Depth of Field Discussion
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Thread: Depth of Field Discussion

  1. #1
    WTF Veteran ama11441's Avatar


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    Nice calculator for DOF!

    http://www.dofmaster.com/dofjs.html
    There are many others like it but this one is mine. John

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    WTF Veteran Wolfdog's Avatar


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    You probably don’t need a wider lens, just a better understanding of “depth of field”, as long as the lens you have has a decent macro and has an f-stop of f16 or greater. Close the aperture or set to a higher f-stop to achieve greater DOF. I’ve taught photography and did much of my own developing. If you get confused, think of it this way. Say you're in a dark room and having trouble focusing. In those conditions you tend to "squint" to get a clearer picture, don't you? The camera is the same way, and shutting "down" [squinting] or closing the aperture thus letting in "less" light with give you a longer focal range or "depth of field".

    Try thinking of it this way; greater/higher f-stop number = greater/higher DOF! That is helpful when you want to keep in focus your subject, the foreground and the background. Most quality SLR lenses have DOF markers right on them, giving you an actual distance or “depth” a given aperture will remain in focus. Thus, f16 [very small aperture opening, “but” greater f-stop equates to greater depth of field] will give you greater depth. By letting less light in, and as such needing to compensate by slowing down the shutter and allowing more time for the exposure, the subject is in focus as well as foreground and background.

    A lens with an aperture set @ f2.8, for example, will be wide open [just like your eyes’ pupils if they were wide open] or “lower” f-stop for “lower” light and a “lower” DOF. Therefore with an f-stop of f2.8 you’ll have less DOF, or only the subject focused upon will come out clearly, and is helpful when wanting to compose shots where the foreground or background can be a distraction.

    The human eye sees in the equivalent to a 55mm lens. A good quality 55mm normally comes with @ f2.8-f16, and better ones down to as far as f1.4 for low-light work. So anything greater than 55mm is considered a telephoto and anything less more a wide angle. Generally, anything less than a 28mm is extreme wide angle and gives you a fish-eye effect.

    As for ISO, the higher you go the grainier the shots will come out. Just think greater ISO, greater light gathering, greater graininess. Higher ISO is good for lower light, but the shortcoming is less clarity. Sticking with a digital setting of ISO 200 or less will give the clearest results, and will need more exposure time and probably a tripod in low light. Anything over 400 ISO tends to get very grainy, as seen in many of my close-ups using my digital pocket camera on this forum, as I tend to use ISO 800 or greater due to lack of proper lighting.

    Dang, I hope this helps. It’s been awhile since I’ve taught photography and almost as long since doing anything but “point-and-click” digital stuff.

    BTW, I found and borrowed a pic of an Olympus OM-1 lens, like the type I use when doing 35mm photography.

    Look at the lens in to following picture. See the "infinity" symbol? Just under it is a red dot with an upward line. Now look to either side of that dot and you'll see f-stops from 5.6, 11, and up to f22. See those bent white lines radiating inward from those numbers? That's the DOF at a given f-stop. For example, say you're focused on a subject that's 4 feet away and imagine the orange number 4 [for 4 feet] is right above the red dot. The red dot represents f2.8, the lowest f-stop this lens is capable of. Still imagining the orange 4 over the red dot, look to each side and you'll see 3 and 6 feet in orange. Now look at the white numbers 22 under that, with the inward bent lines. You can imagine that when focused on 4 feet, and at f22, those white lines reach from the orange 3 and 6, so at f22 you'll have a focal length from 3 to 6 feet when focused at 4 feet. I hope this makes sense, and since I don't see what type of lens you're using and the markings if any it's harder to explain. I hope this gives you a better idea.

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    f stops are rounded up and down using the 1.4 multiplier.

    1 is wide open.

    1.0
    1.0 x 1.4 = 1.4
    1.4 x 1.4 = 2.0
    2.0 x 1.4 = 2.8
    2.8 x 1.4 = 4.0
    4.0 x 1.4 = 5.6
    5.6 x 1.4 = 8.0
    8.0 x 1.4 = 11
    11 x 1.4 = 16
    16 x 1.4 = 22

    you get the picture.

    opening the f stop one f stop, allows twice as much light to the prior f stop.
    closing down the f stop, one f stop, reduces light by half of the prior f stop.

    when discussing f stop (aperture), it has a linear relationship relative to exposure value. open the f stop one stop, with shutter speed and iso constant, then you have an increase 1 ev to the exposure value.

    increase iso from 100 to 200, with f stop constant and shutter speed constant, then you have an increase of 1 ev to the exposure value.

    decrease the shutter speed from 125th to 60th, with f stop constant and the iso constant, then you have and increase of 1 ev to the exposure value.

    so you can have an exposure value of 5 with your camera set to multiple configurations.

    the aperture also has an inverse square law relationship to light.

    if your settings correspond with your light source with your subject at a fixed distance, doubling or halving the distance will not equate to making linear adjustments like on the ev chart. your subject is exponentially brighter or darker with a linear increase or decrease in distance.

    two scenerios where the inverse square law has neglible affect is when shooting macro or when using the sun as your light source.

    i think i confused myself : )

    cl

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