Depth-of-Field

Discussion in 'Casual Photo Conversations' started by alan_marcus|2, Oct 29, 2012.

  1. Many questions on depth of field are posted. Most answers end up quoting tables produced by camera makers and online depth of field calculators. The answers are generally good but I feel compelled to tell you the rest of the story.
    Geometric optics teaches that a lens is only able to form a sharp image of an object at a given distance. Objects at other distances will be unsharp. Matter-of-fact observation reveals this is not factual because objects before and behind the point focused upon appear sharp. Why is this?
    The camera lens is a converging lens. It images by bending light rays inward forming a cone of light. When we focus, we are kissing off the apex of this cone on the surface of film or chip. If the lens was perfect, the apex of this cone would be a point; thus it would have no observable dimension. The reality is this: no one has ever made a lens that yields true points. This is due in part to uncorrected aberrations.
    How big can this imperfect point be and still yield an acceptable image? The answer is based on the resolving power of the human eye. The average observer can resolve a point made by a disk held 3000 diameters distant. In other words, a 10mm disk viewed from 30,000mm (30 meters or 100 feet) appears as a point and not a disk. To make a sharp and clear image the circles produced by the lens must appear as points and not disks. This is a very rigorous standard.
    Sorry, but this standard won't hold up photographically. Film turbidity, image contrast, lens flare, and uncorrected aberration reduce this value to 3.4 minutes of arc, which translates to a circle viewed 1000 times its diameter. This translates to a circle 1/100 of an inch viewed from 10 inches. Now this circle when viewed under a microscope looks flawed; we see scalloped boundaries -- thus its name "circle of confusion".
    What all this means is, if a lens yields producing circles of confusion 1/100 of an inch (.25mm) in diameter, the image viewed from 10 inches (254mm) appears sharp. Now the problem is this: modern cameras have shrunk over the year. Our cameras are miniatures and the standard pertains to contact prints. Our miniature images must be enlarged to make them usable. Modern 35mm cameras must suffer a minimum of 8x magnification to make an 8x10 print. That means the circle of confusion at the image plane of the camera must be 1/800 (0.3mm) in diameter or smaller. What I am trying to say is this: the degree of enlargement and the viewing distance must be taken into consideration when calculating depth of field tables.
    Because depth of field is also subjective (viewing distance, eye sight of observer, contrast of image, and image illuminant, to name a few), the tables are hodgepodges of speculations. Camera makers often express the permissible size of the circle of confusion as a fraction of the focal length. Some specifications I have revealed.
    The most common circle size used for depth of field calculation is 1/1000 of the focal length. Thus, calculations of depth of field for a 50mm lens are based on a circle size of 50mm ÷ 1000 = 0.05mm.
    Leica and Kodak use a more stringent standard 1/1500 and 1/1750 respectively.
    You can do your own calculations:
    P = point focused upon
    Pd = distant point sharply defined
    Pn = near point sharply defined
    D= diameter of permissible circle of confusion
    f= f/number
    F= focal length
    Pn = P ÷ 1+PDf/F2
    Pd = P ÷ 1-PDf/ F2
    More gobbledygook from Alan Marcus
     
  2. veery casual conversation:)
     
  3. Besides all the
    Oops I made a math error.
    That means the circle of confusion at the image plane of the camera must be 1/800 (0.3mm) in diameter or smaller.
    Should read:
    That means the circle of confusion at the image plane of the camera must be 1/800 (0.03mm) in diameter or smaller.
    Sorry of the gobbledygook - Alan Marcus
     
  4. This is why the DOF charts serve a purpose...telling this to a beginner who asks how to obtain DOF, or trying to figure this out on the fly, would be crazy. But it is good to have an understanding of how those charts are created.
     
  5. Of course one can select any COC they desire, more stringent for bigger enlargements. But one has to take into consideration the entire image chain and where the weakest link will be. I still do occasional wet printing in my makeshift darkroom. Are my enlarging lenses any good? I don't know for sure. Is the enlarger properly aligned? Beats me, looks close enough. I'm using a regular glassless negative carrier, should it be glass? How flat must the negative be? The enlarger is a cheap Durst 301 I got for free.
    On to the camera, an aging OM-1, used mostly hand held. Should I use a tripod, at least when I can? I'm 63 and have the first sign of cataracts forming I've been told. Can I still see clearly enough to focus accurately? If so, is the screen and mirror in correct alignment?
    And on and on.
    Sometimes I think the DOF calculations are the least of my problems.
     
  6. I always feel like my train of thought has been derailed when I hear "depth of field". How about "depth of focus", just seems less obtuse, more to the point (of focus), lol.
     
  7. Hi Mendel,
    The accepted jargon:
    Depth of field is that subject distance (zone) before and behind the distance focused upon that is acceptable as to sharpness.
    Dept of focus is that object distance (film to lens or digital chip), a tolerance. The size of the circle of confusion as projected by the lens on film or chip. The zone of depth of focus is dependent only on the aperture setting of lens.
     
  8. You all have too many lenses, have a couple like me and you get a good sense of what you will get at set F stops and distance to subject. ;-)
     
  9. I have found I like f5.6 for portraits and f8.0 for portraits with more then one person. I like f11 for most scenics with my D200 and I like f2.8 if I want to blur out the background more then usual. When I am taking pictures I do not use a calculator or a computer to tell me what I like.
     
  10. I just let AF get on with it :)
     
  11. There are many standard for the size of the COC. Generally the smaller the film/sensor the COC has to be smaller as the image tends to be enlarged more. But for those who are interested in the in focus part of the image (rather than a lot of people nowaday are interested in the out of focus part of the image) then we can take the COC based on the pixel size. To be super critical it can be 1 pixel size as any thing smaller than 1 pixel won't have any detail any way. But we can choose 2 or 3 pixel size or so.
     
  12. Alan,
    I agree with your definition of depth of field: "subject distance before and behind the distance focused upon that is acceptable as to sharpness". This is the most common definition of depth of field. Unfortunately, there are several definitions for depth of focus. One of them (used by some Kodak marketing people) is identical to the common definition for depth of field. (The belief was that depth of focus was better understood.) Putting Kodak marketing speech aside, if we are to speak of depth of focus as distinct from depth of field, then the definition I prefer is: "image distance before and behind the in-focus image that is acceptable as to sharpness". The two concepts are analogous. Depth of field refers to the subject while depth of focus refers to the image. By this definition, depth of focus (like depth of field) depends on both aperture and focal length.
    One of the surprising things to me as a rookie photographic engineer at Kodak (a few decades ago) was that depth of focus got wider with a longer focal length. Depth of focus is only a problem with small formats that typically use short focal length lenses. I specifically recall a roll of super 8 movie film sent in as part of a customer inquiry. (The were called inquiries rather than complaints.) The wide angle shots were terribly out of focus. As the customer zoomed in, the focus improved (but never got to be good). The film cartridge has been discarded by the processing lab, but after obtaining the customers camera, we found that the locater pin had been damaged and the film cartridge wasn't seated at the correct film plane.
     
  13. Ron,
    I too handled customer inquiries for Dynacolor and a major drug chain. Many film cartridge systems lacked sufficient precision to always position and hold the film flat. Worst were the 126 followed by 110. The super 8 cartridge had a cleaver positioning system.
    I related that depth of focus was aperture dependent and not focal length dependent. This is true providing the lenses are operating at the same f/number. Think of two sharpened pencils placed pointy end to pointy end. The film or chip rests at the intersection. If you move upstream or downstream and make a cut, you get a circle. The size of this circle is what we are talking about. The size change is dependent on the focal length divided by working diameter (As you stop down the pencil points become sharper). Stopping down affords greater tolerance as to the placement and flatness of the film or chip.
     
  14. Alan, I don't think you're telling us anything new, although most people certainly don't understand DoF at this depth.
    The COC is an arbitrary figure derived from the focal plane projection of the width of acceptable blur (subjectively considered "sharp enough") on a print of a certain size at a certain distance. it would perhaps better be expressed as a visual angle, but it's not. Therefore with all other things being equal, the COC is proportional to format size. Obviously, as you say, the most appropriate COC depends on many factors and may differ from one application/person to another.
    If you're not familiar with the object field method discussed by Harold Merklinger in 4 articles, it's a great read. See here:
    http://jimdoty.com/learn/dof/dof_merk/dof_merk.html
    Digest those articles, and you will understand DoF at an entirely new level.
     

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