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Thread: Using the Rodenstock Depth of Field Calculator

  1. #1

    Join Date
    Mar 2004
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    1

    Using the Rodenstock Depth of Field Calculator

    I recently adquired this tool to help me to control the depth of field more acurately. However, I don't understand two concepts about 'Optimum f-stop for the desired depth of field': Magnification ratio and Extension difference near point to far point (mm)

    Thanks.

  2. #2

    Using the Rodenstock Depth of Field Calculator

    Are you in the US? If so just call us and we can walk you through it. 800 735-4373.

    Not in the US? Then the Rodenstock distrbutor can also help you.

  3. #3

    Join Date
    Nov 2003
    Posts
    1,221

    Using the Rodenstock Depth of Field Calculator

    I don't have one of those calculators, but I can guess what it does.

    Extension difference from near point to far point must be the distance on the rail between the position where you focus at the farthest point you want in focus and the position where you focus at the nearest such point. This distance is also called the focus spread. It is usually specified in mm. To measure it, you should have a metric scale on the rail or bed.

    Magnification ratio is the ratio of the size of the image to the size of the subject (in the exact plane of focus). (It is also the ratio of the distance from the lens to the film plane to the distance from the lens to the subject plane.) This is only really needed for close-ups. For most normal subjects, you can take it to be zero. There are a variety of ways to estimate it for close-ups. One involves putting a object of known breadth, such as a flat disc, in the plane of exact focus, and measuring the size of its image on the gg. There are also other ways to do it.

    In addition there is another assumed quantity called the diameter of the maximal allowable circle of confusion or coc. This is often chosen for you by the calculator or table, but if you use the formula, you can choose it yourself. One common value for 4 x5 is coc equal 0.1 mm.

    You can estimate the appropriate f-number to get everything in focus as follows.

    Let me first do it for normal subjects not in the closeup range.

    Just divide the focus spread by twice the coc. For example, with coc equal 0.1 mm, suppose you measure a focus spread of 4 mm on the rail. Dividing this by 2 x 0.1 = 0.2 yields 20. That means that if you stop down to f/22, you should have everything from the near point to the far point in focus.

    Now let me do if for close-up subjects. You use the same method, but you also divide the result by one plus the magnification.

    There are some extreme cases where this method doesn't work, but they are rarely encountered in practice, and I doubt if the Rodenstock calculator takes them into account. I think it is a simple slide rule computing the formula mentioned above. But, of course, I could be wrong and stand ready to be corrected.

    There is some debate whether or not this is the right method to use. Paul Hansma has proposed an alternate approach which is more complicated. Many people use tables based on his method. You can find a more extensive discussion of all this elsewhere at this website.

  4. #4

    Join Date
    Mar 1998
    Posts
    1,987

    Using the Rodenstock Depth of Field Calculator

    I do have one. "Optimum f-stop for the desired depth of field" this refers to the f-stop necessary for making what is considered to be "sharp" or "in focus" frm a near point to a far point as determined by you. given format size and printed size (what appears sharp in a 4"x5' contact print maynot appear to be as sharp when that same negative is blown upi to 16" x 20' for example.)

    "Magnification ratio " refers to, as Leonard points out, the scale the object being photographed at is reproduced on the film. If an object 12 inches long is roughly 2" long on the groundglas or film that would be a 1:6 ratio. If a 2" long object is reproduced as 2' long o nth film that would be a 1:1 magnification ratio. if a 2" long object is being reproduced on film as 4' long, that would a 2:1 ratio.

    "Extension difference near point to far point (mm)" refers to how far you have to move the front standard away from the rear standard (or vice versa) to focus on first the far point and then the near point. You measure this with the ruler along the straight edge of the calulator if your camera does not have a scale on the monorail or bed of the camera..

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