Results 1 to 9 of 9

Thread: Confused about the "Circle of confusion"

  1. #1

    Confused about the "Circle of confusion"

    I would like to start off this question by stating that I understand what the "circle of confusion" is and how it is calculated. What I do wish to ask, is for a subjective opinion on your experience and viewpoint on the topic. Should you wish to offer a simple explanation of "cof" for newcomers, I am sure that might also be acceptable and appreciated by some.

    I am wondering what you would consider a very acceptable "circle of confusion" assuming that you are shooting 4x5 negatives and that you are printing three different print sizes? What level of sharpness do you set out as your goal or objective in your negatives? If we could relate the answers to either or all of the sizes 8x10, 11x14 and 16x20 prints that would be most appreciated.

    I am less concerned about getting into the techicalities of the subject than I am about having a reasonable grasp of how it actually applies to the real world.

    Kind Regards,

  2. #2

    Join Date
    Nov 2003
    Posts
    1,219

    Confused about the "Circle of confusion"

    As you probably know, what you consider an acceptable circle of confusion depends on what you intend to do with the image.

    Here is one possible analysis, I sometimes use, but it is by no means the only way to think about it or even the best way.

    I assume my standard is an 8 x 10 print to be viewed from 10-12 inches away, or a larger print to be viewed proportionately further away. (A 16 x 20 print then would be viewed from 20-24 inches away.) The argument for such a point of view is that most people stand about the diagnonal of the print away from it, and this is then a slightly more demanding criterion.

    It seems generally agreed that a circle of confusion of diameter about 0.2 mm is acceptable in an 8 x 10 print viewed close up. (But see below.) That corresponds to a resolution of about 5 lp/mm. Since you have to enlarge about two times to go from 4 x 5 to 8 x 10, that means you shoul divide the coc by two, so a coc of 0.1 mm in the negative (or slide) would be appropriate. To take care of inevitable inaccuracies and things that might go wrong, it would be appropriate to stop down one or two additional stops from what a coc of diameter 0.1 mm would suggest, as long as you stay away from the range where diffraction could be a problem.

    However, there are some obvious problems with this approach. First, many people are drawn in to a print with lots of detail and will try to get as close as they can. The 16 x 20 print may also be viewed from 10 inches. If you expect these "grain sniffers" to be your audience, then you have to compensate accordingly and use the a coc of diameter 0.2 mm divided by the enlargement factor you expect to use.

    Another problem is that some people can resolve more than 5 lp/mm in a print and those people would also be more demanding. The extra one or two stops further stopping down would satisfy them, as long as diffraction doesn't enter seriously into the matter.

    Paul Hansma has suggested an alternate approach. He doesn't worry about how the print will be viewed in advance but always chooses the f-stop so as to minimize the combined effect of coc (defocus) and blur disc (Aery disc) from diffraction. He combines them by a certain mathematical method called root mean square and then chooses the f-stop so this combination is as small as possible.

    You can read about Hansma's approach, together with tables of appropriate f-stops as a function of focus spread, in the large format photography page and the links contained therein. I've also included some discussion of these issues in my essay at www.math.northwestern.edu/~len/photos/pages/dof_essay.pdf

    In practice I use both Hansma's method and the simpler method and usually choose an f-stop in between, depending on just what I want to accomplish.

  3. #3
    wfwhitaker
    Guest

    Confused about the "Circle of confusion"

    Besides the resources offered above, a page which includes a table relating formats, circle of confusion and print size is at http://www.nikonlinks.com/unklbil/dof.htm.



    Not intending to sound demeaning in any way, I learned how to calculate circle of confusion in college 25 years ago and it seemed then to be akin to how many angels could dance on the head of a pin as there's a lot of subjectivity involved. I've been using a view camera for the last 20 years and haven't once felt the need to calculate CoC again. Photography can be as technical as you want.



    Best regards,

  4. #4

    Join Date
    May 2002
    Posts
    1,031

    Confused about the "Circle of confusion"

    Since you've apparently gotten the basic idea, you also understand that the whole thing is very subjective. It boils down to the size of a blur that the viewer will discern as a point when viewing the print. Since some people have better eyesight than others, they might be more demanding.

    One point worth considering, is that there are two factors which contribute to the size of the blur circle on the film: the defocus blur and diffraction. The amount of defocus is determined by subject distance and f/stop (i.e., depth of field) but the diffraction is determined only by the f/stop. As you stop down, the defocus becomes less but diffraction becomes worse. So, while you can decrease the defocus blur by stopping down more, you reach a point where the effects of diffraction start making the overall blur larger even though the supposed DOF is greater. An extreme example is a pinhole. Somewhere on this site, QTL has a link to an article by Paul Hansma that discusses this and provides a means of calculating the optimum f/stop, taking into account both defocus and diffraction. If I have read the article correctly, and assuming Paul's calculations are correct -- I'm not the one to challenge his math } -- then you can use his method to achieve the best possible circle of confusion for a given subject. Err in either direction from the calculated f/stop, and the actual blur circle on the film will be worse.

    I guess this ramble may not really address your original question. Or maybe it does. A test shot, enlarged to the degree in question, should let you judge for yourself whether you need to use a smaller or larger CoC value in your calculations.

  5. #5
    Whatever David A. Goldfarb's Avatar
    Join Date
    Mar 2000
    Location
    Honolulu, Hawai'i
    Posts
    4,658

    Confused about the "Circle of confusion"

    When I'm concerned about DOF or hyperfocal distance, I tend to use the standard CoC value or a standard DOF scale or chart and stop down two stops from the recommended aperture for most purposes. I could recalculate my preferred CoC and make my own tables, but for me this is a much easier way of thinking about it, whether I'm trying to figure out what aperture I need with a view camera or where I should focus with a rangefinder camera.

  6. #6

    Confused about the "Circle of confusion"

    I would be a bit careful using a CoC too far away from the 0.2 mm standard viewed at ~250 mm (technically for ANY print/negative size). The reason being the average resolving ability of a human eye.

    In critical applications it may make more sense, however, in such cases THE most important thing is to get your key object in plane of focus. Since CoC directly affects DoF, it is much safer to use the generally accepted (above) value so as to end up with an image represented as intended for MOST viewers.

    The 250 mm viewing distance comes from a so-called Comfortable Viewing Distance (or a Nearest Distinct Vision), which is the minimum distance at which an average human eye can easily focus. As far as I could find out, this number comes from rather extensive lab tests and personally, I don't see much reason to trying to ammend that. Without question no eye is equal and certain viewing conditions will also affect perception. This is however, a veriable that is beyond our control.

    It just so happened that an 8x10's diagonal comes quite close to that ~250 mm viewing distance and so it's usually used as a reference size. This also matches the Angle of View of a human eye (~50 degrees) so it all comes together. As size changes, so should the viewing distance. Thus one can adjust the CoC based on this (and as explained by other poster above).

    All of this boils down to basically one thing: your photograph will usually be seen as you intended ONLY if the viewing distance matches the above for a CoC of ~0.2 mm.

    Of course you could have your print displayed in a cage and viewers advised to hold their nose right against it, so as to control the viewing distance. This way you could quite precisely control what's rendered sharp and what's not.

    Normally that's not the case and we need to remember, that smaller sizes are often viewed at longer than normal distances and larger ones at shorter.
    Witold
    simplest solutions are usually the most difficult ...

  7. #7

    Confused about the "Circle of confusion"

    Thanks for all of the great replies. You have indeed answered my question. Also the links provided were very helpful as well as informative.

    Hopefully I did not be over do this as I was just looking for a bit more information from the subjective side of the equation.

    Kind Regards,

  8. #8

    Join Date
    Dec 2000
    Location
    Tonopah, Nevada, USA
    Posts
    6,334

    Confused about the "Circle of confusion"

    It seems illogical that a group that has walked the extra mile ie. the large format mile would then take the approach of "sharp enough for this application." Since reading and understanding Mr. Hansma's excellent article, optimum has always been my approach. ie. I strain to get 48 lp/mm in an 8X10 contact even though theoretically 4.8 is "good enough." Having accomplished that goal to some degree I'm currently trying to understand even further what some of the classic lenses (18" Verito, 2 Cooke's with defocus ring, and a 36cm Heliar) can do for me going the other direction. But I want to understand in advance what each lens can accomplish for me in the effects of pre-visualised, planned "defocus." We all started at some point in photography mailing a roll of film off and hoping perhaps with luck we might get something nice back. The point of large format is to be able to have complete control over the process so there is no element of luck left other than the fleeting whimsy of light itself for outdoor type work. So in reference to the original question, I've never worried much about coc because I've worked towards optimum and figured it would always be better than least acceptable. That's just one photographers imperfect approach. Coming full circle I have to say that there have been times when applying Hansma's approach I do the math and the only way to get everything equally focused is f128 and say to myself I'm going to end up with nothing because of diffraction, and move on without doing the shot. 2 different roads to the same place?

  9. #9

    Join Date
    May 2002
    Posts
    1,031

    Confused about the "Circle of confusion"

    Coming full circle ... I do the math

    There are indeed times when even large format cameras can't capture the scene we visualize. Hansma's formula is one to tell us "you can't get there from here."

Similar Threads

  1. Depth of Focus - Circle of Confusion Confusion
    By Wilbur Wong in forum Cameras & Camera Accessories
    Replies: 15
    Last Post: 21-Oct-2005, 10:36
  2. Circle of Scanner Confusion...
    By Gary Albertson in forum Digital Hardware
    Replies: 9
    Last Post: 5-Jun-2001, 19:07
  3. Confused by circle of confusion
    By Stewart Ethier in forum Style & Technique
    Replies: 6
    Last Post: 14-Jul-2000, 14:56
  4. DOF and circle of confusion
    By Roy Feldman in forum Style & Technique
    Replies: 6
    Last Post: 31-Aug-1998, 09:40
  5. Circle of confusion
    By Terry Lorch in forum Lenses & Lens Accessories
    Replies: 2
    Last Post: 12-Jan-1998, 14:09

Bookmarks

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •