DSLR Depth of field

Discussion in 'Digital SLR' started by Darrell, Feb 6, 2005.

  1. Darrell

    Darrell Guest

    Depth of field doesn't change. If you have a 50mm lens it will have the same
    depth of field on a SLR or a dSLR. What does change is the Field of View
    (FOV) so to match the FOV of my Pentax LX with a 50mm lens on my Pentax *ist
    D I would need a lens around 33mm. The depth of field is deeper on a wide
    angle. This is what causes confusion. You would have to calculate or use a
    DOF table to adjust the aperture to match the DOF. You would have to use the
    shorter lens opened up more on the dSLR to match the equivalent lens on a
    35mm SLR
     
    Darrell, Feb 6, 2005
    #1
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  2. Darrell

    Darrell Guest

    In a dSLR the sensor is the same distance as the film, otherwise the lens
    would NOT focus to infinity. The image area is just cropped. The only lenses
    that may be closer to the sensor are the Canon EF-S lenses. The fact all
    other lenses from the makers 35mm SLR cameras function is because the snesor
    and film is at the same distance.
     
    Darrell, Feb 6, 2005
    #2
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  3. Darrell

    Darrell Guest

    What is the CoC for an APS-C/DX sized sensor? I jhave figures for most film
    formats, but I can't recall seeing the dSLR numbers.
     
    Darrell, Feb 6, 2005
    #3
  4. Darrell

    Greg G Guest

    I'm considering buying a DSLR, cheaply. This would mean a not-quite
    current model.

    Someone mentioned that because the image sensor is smaller than 35mm
    film, the depth of field will be greater than I might be used to in
    similar shooting conditions. I'm curious to know if this is true, and
    (briefly, if possible) why.

    Does this mean I'll have trouble blurring the background when I want
    to? To complicate things further, the standard lenses that seem to
    come with these cameras don't seem to be very "fast", although I
    suppose that's probably because they are zooms. This would make it
    even harder to limit depth of field.

    Informed advice please.

    Greg Guarino
     
    Greg G, Feb 7, 2005
    #4
  5. Darrell commented courteously ...
    Because a digital's lens is closer to the "focal plane"
    (sensors) than a 35mm film, correct?

    It interests me that there are probably few people today
    who know that, mathematically, there is no such thing as
    "depth of field".

    The lens is only in focus at one narrowly defined point.
    It was the development of the concept of "circle of
    confusion", where photographers could take advantage of
    the fact that human eyes aren't all that good at resolving
    fine detail (we tend to resolve spots out of focus as
    little blurry circles), that enabled the concept of depth
    of field to be developed and successfully used until this
    day.

    The standard for COC we use today was agreed to many, many
    years ago (I'd have to dust off my old Nikon Handbook from
    1969 to verify the date and the other relavent facts
    here). And, neat thing that it is, if you want more DOF,
    just agree to a bigger COC!
     
    All Things Mopar, Feb 7, 2005
    #5
  6. Darrell

    IMKen Guest

    If you are shooting with a 50 MM lens on a film camera and the same lens on
    a digital then the DOF will be less with the digital as the lens is acting
    as a 75MM on the digital.

    Ken
     
    IMKen, Feb 7, 2005
    #6
  7. Darrell

    Bigguy Guest

    No....
    The lens is still a 50mm lens with a 50mm focal length and the same DoF.
    You are just using the centre two-thirds of the image - i.e it is being
    cropped.

    Guy
     
    Bigguy, Feb 7, 2005
    #7
  8. Darrell

    Paul Mitchum Guest

    Nope. The lens' focal length is the distance between the focal plane and
    lens' optical center. The whole idea of interchangeable lens mounts (and
    thus the SLR concept) is to guarantee the lens focal length, between
    cameras and even manufacturers. So a K-mount (I shoot Pentax) is a
    K-mount is a K-mount in terms of focal length, whether it's on an ME or
    an *istD.

    The field of view of the lens itself doesn't actually change, either.
    Just the recorded portion of it. :) The sensor is smaller, not closer
    to the lens.
     
    Paul Mitchum, Feb 7, 2005
    #8
  9. Darrell

    dylan Guest

    I have a problem here, I agree with your reasoning that changing the sensor
    size shouldn't change the depth of field just the FOV , all we are doing is
    cropping the area, but if I use the online DOF calculatoar at
    http://www.dofmaster.com/dofjs.html it doesn't seem to agree.

    ie enter 50mm 'actual' lens at f2.8, distance 30ft and when using a 35mm
    format the DOF is 20ft and using a 10D it's 12.2 ft.

    Do I misunderstand, is the program wrong or some thing else ??:eek:)

    Thanks
     
    dylan, Feb 7, 2005
    #9
  10. Darrell

    Paul Mitchum Guest

    The depth of field of a lens setting is determined by the circle of
    confusion, not the size of the film. If the COC of both 35mm film and
    the digital sensor were the same, then the DOF would be the same with
    the same lens. But they're not. :)
     
    Paul Mitchum, Feb 7, 2005
    #10
  11. Darrell

    Lourens Smak Guest

    No.. in fact a smaller chip has LESS DoF because the CoC is smaller.
    But, because you need a shorter focal length to get the same image
    projected onto the smaller chip, DoF in the final result will be larger.
    (the FL factor is more important)

    Lourens
     
    Lourens Smak, Feb 7, 2005
    #11
  12. Darrell

    Owamanga Guest

    DOF DOES change between DLSRs and SLRS because the circle of confusion
    is different.

    DOF IS NOT AOV

    I'll post my discussion on this ONCE AGAIN....

    I use a lot of Acronyms here, so briefly:

    CoC - Circle of Confusion
    DoF - Depth of Field
    NL - Near Limit (Closest subject still in focus)
    FL - Far Limit (Furthest subject still in focus)
    HF - Hyperfocal Distance. (Optimum focus point to maximize DoF depth)
    AoV - Angle of View

    CoC on a DSLR with a smaller-than-35mm sensor is different to regular
    35mm. For a typical 1.5 to 1.6 sensor factor your DSLR DOF at f8 will
    appear similar to an f11 shot on a 35mm camera if you adjust lenses to
    give similar Angle of view..

    i.e. DSLR's have a deeper DOF than 35mm SLRs when AoV is matched
    between the cameras.

    Comparing an SLR to an DSLR with a 1.5 sensor ratio and fictitious
    lenses to give identical AoVs using a f5.6 aperture lens focused at
    8m:

    A SLR with a 108mm NL=7.233m FL=8.949m (Depth: 1.716m)
    A DSLR with a 72mm NL=6.830m FL=9.653m = (Depth: 2.823m)

    Even if you stick with the same lens on both cameras,
    say a 50mm f4 focused at 6m, the DoF's differ significantly:

    SLR 50mm f4. HF=22.3m NL=4.73m FL=8.18 (Depth: 3.45m)
    DSLR 50mm f4. HF=31.3m NL=5.04m FL=7.41 (Depth: 2.37m)

    You can use any DOF calculator that allows you to enter a custom CoC
    to represent your sensor size to learn the differences.

    Here's how you can look-up your camera's Coc:
    http://dfleming.ameranet.com/digital_coc.html

    And here is a DOF page + calculator:
    http://www.conent.com/ConAdv/Encyclopaedia/Photography/CNQ_CAPhotography001.asp
     
    Owamanga, Feb 7, 2005
    #12
  13. Darrell

    Owamanga Guest

    Here are some popular ones (not all APS-C/DX):

    Nikon D1H, D1X, D2H, D70, D100 all have a CoC of 0.020
    Canon 1D - 0.023
    Canon 1DS - 0.030
    Canon D30 - 0.019
    Canon Digital Rebel - 0.019
    Sigma SD-10, SD-9 are 0.018
    Olympus E-1 - 0.015
     
    Owamanga, Feb 7, 2005
    #13
  14. Paul Mitchum commented courteously ...
    Maybe I wasn't clear enough. I know that non-SLR digitals
    have lenses closer to the sensors, so focal lengths have
    to be shorter. Hence, most manufacturer's give you the
    35mm equivalent so that old film photographers can relate.

    I'd like to respectfully disagree with your statement that
    focal length is the distance between the FP and the lens'
    optical sensor. Just a single example may suffice. I used,
    but never owned, Nikon's excellet 50-300mm zoom back in
    the 1970's. It was expensive, and it was a beast. But, it
    was also close to twice the length of the 110-300 off-
    brand lens for my Nikon FTN.

    If you meant actual focal length rather than
    magnification, I'd agree with you - i.e., 300mm giving 6X
    magnification compared to 50mm
     
    All Things Mopar, Feb 7, 2005
    #14
  15. Darrell

    Alan Browne Guest

    Yep.
     
    Alan Browne, Feb 7, 2005
    #15
  16. Darrell

    Alan Browne Guest

    Most people don't even get the notion of depth-of-field, never mind graduate to
    DOF being subjective viewing of the final print or projection.
    Plane (which I'm sure you meant). Although for most lenses it will get softer
    at the edges/corners but this is due to lens underperformance, not absense of a
    focus plane.

    Yes, however a larger CoC implies a further viewing distance.

    Here's some empirical stuff to get you reaquainted with the numbers in use.

    http://www.nikonlinks.com/unklbil/dof.htm
     
    Alan Browne, Feb 7, 2005
    #16
  17. Darrell

    Alan Browne Guest

    The final constraint being: when you print it (view it) at a given size.
     
    Alan Browne, Feb 7, 2005
    #17
  18. Darrell

    John Francis Guest


    Having mis-calculated this once, and been called on it, I think I
    might be able to offer a helpful contribution.

    Don't confuse yourself with all these CoC & AOV numbers, etc.

    Your eventual aim, presumably, is to end up with a given final image
    size (either image dimension in pixels, or print size in inches,
    and with your own chosen composition of the image. If that is the case,
    then the *only* variable that affects your DOF is the actual physical
    diameter of the aperture on your lens. That's it. All the other things
    (the film/sensor size, the angle of view of the full image, the amount
    of enlargement & cropping you have to do for composition) cancel out.

    An example; If you take a full-frame image on a 35mm camera
    using a 300/f2.8 lens, and a full-frame image of the same scene
    using your 1.5 crop-factor DSLR and a 200/f2.8 lens, and print
    them both up to the same final size, you'll have more DOF on the
    print from the DSLR; it will match the DOF from a 300/f4 lens on
    the 35mm body (all lenses assumed to be used at maximum aperture).

    Not only that; it will roughly match the DOF from, and angle-of
    view of, a 600/f8 on a medium-format body, or even a 1000/f14
    lens on a 4x5 camera (allowing for the different aspect ratio).
     
    John Francis, Feb 7, 2005
    #18
  19. Alan Browne commented courteously ...

    [snip]
    [snip]

    Good Grief! What did I say "plain"? Yes, of course it is
    "plane"...

    Thanks for the link on DoF
     
    All Things Mopar, Feb 7, 2005
    #19
  20. Darrell

    Paul Mitchum Guest

    You're right. I'm right. We're both right. :)

    My point was that lenses don't have circles of confusion, and COC are
    the fundamental currency of DOF.
     
    Paul Mitchum, Feb 7, 2005
    #20
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