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Thread: DOF and circle of confusion

  1. #1

    DOF and circle of confusion

    A while back I read a chart wich would give the correct F- stop setting based on near to far standards movement. In other words focusing on the nearest point th en measuring the distance on the monorail to the farthest point entering that di stance to a scale which would gine me the proper F stop. Since purchasing the Ro denstock Studio calculator I have learned that monorail angle and repro ratio ar e also factors. My problem is that the Rodenstock calculator is"based on a Circl e of C.. of .03mm for 35mm.IYO is this sufficent for large enlargement? Are ther e other scales out there? I use 4X5

  2. #2

    DOF and circle of confusion

    The circle of confusion for 4x5 is usually set at .15mm or 0.09 mm. You will find some online calculators here: http://home.sol.no/~gjon/depth.htm

  3. #3

    DOF and circle of confusion

    Thanks for the response, I had no idea there were so many DOF scales out there b ut none really answer my question. Based on measurment of movement of lens stand ards ONLY is there a DOF scale out there? Mr. Grepstad's answer was overwhelming , could you narrow it down?

  4. #4

    Join Date
    Feb 1998
    Posts
    287

    DOF and circle of confusion

    An article in Photo Techniques (Mar/Apr 96) on view camera focus used this metho d. Basically, you tape or glue a mm scale on your bed, and then focus on the nea rest object of interest, read the position on the mm scale, then focus on the fa rthest object of interest, and note its position on the mm scale, and then, usin g this focus spread (in mm), refer to a chart for optimum f stop to use, based o n line pairs per mm resolution. I photocopied the chart and taped it to the back of my camera for reference. I hope this helps.

  5. #5

    Join Date
    May 1998
    Posts
    218

    DOF and circle of confusion

    The scale mentioned by Ron sounds as if it is based on the formula t=2.C.N', whe re t=depth of focus (not field), C is diameter of Circle of confusion, and N' is effective aperture, probably assumed here to be equal to the marked aperture. T hus the scale is independant of focal length (neat, huh?). The repro factor is r elevant for closeups.

    Of course, one advantage of LF is movements, especially puting the lens at an an gle to the film. See http://fox.nstn.ca/~hmmerk/ for the gory details.

  6. #6

    DOF and circle of confusion

    Mr.Shaw has gone "above and beyond" by offering to snail mail the article mentio ned in his response. Mr. Gibson your formula sounds great but having attended ar t school I'm afraid I don't have the where with all to figure it out.I know that in 35mm a COF of .03 is low end and .01 equals what film can be resolved at,I h ave no idea how or why this translates to 4x5.Is it possible,Mr. G. to give a ex ample a art school grad can follow? ( I know I should have listened to Dad and t aken some "real" classes, but that was 20 years ago).

  7. #7

    Join Date
    May 1998
    Posts
    218

    DOF and circle of confusion

    If you look at the DoF marking on a 35mm lens, it will look something like this:

    [pre] +------+------+------+------+------+------+ | | | | | | | 22 16 8 V 8 16 22

    [/pre]

    I've only shown marks for f/8 and f/16.

    On a 35mm lens, the scale is on a fixed part of the lens, and distance marks on the focusing ring can be compared to the f/numbers to see the DoF.

    On a 5x4 camera, you may not have a distance scale. Suppose you focus by racking the front back and forwards. With a ruler and pen, you can create a paper scale that looks much like the one above, and hold it against the camera as you rack back and forth. If the total racking distance is the same as the distance betwee n the two f/8 marks, then you have to stop down to f/8 or smaller.

    Now for a bit of theory. Ignore this paragraph if you like. Depth of focus refer s to the total distance we could move the film backwards and forwards, while the image still looks sharp. Call this distance "t". For distant objects, the value of t is approximately twice the diameter of the circle of confusion, multiplied by the aperture number. "t = 2.C.N". The value of C is often taken to be 0.03mm for 35mm cameras, or 0.1mm for 5x4. These numbers are not set in stone: Nikon u sed different values for their cheap (consumer) "E" series lenses than their mor e expensive regular lenses.

    Using C = 0.1mm, here are the value of t for various values of N.

    [pre] N t (mm) === ====== 5.6 1.1 8 1.6 11 2.2 16 3.2 22 4.4 [/pre]

    t is the total distance, so the two f/8 marks should be 1.6mm apart, so each of them is 0.8mm from the central mark. So your scale will look like this:

    [pre] +------+------+------+------+------+------+ | | | | | | | 22 16 8 V 8 16 22

    <0.8mm ><0.8mm > <--- 1.6mm ---><--- 1.6mm ---> <------- 2.2mm ------><------- 2.2mm ------> [/pre]

    Note that these distances are small; you will need a small pen.

    Of course, if you are photographing a landscape, or other plane object, you woul d probably tilt the lens or film, rather than relying on depth-of-field to keep things in focus.

    I suppose Art graduates don't use rulers. Sorry about that.

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