aperture.

Discussion in 'Beyond the Basics' started by duelinthedeep, Feb 25, 2006.

  1. duelinthedeep

    duelinthedeep TPF Noob!

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    Is there a way to know how many feet a certain aperture gives you?
    for example- f/10= 25ft. i have been taking pictures of kids playing and running and its quite hard when they're a certain amount of feet in front of you and you want a shallow background but them in focus.


    thanks in advance:thumbup:
     
  2. santino

    santino TPF Noob!

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    depends on focal length. take a look at your DOF scale on your lens.
     
  3. duelinthedeep

    duelinthedeep TPF Noob!

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    dont have one on my lens....
     
  4. photograph-ny

    photograph-ny TPF Noob!

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  5. Unimaxium

    Unimaxium TPF Noob!

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    The aperture is the not only thing that controls the DOF. The lens focal length and the distance from camera to subject also affect how large your DOF is. Longer lenses cause smaller DOF, and longer camera-to-subject distances cause larger DOF. Thus, it's not possible to tell how many feet you'll get at a given aperture.
     
  6. duelinthedeep

    duelinthedeep TPF Noob!

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    damn!.....

    can't i just use the DOF preview?
    i think it'll be a bit hard trying to press the button and having those kids run somewhere else..
     
  7. Unimaxium

    Unimaxium TPF Noob!

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    Sure. Just keep in mind that things will looks a bit sharper in your viewfinder than they will when they are enlarged, so your DOF will be a bit shallower than it looks. You're right that it would be a pain trying to hold down the DOF preview button while trying to take the shot, but it doesn't sound any harder than trying to calculate how much DOF you have before each shot.

    I think the best way to take a shot of these kids and have a nice blurred background would be to use a long telephoto lens and shoot them from a moderate distance. If you're shooting wide-angle you're pretty unlikely to get a very blurry background. If you don't have a long lens, then just try shooting at the widest aperture you can.
     
  8. Torus34

    Torus34 No longer a newbie, moving up!

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    Let's get down to the practical end of things. I'll assume that you're using a film camera. For digital, all bets are off, as it were.

    You say that you're photographing moving children. This means that all your attention is [or should be] devoted to focussing and framing. All you can do is set your shutter speed so that blur from motion will not be objectionable and then set the aperture to provide the correct exposure for the available light and film speed [ASA].

    You can decrease the sharpness of the background by getting closer to the subject. This will mean that you focus the lens to a shorter distance. It will automatically result in more blur to the background.

    Finally, if the combination of shutter speed and lens opening results in an f stop more than two numbers above the wide-open stop of your lens, consider going to a slower ASA film [eg, from ASA 400 to ASA 200 or 100]. This will force you to open the lens and this, in turn, will help to blur the background.

    Sounds like a lot of stuff to think about right now, but with time, this sort of mental computation should become second nature. The trick with a camera is to learn it so thoroughly that you don't have to stop to think everything through. A camera is a tool, after all -- like a painter's brush or a novelist's typewriter. When you pick up a hammer to hit a nail, you don't stop to consider how to grip the handle.
     
  9. ksmattfish

    ksmattfish Now 100% DC - not as cool as I once was, but still

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  10. duelinthedeep

    duelinthedeep TPF Noob!

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    aperture is beating my brain to a pulp:banghead:
     
  11. Rob

    Rob TPF Noob!

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    Sometimes it is easier to do things the other way around. If you work mostly with a single lens, set your camera in AP mode and work within the normal boundaries and take some shots at different lengths. Although one size does not fit all, I find that in AP mode, I can, for the lens I'm using, guess it right 9/10.

    For example, I always use about f4 with a 50mm with the subject about 8 feet away, and this gives me the correct DOF I'm looking for in a portrait. With my 100-400, f8 is right at most lengths, with the subject about 15 feet away. You only need to get used to your lenses and what's in the background. There isn't time for the science in the field, especially with pesky running kids!! You need to already know the aperture of choice, so experiment and learn the results - you've figured out that it's too shallow/narrow, so make a chart of the ones which worked and the ones which didn't and figure the range out.

    Rob
     
  12. duelinthedeep

    duelinthedeep TPF Noob!

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    sounds like a good idea:cheers:
     

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