What is the depth of focus for a given F stop?

Discussion in 'Digital Photography' started by n, Dec 1, 2003.

  1. n

    n Guest

    Most of my shots are taken at about 3 meters. My effective focal
    lenght is 100-300 on my usual lens.

    Roughly speaking, what depth of field will be in focus at f2.8, 5.6
    and so on?

    How do I do the calculation in advance of seeing the subject through
    the view finder?

    Thanks
     
    n, Dec 1, 2003
    #1
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  2. n

    Robertwgross Guest

    It is for reasons like this that many cameras have a Depth of Field Preview
    button.

    ---Bob Gross---
     
    Robertwgross, Dec 1, 2003
    #2
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  3. There are lots of DOF calculators out there. One that I like is:

    http://www.nikonians.org/html/resources/guides/dof/hyperfocal2.html

    Note that it doesn't do digital cameras.

    If you are using a 1.5/1.6x dSLR, use the 35mm-equivalent focal length and
    the next smaller f-stop. So to figure out the DOF at f/2.8, use f/4.

    If you are using a 2/3" prosumer camera (A1, F717, 5700), use the
    35mm-equivalent focal length and the f-stop four f-stops smaller. So to
    figure out the DOF at f/4, use f/16.

    If you are using a consumer P&S camera (S85, C-5050, G5), use the
    35mm-equivalent focal length and the f-stop four f-stops smaller. So to
    figure out the DOF at f/4, use f/22.

    These are all approximations, but quite reasonable ones. For example, with
    the Minolta A1 at 10 ft, f/4.0 at 150mm equiv. focal length, your DOF will
    be +/- 0.5 feet, and your background will be nicely out of focus for
    portraits, even if dSLR snots such as myself tell you otherwise.

    David J. Littleboy
    Tokyo, Japan
     
    David J. Littleboy, Dec 1, 2003
    #3
  4. The answer is somewhat complex e.g.,
    The Equation for depth of field is:
    DOF = 2NC(D)^2 / F ^2 where:
    N = f-number
    C = circle of confusion (about 0.03 mm)
    D = distance to the subject
    F = focal length of the lens
    ^2 means the quantity is squared
    This gives the entire depth of field, but DOF is not symmetrically
    distributed around D. It is larger behind than in front.
    You can do the math but I have seen a table with these values
    calculated. Try Google with "Depth of Field Table"
    Bob Williams
     
    Robert E. Williams, Dec 1, 2003
    #4
  5. There isn't one on my digital camera ?
    This is rec.photo.digital is it not?
    Paul.
     
    ** Pee Cee **, Dec 1, 2003
    #5
  6. n

    Tony Spadaro Guest

    Tony Spadaro, Dec 1, 2003
    #6
  7. n

    Andy Hewitt Guest

    Doesn't the view on the LCD display show real time preview? It certainly
    does on my Minolta Z1, and I'm pretty sure my Olympus C960 did too,
    although that didn't have any manual over-rides at all.
     
    Andy Hewitt, Dec 1, 2003
    #7
  8. n

    Robertwgross Guest

    Many digital cameras do not have any preview on the rear display. On a D-SLR,
    the optical viewfinder must be used to line up the shot. On a good D-SLR, the
    Depth of Field Preview button will show how much is present. Then only after
    the shot does the rear display light up to show the image captured.

    ---Bob Gross---
     
    Robertwgross, Dec 1, 2003
    #8
  9. n

    Andy Hewitt Guest

    Blimey, is that really so?

    I tested this on my Z1, and in manual mode you can play around with all
    the settings and immediately see the effect in the viewer before taking
    the picture.

    Actually I just went off and rechecked, and I can see the effect on
    pressing the shutter release halfway.
     
    Andy Hewitt, Dec 1, 2003
    #9
  10. n

    Robertwgross Guest

    Blimey does not post here. Yes, it is true. On a D-SLR camera, there is a
    mirror in the way of the image sensor, so the subject view is seen on the
    optical viewfinder and nothing on the rear display. When the shutter button is
    pressed, the mirror flies up, the shutter opens, and the image is captured on
    the sensor. Then maybe one second later, the image shows up on the rear
    display.
    That is why there are many grades of digital cameras out there. Some users like
    some features, and some users like other features.

    ---Bob Gross---
     
    Robertwgross, Dec 1, 2003
    #10
  11. n

    Andy Hewitt Guest

    Haha, OK.
    I see, so they aren't really an SLR then. At least in the respect of a
    true SLR anyway, where the image you see in the viewfinder is from the
    lens, and therefore what will reach the film once the shutter is
    released.

    Surely though, and thinking about this, this is actually the way a
    traditional SLR works, the only difference being a CCD is used instead
    of a film. What passes the mirror, and reaches the viewfinder, should be
    the same image that reaches the CCD/film.

    Indeed, the viewfinder on the digital should be more accurate, an SLR
    can obviously offer a DOF previewm and at the same time give a visual
    indication of the apature setting too. However it cannot allow for the
    shutter speed. A digital camera could do this, and could give a truer
    preview of the image.

    I appreciate that the circuitry, and journey of the light would differ
    slightly, but this could be corrected within the software. I would
    certainly expect it to with the cost of a reasonable DSLR being what it
    is compared to a tradtional SLR (even in their peak times).
    Personally I think they have made it far too complicated now. When I
    bought my first SLR back in the 80's, I only had to worry about whether
    to have a manual, or auto, and what shutter speed range I needed. Your
    budget dictated the build quality. It's certainly not so easily defined
    now.
     
    Andy Hewitt, Dec 1, 2003
    #11
  12. n

    Robertwgross Guest

    The D-SLR is very similar to the film SLR, so I think you are getting it
    confused. To a certain extent, if you take a film SLR, take out the film
    transport mechanism, and then insert a digital sensor (and associated image
    processing), then you've got the D-SLR. With the SLR or D-SLR, the image in the
    optical viewfinder is what is coming through the lens.

    In contrast, the non-SLR digital camera handles lots of things differently, and
    often there is a digital display preview of the subject.

    ---Bob Gross---
     
    Robertwgross, Dec 2, 2003
    #12
  13. n

    MikeWhy Guest

    The lens typically doesn't stop down to the selected aperture until you snap
    the shutter or push the DoF preview button. All SLRs I've ever seen,
    including the cheapest of the cheap digital ones, have a depth of field
    preview button. (Push the button, and see.)
     
    MikeWhy, Dec 2, 2003
    #13
  14. n

    Jules Baby Guest

    DOF relies on 3 factors:-
    1) Aperture or f-stop: wider = less DOF eg.f2
    2) Lens focal length: longer = less DOF eg.300mm
    3) Camera to subject distance: closer = less DOF


    My advice...do lots of tests to get a feel for it.
     
    Jules Baby, Dec 2, 2003
    #14
  15. n

    Andy Hewitt Guest

    Yes I see that. So of course you can't see what the exposure will do for
    you. Understood now.
    Yes.

    Cheers.
     
    Andy Hewitt, Dec 2, 2003
    #15
  16. n

    Andy Hewitt Guest

    Yes, I have that on my SLR too, I just thought there'd be more to the
    digital version.
     
    Andy Hewitt, Dec 2, 2003
    #16
  17. n

    Bob Salomon Guest

    Do you really need to know what the depth of focus is? That is the range
    of sharpness BEHIND the lens where the film or imaging sensor must lie
    to have an in focus picture. The longer the lens the LARGER the depth of
    focus.

    Or do you want to know what the depth of field is? That is the area in
    FRONT of the lens that ranged from the front of the point the lens is
    focused on to behind the point the lens is focused on. The shorter the
    lens the GREATER the depth of field. The smaller the aperture the
    greater the depth of field. The larger the circle of confusion the
    greater the depth of field.

    However judging depth of field on an SLR is deceptive because you are
    seeing it only at the magnification that the viewfinder uses. If you
    make a print that has more magnification then the viewfinder then you
    will have LESS depth of field then you saw in the finder.
     
    Bob Salomon, Dec 2, 2003
    #17
  18. n

    Andy Hewitt Guest

    Who mentioned that?
    Yes, I know that much.
    But not the otherway around I would suspect. You can't make something in
    focus that already isn't.
     
    Andy Hewitt, Dec 2, 2003
    #18
  19. n

    Bob Salomon Guest

    It is the subject line of this thread.
     
    Bob Salomon, Dec 2, 2003
    #19
  20. n

    Andy Hewitt Guest

    So it is, I hadn't even considered a difference. I think we all just
    assumed depth of field was meant.

    Question is, are we all wrong, or are you being a pedant? :)
     
    Andy Hewitt, Dec 3, 2003
    #20
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