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    Depth of Field for Digital Cameras - A Brief Primer
    by magnetic9999


    starring ...
    Erni, the blue Scandinavian egghead ..
    and dummy-X,
    wooden hip hop guru


    If you already know what depth of field is and how to control it, the important point to get from this primer is that a digital camera has more depth of field at the same f/stop than a 35 mm camera. It is about a 5 stop difference for most cameras, and as low as a 4 stop difference for cameras with larger sensors, like the Olympus E-10/E-20.

    IF YOU'RE NOT 100% SURE about what Depth of Field is, read on ...

    Everybody has probably seen photographs in which every element from foreground to background is in sharp focus, and other pictures in which only the subject is in sharp focus while everything else is blurry. The first picture is said to have more depth of field than the latter, which has shallow depth of field. For those that don't know, depth of field, or DOF, is how deep the area in focus is, when you focus on a given subject. It's a very powerful artistic tool.

    You can use your depth of field to make something in the foreground in focus, and blur out the background, thereby bringing more attention to your subject. Or you can open your depth of field way up and put your significant other and that mountain 10 miles behind her/him all into sharp, startling, crisp focus.

    Look at the picture of dummy-X above: he's in focus, but the ceiling and background behind him are not. That means the depth of field (DOF) is narrow and it's obviously centered on him.

    With Erni, though, on his pedestal, his background is almost as in-focus as he is. The DOF is a bit deeper or wider.

    A really wide DOF might mean that grandma standing 5 feet away from you is in focus, and so also is the tree 50 feet behind her. But a narrow DOF might mean that even though you've focused sharply on the tip of grandma's nose, her eyes, only an inch behind, are slightly out of focus.

    How does one control this? By changing your aperature. In general, the bigger the aperature, the narrower the depth of field. (Don't know what an aperature is? Click here). The opposite is also true: the smaller your camera aperature, the wider or deeper your DOF.

    Easy enough. Now here's the confusing part for a lot of people. A smaller number is a bigger aperature. Got that? An aperature of 2.8 is a BIGGER hole/opening than an aperature of 16. Why? Because it's really a fraction and they've left out the numerator. Just like 1/2 is bigger than 1/16. Same thing. Still with us?

    Now back to digital cameras. Since they use a smaller area to capture the image than the 35mm film rectangle, they give more DOF at the same aperature. Because remember, the smaller the opening, the deeper the DOF. In other words, if you take a 35mm camera and set the same f/stop as your digital, point them both at the same thing, more will be in focus from front to back in the scene with the digicam. As a rule of thumb, the DOF on a digicam is the same as the DOF on a 35mm set 5 stops smaller.

    What does that mean? Well, take a look at your aperatures on the camera. If you've ever noticed, they seem to go in certain consistent steps: 22, 16, 11, 8, 5.6, 4, 2.8, 2.0. Each one of those steps is an f/stop, and it basically means, going from left to right, twice as much light is being let in at each step. In other words, f/16 lets in twice as much light as f/22. Better cameras give you more in-between f/stop settings.

    So if you set your digicam aperature to 2.0, and count 5 steps higher (i.e. to the left), you'll see the equivalent aperature on a 35 mm camera is 11. If you have a 35mm camera you can look at the DOF marks on the lens and you'll know exactly how much depth of field, in both meters and feet, you would then have.


    What happens if you set your digicam aperature to f/16?  Start counting steps to the left and you run out right away. Basically this means that setting your digicam to f/16 though gives you a much larger, deeper DOF than you could ever get with a 35 mm camera. This can be great for getting lots of stuff in focus. However, the downside is if you want really shallow, or narrow DOF, it's a lot harder, because it's always deeper, not narrower, thanks to those small digicam sensors.

    The best thing to do to really grasp this is set your camera up pointing at something, and take a bunch of shots of that same thing, changing the aperature each time, and watch how your picture changes. Best way to do this is use aperature-priority mode (A mode), if your camera has it.  That way, the camera will automatically change the shutter speed to compensate for the changes in amount of light you're letting in, and all YOU have to worry about is the aperature and the depth of field.

    Good luck!

    - Kollin B., magnetic9999

       
    Examples
    (click to enlarge)

    At this wide aperature (f/2.8), Erni can be seen sharply, running from dummy-X, who's very blurry.

    At this intermediate setting (f/5.6), dummy-X is a lot sharper, yet his shadow and the blinds are still fuzzy.

    Here's a very narrow aperature (f/11). Now everything is in focus, including the blinds upon which the dummy's (nothing personal) shadow is cast.




    More Examples:


    Narrow DOF: focused on the outer edge of the candle.  Wick is fuzzy.
           
    Narrow DOF: focused on just the wick, outer tim is now fuzzy.

    Ultranarrow DOF shows as stripe of in-focus text
    (easier to see on blow-up).
           
    Ultranarrow DOF keeps only front of rings in focus.



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