial: What is "focus distance (to subject)" ?

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    DOF Tutorial: What is "focus distance (to subject)" ?

    Hi, I am a new member of this forum after having been attracted by the excellent tutorials - congratulations. There is a lot to learn, even for those who have been taking pictures for a while!

    In the Depth of Field tutorial the term "focus distance (to subject)" is used. Where from on the camera side this distance is measured is of little relevance for large camera to subject distances. For shorter distances this matters.

    I have a Nikkor Micro 70-180 mm lens, a zoom lens that allows taking images from short distances. It is mounted onto a Nikon D700 camera and I want to take pictures from a distance of between approx. 75 and 100 cm (measured from the sensor plane). This corresponds to about 50-75 cm distance from the front element of the lens to the subject).

    Questions:

    1) From where on the lens or camera should the focus distance be measured to use the DOF calculator?

    2) Is this (or any) calculator accurate for my setup?

    3) If so, down to what focus distance is this (or any) calculator accurate?

    Many thanks for your replies.

    Jean-Claude

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    Moderator Dave Humphries's Avatar
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    re: DOF Tutorial: What is "focus distance (to subject)" ?

    Hi Jean-Claude,

    I believe the "Macro DoF" calculator on the Macro tutorial page is better for what you want. It is just below the snake image, half way down the page.

    1) It says beneath it what the distances are.
    2) As long as you dial in your sensor format size, it should work.
    3) Afraid I don't know, this is one for Sean (the author and site owner)

    Welcome to the CiC forums from ...

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    arith's Avatar
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    re: DOF Tutorial: What is "focus distance (to subject)" ?

    Lenses of the same focal length can be different lengths and so I think it goes on sensor to object distance. I think your camera is full format like the 1Ds and this calculator might help:

    http://www.dofmaster.com/dofjs.html

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    re: DOF Tutorial: What is "focus distance (to subject)" ?

    Hi, Jean-Claude, and welcome to CiC!

    Can you tell us a little about why you want to measure DOF so exactly? DOF is not exact, by it's nature: it's based on what parts of the image are "acceptably sharp." When you focus on some point at the middle of the thing you're trying to photograph, that will set the center of the DOF at that point. Then the question is whether the DOF covers what you want.

    Cheers,
    Rick

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    Re: DOF Tutorial: What is "focus distance (to subject)" ?

    Thank you all for your replies.

    David, thank you for putting me on the Macro tutorial which I have used to calculate the magnification factor which I then used in the DOF calculator. The numbers I obtained with a focal length of 70 mm and focal distances of 60 to 85 cm (f/5.6 to f/11) are 0.16 to 0.1 for magnification (this is obviously at the limit of macro photography). DOF was 33% to 38% smaller than the DOF obtained with the calculator included in the DOF tutorial. Interesting. I guess the truth lies somewhere between the two and we cannot get any further without having more data on my lens, such as pupil size.

    Thank you Steve, I did the same calculations with the DOFMaster calculator you suggested. These results are 4-5% lower than those obtained with the calculator included in the DOF tutorial.

    Rick, your point is well taken for many reasons, e.g. the fact that DOF figures are based on assumptions made in people with less than optimal eyesight, etc. What I am trying to figure out is which of my lenses I should use for taking pictures of small animals in a confined space and how to arrange this space. The 70-180 micro is a superior (sharper) lens to e.g. my Nikkor 50 mm AFS 1.4 but 70 mm is a bit long for the shooting circumstances. The zoom is of course a major asset. For these reasons, and besides taking pictures using a stuffed toy model and checking out the results on my screen, I like to have a formula where I can see what happens when I change the variables and this formula should produce results representing reality as closely as possible.

    All your answers have been very useful to me. Thank you again.

    Jean-Claude

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    Re: DOF Tutorial: What is "focus distance (to subject)" ?

    Quote Originally Posted by jcs1 View Post

    Rick, your point is well taken for many reasons, e.g. the fact that DOF figures are based on assumptions made in people with less than optimal eyesight, etc. What I am trying to figure out is which of my lenses I should use for taking pictures of small animals in a confined space and how to arrange this space. The 70-180 micro is a superior (sharper) lens to e.g. my Nikkor 50 mm AFS 1.4 but 70 mm is a bit long for the shooting circumstances. The zoom is of course a major asset. For these reasons, and besides taking pictures using a stuffed toy model and checking out the results on my screen, I like to have a formula where I can see what happens when I change the variables – and this formula should produce results representing reality as closely as possible.
    If the formula will be helpful to you, Jean-Claude, you should be able to use your test space to determine where the "zero" point is for your lenses. If you put a ruler on the floor of your space, you can see where equally clear numbers are in front of and behind your focus point. Similar to the lensalign gadget. You can figure the angle of the camera relative to the floor and correct to real distance. Then see what point in the camera (front, in the lens, sensor plane) makes DOF calculations make sense.

    Or you can use the ruler to directly calibrate DOF of the confined space for some apertures and camera distances you'll find useful.

    Cheers,
    Rick

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    Re: DOF Tutorial: What is "focus distance (to subject)" ?

    Hi Rick,

    Thank you for your precious advice. It is midnight here but tomorrow I will check it out and see how far I get with the ruler or if I really need a LensAlign as the manufacturers seem to believe.

    Good night.

    Jean-Claude

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