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Thread: Circle of Confusion

  1. #1

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    Circle of Confusion

    The Luminous Landscape state that a Circle of Confusion is a group of photographers sitting around discussing Depth of Field. As this appears to be quite true I want to see if I have finally got somwhere near the truth.
    Here goes- When viewing an 8x12 print ,viewed from 12 inches away, a point of light that is larger than 0.020 mm will look blurred. this was because, when the picture was taken the focal point was ahead of, or behind, the sensor or film. Thus if one consides the whole picture to be a series of points of light of varying intensity the whole picture will be blurred. The amount of blurr with vary with the size of the CoC. I hope this is understandable and if so, have I got it right? Also, how does this affect depth of field, or if one has a reasonably accurate set of DoF tables does it matter?
    ATB Paul

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    crisscross's Avatar
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    Re: C of C

    QED

    More seriously, you are rarely able to determine shoot settings purely for DOF any more than you are able to re-organise the light to get them perfect (outdoors, not studio). The required degree of sharpness for prints also varies according to both subject and viewing conditions.

    So it is one of the many things in the back of your mind when you are about to shoot. If it is a subject that is only there for a second, you do your best. If it going to be there for ages, you take a few alternative settings.

    There is a useful free DOF calculator widget for mac, I expect similar for PC, to run through a few of your more common situations or when analysing a shot you have in PP so as to do better next time.

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    Re: Circle of Confusion

    Thanks for your response Crisscross. I am not sure that I was very clear with my question. The basis of the question was - Have I finally come to understand what the CoC really is and, if so, does it really matter when shooting if you can determine the resulting DoF with reasonable accuracy.

  4. #4

    Re: Circle of Confusion

    Paul this is how I calculate the depth of field

    To calculate the depth of field that will be in focus first calculate the working lens diameter. This is achieved by dividing the focal length by your chosen f stop (lets call this WD)
    Next set your focal distance (lets say 15 feet or 180 inch)
    Next chose how fine the resolution is required (lets say 0.1mm)
    Now lets say the lens chosen is 50mm and the f-stop is f5
    This gives a working diameter of 10mm (WD=10mm)
    We now have the figures required to calculate the depth of field

    0.1
    ------------------- X 180 = 1.8inch either side of the point of exact focus
    (WD _10mm)

    Giving a total depth of field of 2 x 1.8inch = 3.6inches

    Circle of Confusion

    As these calculations where for a full frame a 35mm camera the total depth of field for
    A crop sensor will be the total depth of field times your cameras crop factor.
    For the Canon aps sensors this would be 3.6 X 1.6 = 5.76inch
    And for Nixon aps sensors is would be 3.6 X 1.5=5.4inch

    John

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    crisscross's Avatar
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    Re: Circle of Confusion

    Quote Originally Posted by Paul View Post
    Thanks for your response Crisscross. I am not sure that I was very clear with my question. The basis of the question was - Have I finally come to understand what the CoC really is and, if so, does it really matter when shooting if you can determine the resulting DoF with reasonable accuracy.
    I want to turn the question upside down, because if you try to fill your head with an encyclopaedia of photography with calculator functions, you may not have the time, energy or creativity left to take decent photos.

    So suggest starting with a different question: what do you want to photograph and why?

    At one extremes you have macro (including quasi macro) photography of very small objects where it is normally extremely difficult to organise adequate DOF and you deal with concentration of items on a plane with everything in front or behind out of focus. For flowers, a common style has arisen where it is acceptable, almost manadatory, to have certainly the background out of focus. For insects you have to work at it harder as they look silly unless all in focus.

    At the other extreme you have landscapes, especially sea or river based 'landscapes' where you are trying to keep the content of several square miles in focus IF that is what you try to do. Use of Wide Angle (WA) lenses is AN answer as it allows the rock you are nearly tripping over to be in focus as well as the horizon 2 miles away. BUT it can bring in a total distortion of the real feel of being there and make buildings lean in alarmingly. IMO it is better to concentrate on just enough content to have a main subject and some contextual background, ideally a foreground line to achieve 3Dness. I find it quite useful to stay within what my 80(-400) lens can accommodate unless really necessary to go wider.

    In between and difficult are building interiors and streetscapes, where I suggest (and I think manufacturers are also) 18mm as the limit of WA.

    When you have decided what you are photographing, you can use a calculator for a set of typical situations/lenses; still not worrying where exactly you are losing focus to 3 decimal places.

    Finally you can refine so that you can flip quickly between lenses and other settings according to whether the sky has suddenly filled with stunning clouds and rainbows or a small bird or butterfly has settled a few feet away. Hope to do that during the rest of my life

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    Re: Circle of Confusion

    Further to you point(s) Crisscross, I am not guilty of carrying out large numbers of calculations when taking pictures. I am more inclined to shoot first and ask questions afterwards. Although, I do believe that one should follow a reasonably structured work-flow. My interest in the CoC was more academic. I read the tutorial on depth of field and hyperfocal distance and did not fully understand what the CoC was exactly and what its implication is when taking photographs. So my question still remains. Based on my description in my first post-do I understand what the CoC is, and when taking pictures and guesstimating depth of field, some times using a handy-andy,does the CoC matter at that point in the process?
    Thankyou for taking the time to respond in your considered manner.

    ATB Paul

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    Re: Circle of Confusion

    Paul have a look at what's available on this page http://www.dofmaster.com/You will be able to print out quick reference charts to help you with DOF and custom charts for your lenses for Hyperfocal distance.

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    Re: Circle of Confusion

    Thanks Bill44.
    I went to the recommended websites and found the info generally very useful. I bookmarked the site for future reference. Thanks again,
    ATB

  9. #9

    Re: Circle of Confusion

    Thanx for that link it seems quite informative, I am too like Paul confused a bit about this Circle of confusion, but from what I understand, at the focus point everything is sharp but the sharpness becomes less as you go away from that point so the places around that focal point would be represented by a lot of blurry circles of confusion, although they are blurred they still fool our eyes and we perceive them as sharp, but as you go away from the focal point you reach a place where the blurriness is so much that the human eye can not be fooled any more. That is how I understand the issue and hope I am not wrong!

  10. #10

    Re: Circle of Confusion

    Concerning depth of field, I wonder if people with Cameras that have DoF preview function can tell me how much useful it is for them when they are in action. And can a small area like the viewfinder give the photographer enough space to be able to make decisions about the DoF? For I have never used that feature before and wonder if lacking it would put me at a great disadvantage!

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    Re: Circle of Confusion

    My old Pentax MX film camera had a depth of field preview lever, which I used often. By looking through the viewfinder while depressing the lever, I could get a rough idea of the depth of focus. The problem was that the viewfinder would get dark when using small apertures due to the reduced light. Also, it was hard to see what was in focus through the small viewfinder. I assume that modern digital cameras with the feature would be similar. My current digital camera, Sony A350, does not have a depth of field button; however, I can review the photos after they are taken to check what was in focus. By magnifying the photo and scrolling around, I can get a good idea of the depth of field. It takes a little time but it works. If the DOF is not what I was expecting, I can adjust the aperture and take another photo.

    Is a depth of field preview essential? No, not with a modern digital camera that allows reviewing the photos after the fact. However, its still a good feature to have.

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    Re: Circle of Confusion

    Quote Originally Posted by Paul View Post
    My interest in the CoC was more academic. I read the tutorial on depth of field and hyperfocal distance and did not fully understand what the CoC was exactly and what its implication is when taking photographs. So my question still remains. Based on my description in my first post-do I understand what the CoC is.
    Yes, if an answer, yours would pass the exam question, IMO.

    From an academic viewpoint, there would be some that would argue that the 8 x 12 print should be viewed at 14.5 inches (which is the diagonal length of the print). So you might loose an half mark for that.

    And there will be others that will argue 0.019mm is the correct CoC for an APS-C camera.

    You did not define in your example the format of the camera used, that is important and you would likely loose a partial mark for that, too.

    ***

    Quote Originally Posted by Paul View Post
    and when taking pictures and guesstimating depth of field, some times using a handy-Andy, does the CoC matter at that point in the process? ATB Paul
    Yes the CoC does matter, because it changes with the camera format and the correct numeral representing the CoC needs to be put into the calculator, if it is a software item; or be selected in the table or on the dial if it be it a physical DoF Set of Tables or DoF Dial.

    WW

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    Re: Circle of Confusion

    Quote Originally Posted by Priapus View Post
    Concerning depth of field, I wonder if people with Cameras that have DoF preview function can tell me how much useful it is for them when they are in action.
    Personally, I've never found mine to be of any benefit in the field. The best use I've found for it is to demonstrate how critical exposure is on a digital camera ... stop down 2 stops from max wide-open, and see how little the brightness changes in the viewfinder when the DoF preview button is pressed. A full 3/4 of all recordable info is captured between the DIFFERENCE between the two levels.

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    William W's Avatar
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    Re: Circle of Confusion

    Quote Originally Posted by Priapus View Post
    Concerning depth of field, I wonder if people with Cameras that have DoF preview function can tell me how much useful it is for them when they are in action. And can a small area like the viewfinder give the photographer enough space to be able to make decisions about the DoF? For I have never used that feature before and wonder if lacking it would put me at a great disadvantage!

    I think most will agree that a DoF Preview is of limited use on and APS-C body as mostly the view finders are quite small and dark. Also less useful in darker situations and or when using slow lenses.

    135 format (“Full Frame”) DoF preview becomes more useful, but it also depends on the quality of the screen – most DSLR 135 format cameras have reasonable focusing screens but some older film cameras do not.

    When moving to medium format (645, 6x6, 6x7, 6x9) DoF preview is very useful, and in the larger formats, in view and field cameras is used all the time.

    Practically: I rarely use the DoF preview on my DSLRs, even the 135 format – when used it is a rough check – not an accurate measure: if accuracy is required I suggest using a table and a tape measure, as it is likely you will be using very shallow DoF from wide aperture, anyway.

    WW

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    William W's Avatar
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    Re: Circle of Confusion

    An aside and not meant pedantically: Depth of Field and Depth of Focus, are different.

    WW

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    Re: Circle of Confusion

    Quote Originally Posted by Colin Southern View Post
    The best use I've found for it is to demonstrate how critical exposure is on a digital camera ... stop down 2 stops from max wide-open, and see how little the brightness changes in the viewfinder when the DoF preview button is pressed. A full 3/4 of all recordable info is captured between the DIFFERENCE between the two levels.
    Clever.

    Thank you.

    I will use that in a demonstration / talk, I am doing next week.

    Bill

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    Re: Circle of Confusion

    Thankyou William W. This is the response I was looking for. With respect to taking the picture, I use a DSLR with a 1.6 crop factor. The DOF table that I use is calculated at 0.019. My original statement of 0.020 was an error on my part.( I promise to try harder in future). Although I haven't finished with this subject yet, I am trying to formulate my next question so that I don't look too stupid. Thanks again,ATB

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    William W's Avatar
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    Re: Circle of Confusion

    Quote Originally Posted by Paul View Post
    I use a DSLR with a 1.6 crop factor. The DOF table that I use is calculated at 0.019. My original statement of 0.020 was an error on my part. (I promise to try harder in future).
    Bonus half mark for rounding up.

    Quote Originally Posted by Paul View Post
    Although I haven't finished with this subject yet.
    Ha! When you think you are finished, begin researching the origins / history of it – there is as much skulduggery in it as the politics behind attempting a universally accepted “Standard Apertures” and “Standard Film Speeds”, over the years.

    Looking forward to the next question.

    WW

  19. #19

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    Re: Circle of Confusion

    Concerning the DOF preview button. For what its worth I have not had any success with it. However, I read quite recently infact, that it is helpful when setting the height of a grad. ND filter with respect to its position in the scene. I tried it without a tripod but its a little clumsy. I will try again using a trypod to see if it works.

    ATB Paul

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