Avogadro's 16 rules of DOF

Discussion in '35mm Cameras' started by Avogadro, Sep 1, 2003.

  1. Avogadro

    Avogadro Guest

    Everything about depth of field was probably very well understood a
    couple hundred years ago - which is before cameras were invented. And
    yet today, DOF is poorly understood by most photographers. Why would
    that be?

    Well, DOF equations are not that complex, but most photographers are
    not really very good mathematicians, and most would hesitate to
    undertake DOF calculations. Besides, there is the difficulty of
    applying the results, because they involve concepts that can be
    unfamiliar, like the circle of confusion, resolution, subtended
    angles, etc.

    Because of these things, it is easy to make statements about DOF that
    are quite confusing. For instance, what would you think about the
    following:

    o Wide angle lenses have more depth of field than telephoto lenses.

    o All lenses have the same depth of field at the same aperture.

    Both statements are quite correct when applied to many everyday
    photographic situations. Notice, though, that they disagree!
    Accordingly, statements like the above incite hours of discussion on
    r.p.e.35mm, often liberally spiced with insult, ridicule, and
    diatribe.

    DOF is the zone in the picture, from near to far, that appears to be
    in sharp focus. I think most of us, when presented with an actual
    photo, would have trouble deciding exactly what is sharp and what
    isn't. Left to ourselves, most would pick different zones of
    acceptable sharpness. And it would vary by our mood too, to say
    nothing of the size of the picture, the lighting, the subject matter,
    and whether the photo is even critically sharp to begin with. So,
    although we have equations that can calculate DOF very exactly, it is
    actually a somewhat fuzzy quantity.

    Although it is fuzzy, there is good justification for understanding
    DOF well enough to be able to make decisions in our photography... to
    answer questions like, "Will I get better DOF with 35mm or with medium
    format?", "Do I have a hope of getting the front and back flower
    petals in focus at the same time?", "Should I switch to a wide angle
    lens?", and so on.

    Anyone can download DOF calculators from the Internet and do the
    calculations. Or you can punch the equations into a spreadsheet and do
    the calculations there. (Caution: many of the equations in books and
    Web sites are simplified versions and will not always give exact or
    consistent answers.)

    But when we find ourselves at a shoot with a DOF issue, there's no
    time for calculations. So here then are some RULES OF THUMB to guide
    you in your shooting. (Photographers love rules.)

    1) Wide angle lenses have more depth of field than telephoto lenses,
    at EQUAL CAMERA-SUBJECT DISTANCES. (We are assuming all other factors
    are the same.) In general use, wide angle lenses rarely present DOF
    problems, but long teles sure do.

    2) All lenses have the same depth of field at the same aperture, if
    the IMAGE SIZE IS KEPT EQUAL. Thus, if you want more DOF when shooting
    a portrait, changing focal lengths won't help. The only recourse is to
    stop down.

    3) A portion of the zone of acceptable sharpness will be on the near
    side of the plane of focus, and a portion will be on the far side. The
    DOF on the far side is always bigger, sometimes by a microscopic
    amount, sometimes by an infinite amount. The old rule that near DOF is
    1/3 and far DOF is 2/3 is ALMOST ALWAYS WRONG, but is sometimes
    approached reasonably for in-between DOF situations (subject a few
    feet or meters away, limited light).

    4) When DOF is shallow (such as in close-up photography or with long
    telephoto lenses), the near and far zones are about EQUAL in size.

    5) When DOF is deep (such as when shooting scenics with wide angle or
    normal lenses stopped down to f:8 or smaller), the far zone can extend
    beyond the horizon. The total DOF is INFINITE (but the near zone is of
    course limited).

    6) You will get more DOF with smaller image formats. The 35mm format
    is relatively small and is great for DOF. Large format cameras can
    give very narrow DOF, and this can be effective in portraits when used
    skilfully. Most film formats are larger than the retina of the human
    eye, so DOF in photos often seems shallow compared to what the eye
    sees.

    7) Cameras with swings and tilts can place the plane of focus on quite
    a slant, and the DOF follows... this can allow you to get a whole rug
    in sharp focus from front to back. DOF apparently increases, but
    doesn't.

    8) Comparing 35mm and 120 formats (each using lenses with similar
    angles of view), you will get about the SAME DOF if the film in the
    larger camera is TWICE as fast (because it allows you to stop down one
    stop).

    9) When DOF is shallow, you can get roughly DOUBLE the total DOF by
    closing down TWO f-stops. If you want to keep the shutter speed the
    same, use faster film. Thus, quadrupling the ASA doubles the DOF.

    10) When doing close-up or macro photography, if you want to calculate
    the DOF, use the MAGNIFICATION that you are shooting at. Don't try to
    use lens-subject distance (because most people won't know where the
    front nodal point is). ("Magnification" is the size of the image on
    the film compared to the actual subject size. Thus, in 35mm
    photography, a magnification of 0.5 means the subject measures
    48x72mm. Many macro lenses have a magnification scale... if not,
    magnification can be estimated fairly easily.)

    11) With a tele-converter, the DOF will be for the focal length of the
    combined assembly.

    12) Cropping an image when printing magnifies the unsharpness, so you
    should have stopped down more when shooting to compensate.

    13) Poor quality lenses give deeper apparent DOF... the eye is more
    tolerant because of the lack of tack-sharpness. The same applies to
    pictures that are unsharp because of camera movement, poor enlarging
    techniques, etc.

    14) When DOF is shallow, you can't just keep stopping down to get
    everything in sharp focus. Macro photography is a common application
    when even at f 22, things may not all be in focus, and you will have
    to compromise. At small lens openings, sharpness suffers because of
    diffraction (due to the wave nature of light). Diffraction becomes
    noticeable at f:16-22 and gets worse as the aperture becomes smaller.

    15) In actual shooting situations at a particular f stop, you can
    maximize the total DOF by setting the infinity mark on the focus scale
    opposite the far DOF limit (assuming these are marked on your lens).
    This is the same as focusing on the hyperfocal distance. You can get
    the hyperfocal distance using those downloaded programs or equations
    we talked about before.

    16) Some lenses have DOF preview. This is a nearly useless feature
    unless you train yourself to use it... because the viewfinder image
    darkens a lot, and because the eye can often see a bit past the
    viewfinder screen to view the aerial image, making the DOF seem deeper
    than it is.

    Avogadro
     
    Avogadro, Sep 1, 2003
    #1
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  2. Do you by any chance have the correct formulae for calculating DOF?

    Thanks,
    Aaron Queenan.
     
    Aaron Queenan, Sep 1, 2003
    #2
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  3. In a British autumn, depth of field is greater, because all the rain makes
    the mud deeper and it goes further up your wellies

    8^)
     
    Tony Parkinson, Sep 1, 2003
    #3
  4. Avogadro, thanks for this excellent summary. I think (point 6) that the
    human eye effect may have a lot to do with the ability of the eye to
    accommodate rapidly to "see" different parts of the scene sharply
    without the user being conscious of it - but your thought about format
    size may apply too.

    We've got your number*

    *6.023 x 10^23, IIRC.
     
    David Littlewood, Sep 1, 2003
    #4
  5. Avogadro

    Dallas D Guest

    I like mashed avo on toast with salt and pepper.

    It also goes nicely with a French Salad.

    It does nothing for my DOF though...
     
    Dallas D, Sep 1, 2003
    #5
  6. Thank you; those rules were useful. Some, like #2, are not intuitive and
    were unbeknownst to me. -Kevin
     
    Kevin Neilson, Sep 1, 2003
    #6
  7. Avogadro

    Ken Cashion Guest

    It is actually straightforward once the acceptable circle of
    confusion is established. This is just a dimension that if present on
    a print, someone would call it a dot rather than a disc.
    This depends on how big a print is viewed at what distance.
    If it is about .001" on a negative and that negative is
    enlarged to an 8x10 and viewed at a comfortable distance in our hands,
    would we agree that it was a tiny little bitty dot or a blob. See,
    these are rather arbitrary terms.

    Ken Cashion
     
    Ken Cashion, Sep 2, 2003
    #7
  8. Avogadro

    Alan Browne Guest

    What Avogadro left out was that the CofC for 35mm is 'typically' 0.035mm.

    Download fcalc from www.tangentsoft.net and it will do the calcs, and
    contains all of the formulae and CofC's for most common film formats.

    Cheers,
    Alan
     
    Alan Browne, Sep 2, 2003
    #8
  9. Avogadro

    Alan Browne Guest

    OOOPPPS ...meant 0.025mm

     
    Alan Browne, Sep 2, 2003
    #9
  10. Avogadro

    T P Guest


    Given the very wide ownership of cameras today, it is not surprising
    that such a high proportion of camera owners are profoundly ignorant.

    Compare this to the time before plastic junk SLRs took hold of the
    market (late 1970s, early 1980s) when a much higher proportion of
    amateur photographers were drawn from the educated professional
    classes ...

    ;-)
     
    T P, Sep 2, 2003
    #10
  11. Avogadro

    Avogadro Guest

    Yes.

    (Had to think about that one for 6 days.)

    (Actually, was away.)

    Suggestion: shoot them both at 75mm, if you have both. See for
    yourself. Don't believe everything you hear, try it.

    Avogadro
     
    Avogadro, Sep 15, 2003
    #11
  12. Hi!


    [snip]

    : Just remember that you won't always know the primary use that a shot
    : will be put to, and that most shots will be viewed under many
    : different situations.

    If that's truly the case for a given shot, there's only one rational thing to do- Use a DoF scale calculated with a CoC diameter that will support the largest print possible for the format.

    That makes it really difficult to shoot because it pushes your Near Sharps further away from the camera than they would be if larger CoC's were permissible. It is much "smarter" to make up your mind what size print you'll be making and what viewing distance it must suffer at your chosen print resolution before you make the exposure.

    Think about it. If you produce CoC's that are so large as to be visible in the final print for ANY of the "many different situations" in which the print may be viewed, you will choose to completely avoid the unacceptable "situations" by making the print no larger than the CoC's will allow -or- you will compromise the quality of your work. It's got to be one or the other.

    If we are dedicated to producing high quality images, we won't be enlarging the CoC's beyond the point where print resolution falls below our chosen minimum (5 to 8 lp/mm for a 10-inch viewing distance, or the equivalent at greater viewing distances). So why not make up our minds BEFORE we shoot? By doing a little bit of homework in advance, we can equip ourselves with the ability to determine pricesely how much DoF is necessary for each print size we're likely to produce. If the subject space demands m
    ore DoF that can be had for an 11x14-inch print, then so be it - we'll make an 8x10 print and switch to the more generous DoF tables/calculations had with larger CoC's.

    : Also, for many shots, conditions won't allow you
    : realize your ideal goals, and compromises will be needed. Therefore
    : the numbers have to be regarded as rough guides in most cases.

    Compromise the enlargement factor or restrict the viewing distance (if you can), but don't compromise the image quality.

    The numbers are not rough guides. They can be trusted as a rock solid truth that you can exploit to great benefit and control with amazingly little effort (read my first post). Ignoring the truth won't make it go away. The truth serves those who submit to it. Accepting poor quality in a print that could have been sharper at the desired enlargement factor isn't compromise - it's ignorance and, in my opinion, ignorance is forgivable only when it's not willfull. It's hard to summon sympathy for an "artis
    t" who is disappointed on occaision with results he left to chance while chosing to ignore the principles that influence his medium. Self-expression can not precede mastery of brush and pigment.

    Mike Davis


    --
     
    Michael K. Davis, Sep 22, 2003
    #12
  13. Avogadro

    Tony Spadaro Guest

    The old "everything from here to Jupiter must be in focus" bull. As
    enlargement increases so does viewing distance - at least by anyone who
    isn't anal retentive.

    --
    http://www.chapelhillnoir.com
    home of The Camera-ist's Manifesto
    The Improved Links Pages are at
    http://www.chapelhillnoir.com/links/mlinks00.html
    New email - Contact on the Menyou page.
    to do- Use a DoF scale calculated with a CoC diameter that will support the
    largest print possible for the format.
    further away from the camera than they would be if larger CoC's were
    permissible. It is much "smarter" to make up your mind what size print
    you'll be making and what viewing distance it must suffer at your chosen
    print resolution before you make the exposure.
    in the final print for ANY of the "many different situations" in which the
    print may be viewed, you will choose to completely avoid the unacceptable
    "situations" by making the print no larger than the CoC's will allow -or-
    you will compromise the quality of your work. It's got to be one or the
    other.
    enlarging the CoC's beyond the point where print resolution falls below our
    chosen minimum (5 to 8 lp/mm for a 10-inch viewing distance, or the
    equivalent at greater viewing distances). So why not make up our minds
    BEFORE we shoot? By doing a little bit of homework in advance, we can equip
    ourselves with the ability to determine pricesely how much DoF is necessary
    for each print size we're likely to produce. If the subject space demands m
    make an 8x10 print and switch to the more generous DoF tables/calculations
    had with larger CoC's.
    can), but don't compromise the image quality.
    truth that you can exploit to great benefit and control with amazingly
    little effort (read my first post). Ignoring the truth won't make it go
    away. The truth serves those who submit to it. Accepting poor quality in a
    print that could have been sharper at the desired enlargement factor isn't
    compromise - it's ignorance and, in my opinion, ignorance is forgivable only
    when it's not willfull. It's hard to summon sympathy for an "artis
    chosing to ignore the principles that influence his medium. Self-expression
    can not precede mastery of brush and pigment.
     
    Tony Spadaro, Sep 22, 2003
    #13
  14. Avogadro

    Avogadro Guest

    Yes...

    With reference to Mike Davis's post, DOF is not a goal to always be
    maximized... and often is intentionally reduced for effect.

    DOF is just one of many tools. Just try to understand a bit how it
    works and use it intelligently, balanced off against other objectives.

    Avogadro
     
    Avogadro, Sep 22, 2003
    #14
  15. Avogadro

    Avogadro Guest

    If image perfection is your goal, what are you doing in a 35mm
    newsgroup?

    Avogadro
     
    Avogadro, Sep 22, 2003
    #15
  16. Unless, of course, enlargment increases to get a tighter crop.

    Only the analy retentive never cr*p. ;-)
     
    Q.G. de Bakker, Sep 22, 2003
    #16
  17. Hi Tony!

    : The old "everything from here to Jupiter must be in focus" bull.

    When did I insist that everything in the image be equally sharp? Selective focus is an absolutely valid choice for many subjects, but controlling selective focus to yield predictable results requires even more precision than a "here to Jupiter" shot (which can be had by stopping down further than necessary.)

    : As
    : enlargement increases so does viewing distance - at least by anyone who
    : isn't anal retentive.

    If that were true we'd all be shooting with Minox cameras.

    Mike Davis
    --
     
    Michael K. Davis, Sep 22, 2003
    #17
  18. Avogadro

    Tony Spadaro Guest

    No - because a Minox can't deliver a picture that is acceptable at minimum
    viewing distance.

    --
    Camera-ist's Manifesto
    The Improved Links Pages are at
    http://www.chapelhillnoir.com/links/mlinks00.html
    New email - Contact on the Menyou page.
    Selective focus is an absolutely valid choice for many subjects, but
    controlling selective focus to yield predictable results requires even more
    precision than a "here to Jupiter" shot (which can be had by stopping down
    further than necessary.)
     
    Tony Spadaro, Sep 22, 2003
    #18
  19. Hi!

    : If image perfection is your goal, what are you doing in a 35mm
    : newsgroup?

    : Avogadro

    Wow! Why did you post the original article? Weren't you trying to teach?

    With a just a few minutes effort, anyone interested in getting a real handle on DoF can follow the recommendations in my first post to this thread to easily work out how much they have to offset their engraved DoF scales for various print sizes. If they would prefer to use customized CoC diameters in a spreadsheet or a handheld calculator, they're welcome to do that instead. How they employ this knowledge is totally up to them - be it for selective focus or whatever. I haven't even stipulated that any p
    articular print resolution must be used - I've only stated what I prefer and handed you the means to tightly control your shooting to achieve whatever degree of sharpness you prefer.

    Surely there are some 35mm shooters who frequent this newsgroup who are tired of rolling the dice every time they select an aperture and set their focus.

    Mike Davis
    --
     
    Michael K. Davis, Sep 22, 2003
    #19
  20. Avogadro

    Tony Spadaro Guest

    Ever hear of a DOF preview button? Works a lot better than those idiotic
    charts and calculators.

    --
    http://www.chapelhillnoir.com
    home of The Camera-ist's Manifesto
    The Improved Links Pages are at
    http://www.chapelhillnoir.com/links/mlinks00.html
    New email - Contact on the Menyou page.
    handle on DoF can follow the recommendations in my first post to this thread
    to easily work out how much they have to offset their engraved DoF scales
    for various print sizes. If they would prefer to use customized CoC
    diameters in a spreadsheet or a handheld calculator, they're welcome to do
    that instead. How they employ this knowledge is totally up to them - be it
    for selective focus or whatever. I haven't even stipulated that any p
    and handed you the means to tightly control your shooting to achieve
    whatever degree of sharpness you prefer.
    tired of rolling the dice every time they select an aperture and set their
    focus.
     
    Tony Spadaro, Sep 22, 2003
    #20
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