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Thread: Choice for the Circle of Confusion?

  1. #1

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    Choice for the Circle of Confusion?

    I'm starting a project to calculate my own DOF tables. And of course, it all comes down to one's choice for the circle of confusion, "c".

    What's a typical choice for "c"? I'm getting my formulae from an article that's decades old (not sure of the source), and they recommend using c=0.033mm. Does this value sound about right? (Not sure.) I would probably select a single value of "c" for the negative. If I decide to take into account enlargement factors for different formats, I can use multiples of this value.

    I plan to calculate separate tables for each lens, and for each f-stop. Each table would include the near and far distances of acceptable sharpness, and the hyperfocal length.

  2. #2

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    Re: Choirce for the Circle of Confusion?

    How much do you want to enlarge? That's what determines how fuzzy is too fuzzy.

  3. #3
    Tin Can's Avatar
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    Re: Choirce for the Circle of Confusion?

    This is interesting Neil.

    I now need to shoot hyper-focal all the time in the field.

    I plan to buy a digital rangefinder and calibrate scales on my field cameras.

    Eyesight is requiring point and shoot LF as I learned yesterday shooting in high glare conditions. Even my 20X reversed Nikon 50 mm lens that works in studio was useless in the field.

  4. #4
    Jim Jones's Avatar
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    Re: Choirce for the Circle of Confusion?

    To make universal DOF tables we should consider all possible uses of all the interpretations of all the images of all the subjects we may photograph. That's quite a task. Better yet, study several of the experts who have researched and written extensively on the subject, such as Harold Merklinger. Then you will be able to derive rules of thumb that let you solve many DOF problems, perhaps without tables or calculators. Other photographers may prefer different solutions on the same occasion. For example, the hyperfocal distance is C multiplied by the diameter of the lens entrance pupil (the aperture as viewed through the front of the lens). C = maybe 1000 for casual work, perhaps less for a cell phone, and 2000 or more for critically sharp images. Another one: when using a lens for images to be viewed at a distance that provides correct perspective, stopping the aperture down below an entrance pupil of about 1/4 inch or 6mm invites unsharpness due to diffraction. If these rules sound too simple, remember that the mathematics of DOF involve only two parameters: the lens entrance pupil and the distance to the subject. Complicating this are the diverse preferences of photographers. If the rules of thumb cited above seem too simple, perhaps I erred in formulating them without the use of a calculator about 50 years ago, or perhaps the reader hasn't fully absorbed all that experts have written on DOF. It might be more convenient for most photographers to just rely on existing charts.

  5. #5
    Tin Can's Avatar
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    Re: Choirce for the Circle of Confusion?

    Jim, I just checked my Calter II 150mm f5.6 lens at f22 which I shot yesterday with the 2 5x7 I posted today. Close to 1/4". My scans look not sharp. I don't really care about scans and my technique is weak.

    I will 'see' how contact prints look tomorrow.

  6. #6

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    Re: Choirce for the Circle of Confusion?

    I have found the article on focusing the view camera right here on the LF home page to be quite helpful.

    Here's the link: https://www.largeformatphotography.info/fstop.html

    Best,

    Doremus

  7. #7

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    Re: Choirce for the Circle of Confusion?

    Quote Originally Posted by Doremus Scudder View Post
    I have found the article on focusing the view camera right here on the LF home page to be quite helpful.

    Here's the link: https://www.largeformatphotography.info/fstop.html

    Best,

    Doremus
    Doremus,

    Thanks! I didn't realize these recommendations were available.

    c=0.033 for the negative it is. The article recommends 0.1mm for 4x5, and I figure that I enlarge by about 3X for this format. So, that's essentially two sources that make the same recommendation.

  8. #8

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    Re: Choirce for the Circle of Confusion?

    Hi Neil

    Very often, recommendations for the choice of a circle of confusion as found on the Internet come with no justification nor indications of the validity for the recommended value.

    0.033 mm is the classical value for 35 mm photography and DOF scales for 35mm standard lenses have been computed for decades based on this value.
    The justification and rules to choose you own value for "c" are quite simple, and from a basic value, it is easy to modify if according to your specific needs.

    First come the angular resolution of the human eye, a common value is an angle of 2 minutes of arc, this is approx 1/1720 radian 1/(30*180/3.14)
    This is already quite stringent: 2 minutes of arc for a graduation means that each tick on a metric ruler with graduations in mm, as seen from a distance of 1.72 m, is separated by an angle of 1/1720.

    Well, let's admit this starting value of 1/1720.

    Now we look at a 8x10" print from a distance equal to the diagonal of the image format, i.e. about 30 cm / 12 inches.
    The circle of confusion on the print is 300 mm / 1720 = 0.17 mm
    Hence with a contact print recorded on a 8x10" film, in those viewing conditions, c=0.17mm.
    For a 4x5", enlarged 2x, c = .17/2 = .085 mm
    More generally, if D is the diagonal of the format, c = D/1720
    Take D = 43 mm for 35 mm [24x36 mm] photography, we get 43/1720 = 25 microns
    Take D = 50 mm like the focal lenght of so many 35 mm cameras, we get 50/1720 = 30 microns.

    Now you are all set, if you work with the 4x5 format with 8x10" prints and only allow the public to look at your work from a distance of 30 cm /12 inches, take c = about 0.09 mm, but if you still allow a distance of 30 cm but on a bigger 16x20" print, you'll have to choose c = 0.09/2 = 0.045 = 45 microns. DOF will be very shallow under those conditions!

  9. #9

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    Re: Choirce for the Circle of Confusion?

    Quote Originally Posted by Emmanuel BIGLER View Post
    Hi Neil

    Very often, recommendations for the choice of a circle of confusion as found on the Internet come with no justification nor indications of the validity for the recommended value. . . .
    Emmanuel,

    Thanks very much; I can see this logic. It becomes apparent how depth of field extends originally from the print, and then to the negative. I wasn't very clear on that.

  10. #10
    Drew Wiley
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    Re: Choice for the Circle of Confusion?

    Decent pocket-sized electronic distance meters can now be had for $200 or so. More expensive ones with all kinds of bells n whistles and very long distance ranges are not necessary, cause you'll be in optical infinity territory way out there anyway. But don't go cheap. Anything for a hundred bucks or so will be an inaccurate worthless toy.

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