Focal length and depth-of-field

Discussion in 'Digital Photography' started by Peabody, Aug 13, 2009.

  1. Peabody

    Peabody Guest

    As may become obvious, I'm fairly new to photography, currently
    using a Canon A590 P&S, and am looking at getting a DSLR. But I
    need help understanding something, because it could affect what I

    It seems to me that the one thing I can't get with my A590 that I
    could get with a larger-format camera is shallow depth of field -
    those wonderful blurry backgrounds. Even when I open up the A590
    all the way, everything is still pretty much in focus. Well, not
    exactly in focus, but not exactly blurry either. And my
    understanding of this has always been that this was because the lens
    used on my camera, being appropriate for a smaller camera and a very
    small sensor, had a much shorter focal length, and that, all else
    being equal, the shorter the focal length of a lens, the deeper the
    depth of field of the image.

    But I see discussions of this issue which say it's not true - that
    crop sensor cameras have the same depth of field as full frame.
    Then it turns out that what they really means is they have the same
    depth of field if they're using the SAME lens. Well duh.

    And then I see something like this from Scott Bourne's Photofocus
    site that makes it look as though the sensor size determines depth
    of field:

    "The larger the sensor, the shallower the depth of field for a given
    subject size. Small P&S cameras have large Depth of Field (DOF.)
    With a crop sensor, you have larger DOF. This means it’s harder to
    get a good bokeh and harder to control the backgrounds."

    Well, having thought about all of this, I've just about concluded I
    was right in the first place. The sensor size per se doesn't
    determine the depth of field, but the focal length of the lens does
    (all else like aperture and distance being held constant).

    But I want to do a thought experiment, and get the experts here to
    tell me if I'm right.

    There are two cameras. The first is a full-frame DSLR with a fixed
    50 mm lens. It's focused on a "test pattern" subject 10 feet away.

    The second camera is a 1.6 crop sensor camera, focused on the same
    subject at the same distance. The field of VIEW of photos from the
    two cameras is the same. I'm not sure of the math, but I think this
    means the actual focal length of the lens on the second camera is

    If both lenses are opened up to the same maximum aperture, I believe
    the depth of field from the full-frame camera will be significantly
    shallower than from the crop-sensor camera. Is that right?

    I understand that a number of crop sensor cameras use standard
    full-frame lenses, but not all of them do, and I just want to be
    sure I understand what I'm giving up in cases where both the body
    and the lens are reduced in size proportionately.

    And if there's a link to something that explains this in detail,
    including the optics involved, that would be great.

    Peabody, Aug 13, 2009
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  2. Peabody

    Miles Bader Guest

    I'm no expert, but I can look at wikipedia...

    So take the below with a grain of salt, but my impression is that:

    1) The DOF varies linearly with the sensor size (keeping the lens
    focal length and aperture constant), simply because a larger sensor
    is magnified less to create the final image, meaning that a "blur
    spot" can be proportionally larger before being considered out of

    2) It also seems that DOF varies roughly with the inverse-_square_ of
    the lens focal length (keeping the aperture constant).

    To keep the angle-of-view constant, when changing the sensor size, you
    linearly vary the lens focal length according to the change in sensor
    size. When doing this, the two above factors will oppose each other,
    but the second one will "win", since it's a square factor, whereas the
    first is linear.

    Thus, keeping the angle-of-view constant, while changing the sensor
    size, the DOF will vary inverse-linearly with the sensor size.
    Yes; using the above rule-of-thumb, the DOF in the 1.6-crop image will
    be 1.6 times the DOF in the full-frame image. To achieve the same DOF,
    you'd need to use an f-number 1.6 as big -- e.g., instead of f2.8, you'd
    need to use f1.7.
    I don't think it really matters; the only real difference is that
    "non-full-frame capable" lenses don't have as large a usable image
    field. The behavior in the usable portion should be the same as a
    full-frame lens.

    Miles Bader, Aug 13, 2009
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  3. Peabody

    me Guest
    me, Aug 13, 2009
  4. Peabody

    PatM Guest

    Increasing focal length may decrease DOF, but if you move back and
    reframe in exactly the same manner, then moving back increased your
    DOF and in the end you'll get the same DOF for same given aperature.

    This is something that DSLR
    PatM, Aug 13, 2009
  5. Peabody

    Don Stauffer Guest

    For similar fields of view, indeed focal length scales with format size.
    So smaller cameras do have a hyperfocal distance that kind of scales
    with format size. The result is that since the photographer doesn't
    scale down, he sees a nearer hyperfocal distance and finds it harder to
    create a thinner area of good focus.
    Don Stauffer, Aug 13, 2009
  6. Peabody

    Cripes Guest

    What planet in what universe with new laws of physics are you on,
    pretend-photographer troll?
    Cripes, Aug 13, 2009
  7. Peabody

    PatM Guest

    Say you frame a person in your viewfinder. You are at 50mm f/4. You
    have a given DOF. If you zoom to 100mm, keep f/4, and move back to
    get the same framing; then you'll have the same DOF. Try it sometime.

    That is why it seems like big lenses have small DOFs compared to
    smaller lenses. If you are focusing on something from 5' away, DOF of
    1' seems pretty good. But if you have a 300mm lens and are using the
    same framing, that DOF of 1' seems pretty darned small.

    Meanwhile, this is the last post I will make on the subject because I
    do not intend to get into a flaming war with you, which I see from the
    tone of your post is what you are trying to start. Sorry, but I'm not
    taking your bait. Rather, I'll go grab my camera and take pictures.
    You should try it sometime. Taking pictures is much more fun than
    PatM, Aug 13, 2009
  8. Peabody

    Dudley Hanks Guest

    While I tend to agree with a lot of what you postulate, your statement that
    you can change DOF by changing focal length without losing aperture just
    doesn't jive that well with reality.

    The very fact that you've changed focal length entails changing the
    aperture -- especially with smaller sensored cams using variable aperture
    tele lenses.

    If you're small sensor has a lens rated at, let's say, f/2.8 - 5.6, with a
    (35mm equiv) focal length of 28mm to 130mm, then simply zooming from 28 to
    max lenth automatically drops your aperture size down to f/5.6 so you're
    going to have to compensate with a change in ISO or shutter speed.

    You'd need a rather expensive constant apeture lens (usually only found on
    higher end DSLR's) to accomplish your experiment. And, of course, if you
    had such a rig, you more than likely wouldn't be overly worried about
    limited DOF problems in the first place...

    Take Care,
    Dudley Hanks, Aug 13, 2009
  9. Peabody

    Jerry Tolls Guest

    One of my P&S cameras is f/2.0 at the 36mm wide end and f/2.4 at the 324mm
    zoom end. Another has f/2.7 on the 36mm wide end and f/3.5 on the 432mm
    zoom end. Just two examples from those I own. In both that is plenty of
    focal-length change to select and implement any DOF that I desire and
    require. Where on earth do you get your "let's say" figures of f/2.8-f/5.6
    for a mere 28mm-130mm (4.5x) zoom range on any P&S camera? Are you
    selecting your chosen values based on the severe limitations in one of your
    own DSLR pieces of glass? DSLR glass is horrendously poor when it comes to
    holding aperture from wide to full zoom. It's a size, weight, and cost
    constraint that they'll never get around.

    Must you DSLR-Trolls always try to fabricate wild imaginings to support why
    you wasted that much money on the body and the expensive accessory glass to
    make it at least as functional in image quality as a good P&S camera?

    In both of the above mentioned P&S cameras this is less than a 2/3rds EV
    change going from wide to full zoom. Well within the realm of an acceptable
    exposure margin of error, even if using locked-in manual exposure settings
    for any image. Hardly a cause for hysterically running for the hills to go
    buy a DSLR body plus the overpriced, always have to keep changing them,
    hunks of glass to try to get equivalent images from it. DSLRs frequently
    judge the wrong exposure with even more EV offset than that. 1, 2, and even
    3 full EV stops wrong exposure frequently common in DSLRs. Look at your own
    photos posted in the past as perfect living proof exposure errors as bad.
    We've all seen them so you can't lie about it.
    Jerry Tolls, Aug 14, 2009
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