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Thread: Understanding DOF vs. format size

  1. #1

    Question Understanding DOF vs. format size

    I came across this explanation in Wiki. I was wondering how come different format cameras can achieve different DOF at the same focal length & Aperture. Frankly, I still cannot fully understand the relation between subject distance, lens focal length and Aperture.

    At times, my shots will appear OK (once I get the correct focal length with f2.8) but at certain times, no matter how I shift, the focus is always at the wrong place. I hope somebody can explain to me what the article below is about....

    To a first approximation, DOF is inversely proportional to format size. More precisely, if photographs with the same final-image size are taken in two different camera formats at the same subject distance with the same field of view and f-number, the DOF is, to a first approximation, inversely proportional to the format size. Strictly speaking, this is true only when the subject distance is large in comparison with the focal length and small in comparison with the hyperfocal distance, for both formats, but it nonetheless is generally useful for comparing results obtained from different formats

    To maintain the same field of view, the lens focal lengths must be in proportion to the format sizes. Assuming, for purposes of comparison, that the 45 format is four times the size of 35 mm format, if a 45 camera used a 300 mm lens, a 35 mm camera would need a 75 mm lens for the same field of view. For the same f-number, the image made with the 35 mm camera would have four times the DOF of the image made with the 45 camera.

    In many cases, the DOF is fixed by the requirements of the desired image. For a given DOF and field of view, the required f-number is proportional to the format size. For example, if a 35 mm camera required f/11, a 45 camera would require f/45 to give the same DOF. For the same ISO speed, the exposure time on the 45 would be sixteen times as long; if the 35 camera required 1/250 second, the 45 camera would require 1/15 second. In windy conditions, the exposure time with the larger camera might allow motion blur. Adjusting the f-number to the camera format is equivalent to maintaining the same absolute aperture diameter.

    The greater DOF with the smaller format can be either an advantage or a disadvantage, depending on the desired effect. For the same amount of foreground and background blur, a small-format camera requires a smaller f-number and allows a shorter exposure time than a large-format camera; however, many point-and-shoot digital cameras cannot provide a very shallow DOF. For example, a point-and-shoot digital camera with a 1/1.8″ sensor (7.18 mm 5.32 mm) at a normal focal length and f/2.8 has the same DOF as a 35 mm camera with a normal lens at f/13.

    In some cases, camera movements (tilt or swing) can be used to better fit the DOF to the scene, and achieve the required sharpness at a smaller f-number.

  2. #2
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    Default Re: Understanding DOF vs. format size

    Simply put it this way. A larger sensor yields a shallower DOF as compared to a smaller sensor using the same aperture.
    Canon EOS 5D, 24-70 f/4 L IS, 50 f/1.2 L, 70-300 f/4-5.6 L IS, 600EX-RT. Sigma 12-24 f/4.5-5.6 EX.

  3. #3

    Default Re: Understanding DOF vs. format size

    i dunno how to explain the science of why the DOF varies with format...but in short...DOF will vary with these variables:

    - Focal length (longer = shallower)
    - Aperture (wider = shallower)
    - Film plane / sensor size (bigger = shallower)

    that's why its more difficult to shoot with medium/large format as well, unless you stop down....and also the reason why large format lenses have seemingly ridiculous tiny apertures to maintain DOF for large scenes.

    I guess maybe the film plane affects DOF, because of the image circle size projected by the appropriate format's lens. maybe...any experts here?

  4. #4

    Default Re: Understanding DOF vs. format size

    Quote Originally Posted by Snoweagle View Post
    Simply put it this way. A larger sensor yields a shallower DOF as compared to a smaller sensor using the same aperture.
    , so in other words, P&S cameras with tiny sensor will have deeper DOF given the same aperture......but of course with loads of noise too.

    So ppl with large sensors (e.g. DSLR vs P&S) will need to 'move' closer to the target in focus either physically or zooming in with the same aperture to achieve deeper DOF right?

    How do you guys get the correct focal distance? I have problem judging the distance to get the DOF I want especially outdoor shoots and my viewfinder isn't helping anyway in telling me the DOF.

  5. #5

    Default Re: Understanding DOF vs. format size

    try the DOF preview button if you have one

  6. #6
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    Default Re: Understanding DOF vs. format size

    Quote Originally Posted by windwaver View Post
    , so in other words, P&S cameras with tiny sensor will have deeper DOF given the same aperture......but of course with loads of noise too.

    So ppl with large sensors (e.g. DSLR vs P&S) will need to 'move' closer to the target in focus either physically or zooming in with the same aperture to achieve deeper DOF right?

    How do you guys get the correct focal distance? I have problem judging the distance to get the DOF I want especially outdoor shoots and my viewfinder isn't helping anyway in telling me the DOF.
    Go back to the elementary rules of DOF. f/8 or f/11 outdoors for maximum DOF, and try to go with as short a focal length as possible.

  7. #7

    Default Re: Understanding DOF vs. format size

    Quote Originally Posted by calebk View Post
    Go back to the elementary rules of DOF. f/8 or f/11 outdoors for maximum DOF, and try to go with as short a focal length as possible.
    , I though to get less DOF, your aperture must be smaller (my max is f2.8)? I'm not shooting landscape. My question is more like how to judge the correct focal length in order to blur out the background with a fix aperture (e.g. f2.8).

  8. #8

    Default Re: Understanding DOF vs. format size

    Quote Originally Posted by windwaver View Post
    , I though to get less DOF, your aperture must be smaller (my max is f2.8)? I'm not shooting landscape. My question is more like how to judge the correct focal length in order to blur out the background with a fix aperture (e.g. f2.8).
    Usually you switch the aperture rather then the focal length. Any reason why you want to do something like that?

    Oh, distance from subject is another way to reduce or increase DOF. Thus, the three variables of DOF are focal length, aperture size and distance from subject.

    Samuel
    f/8 and be there.

  9. #9
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    Default Re: Understanding DOF vs. format size

    Quote Originally Posted by windwaver View Post
    , so in other words, P&S cameras with tiny sensor will have deeper DOF given the same aperture......but of course with loads of noise too.

    So ppl with large sensors (e.g. DSLR vs P&S) will need to 'move' closer to the target in focus either physically or zooming in with the same aperture to achieve deeper DOF right?

    How do you guys get the correct focal distance? I have problem judging the distance to get the DOF I want especially outdoor shoots and my viewfinder isn't helping anyway in telling me the DOF.
    Distance between u and your subject and also between the subject and the background will also determine how much DOF as well. U cannot compare a P&S cam with a DSLR in terms of DOF, else everyone would get a P&S and save more money if comparing to the equivalent.
    Canon EOS 5D, 24-70 f/4 L IS, 50 f/1.2 L, 70-300 f/4-5.6 L IS, 600EX-RT. Sigma 12-24 f/4.5-5.6 EX.

  10. #10

    Default Re: Understanding DOF vs. format size

    How does one match the FOV of 35mm with 4x5? Different aspect ratio what...must crop crop...

  11. #11

    Default Re: Understanding DOF vs. format size

    Quote Originally Posted by alternatve View Post
    Usually you switch the aperture rather then the focal length. Any reason why you want to do something like that?

    Oh, distance from subject is another way to reduce or increase DOF. Thus, the three variables of DOF are focal length, aperture size and distance from subject.

    Samuel
    Well, cos I'm widest at f2.8, thus the only way for me to get the correct DOF is to shift the focal length/distance from subject.

  12. #12

    Default Re: Understanding DOF vs. format size

    Quote Originally Posted by windwaver View Post
    Well, cos I'm widest at f2.8, thus the only way for me to get the correct DOF is to shift the focal length/distance from subject.
    But having said this, my question again is how you guys judge the correct distance to determine how much background you can blur? It's like a try & error for me most of the time during outdoor shoots (my viewfinder doesn't help much).

  13. #13
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    Default Re: Understanding DOF vs. format size

    Quote Originally Posted by windwaver View Post
    But having said this, my question again is how you guys judge the correct distance to determine how much background you can blur? It's like a try & error for me most of the time during outdoor shoots (my viewfinder doesn't help much).
    Er normally we frame for the shot with the correct focal length, then use the appropriate largest aperture possible.

  14. #14

    Default Re: Understanding DOF vs. format size

    Quote Originally Posted by windwaver View Post
    Well, cos I'm widest at f2.8, thus the only way for me to get the correct DOF is to shift the focal length/distance from subject.
    Dude.....

    What's the aperture control on the lens for? It isn't just a nicely painted set of number or some digits on the LCD panel.... Change the aperture and you change the DOF as well. In fact, the aperture is the most direct way of changing DOF... Try it on for size.

    Samuel
    f/8 and be there.

  15. #15

    Default Re: Understanding DOF vs. format size

    Quote Originally Posted by windwaver View Post
    Well, cos I'm widest at f2.8, thus the only way for me to get the correct DOF is to shift the focal length/distance from subject.
    Dude....

    There IS an aperture control dial on your camera! Aperture is one of the most direct way of changing DOF. Why you cannot see it is because you ARE viewing and composing wide open so that you have a clearer viewing screen. Depress the DOF button to see the effects of the aperture you selected...

    Samuel
    f/8 and be there.

  16. #16

    Default Re: Understanding DOF vs. format size

    actually, i realise u are confusing yourself with the different subjects involved. stick to aperture and focal length. ignore format size for the moment since a comparison with larger formats are irrelevant to what u really need to know. trying to know why larger formats affects the dof will only confuse u more.

    i would suggest stopping down if most of your shots end up getting the focus on the wrong place after recomposing or use a tripod. im ignoring the possibility u have a lens that could be front or back focusing.

  17. #17

    Default Re: Understanding DOF vs. format size

    Quote Originally Posted by alternatve View Post
    Dude....

    There IS an aperture control dial on your camera! Aperture is one of the most direct way of changing DOF. Why you cannot see it is because you ARE viewing and composing wide open so that you have a clearer viewing screen. Depress the DOF button to see the effects of the aperture you selected...

    Samuel
    Hey buddy, I've already mentioned f2.8 is my widest and my viewfinder doesn't help (there's no DOF button) when I half-press the shutter-release button.

  18. #18

    Default Re: Understanding DOF vs. format size

    Quote Originally Posted by flipfreak View Post
    actually, i realise u are confusing yourself with the different subjects involved. stick to aperture and focal length. ignore format size for the moment since a comparison with larger formats are irrelevant to what u really need to know. trying to know why larger formats affects the dof will only confuse u more.

    i would suggest stopping down if most of your shots end up getting the focus on the wrong place after recomposing or use a tripod. im ignoring the possibility u have a lens that could be front or back focusing.
    I understand where you're coming from buddy. What I need to know is how you guys get the correct DOF without the DOF preview button?

  19. #19

    Default Re: Understanding DOF vs. format size

    Hi,
    Quote Originally Posted by windwaver View Post
    Hey buddy, I've already mentioned f2.8 is my widest and my viewfinder doesn't help (there's no DOF button) when I half-press the shutter-release button.
    Then don't start with f2.8 lah... start with f4 instead. Take a shot and check the depth of field of the image by viewing the image... you using a digital camera, right??

    If you want more depth of field, do the following:
    1) stop down (larger f number) or
    2) walk further away or
    3) zoom out

    If you want less depth of view, do the following:
    1) open up or
    2) walk closer or
    3) zoom in

    Have a nice day.
    Last edited by weixing; 24th June 2008 at 12:21 PM.

  20. #20
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    Default Re: Understanding DOF vs. format size

    Quote Originally Posted by windwaver View Post
    Hey buddy, I've already mentioned f2.8 is my widest and my viewfinder doesn't help (there's no DOF button) when I half-press the shutter-release button.
    What camera are you on? I think you are not getting what everyone is saying here, to be honest.

    Why are you putting all emphasis on aperture, then using your focal length to control your DOF? You should be focusing on getting your shot right first in terms of composition, then worry about your exposure settings after your shot is composed.

    You are shifting focal length to fit f/2.8. You should be shifting aperture to fit your shot size.

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