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Thread: DOF and f number

  1. #1

    Default DOF and f number

    can a F3.5 camera give a shallow DOF say, 35mm with the object being at 1-2 meter away?

    my friend says that his digicam with F2.8 cant give shallow effect, so i need to go for faster lens.

    isnt this incorrect? thanks

  2. #2
    Join Date
    Jan 2002
    Not Here Not There


    Hi Bordeaux78,

    The depth-of-field is not solely determined by the aperture width (F stop) but also by the Focal Length. I believe your friend digicam has a very short focal length, that's why he/she cannot get a shallow DOF effect at F2.8.

  3. #3


    To add to that, I believe that digicams have a much larger DOF than film cameras, even at the same f-stop and focal length.

  4. #4


    3 factors for DOF
    1) focal length - longer length, lesser depth
    2) Aperture - smaller aperture, more depth
    3) object distance - the nearer, the lesser depth..

    there is a formula somewhere...

  5. #5


    Surf over to this URL to get the answer to your qns. BTW, the bit about the 5 stop rule isn't too 'objective' IMO, so take with a pinch of salt.

    The formula is located at the bottom of the page.
    BTW, there are more than 3 factors that affect DOF.

  6. #6


    Originally posted by Lennier

    The formula is located at the bottom of the page.
    BTW, there are more than 3 factors that affect DOF.
    the formular is as complicated as A maths.

  7. #7


    Like what skf said, those are the 3 rules to remember. In addition, most digital P&S will never get good background blur. For example, the G1 has a focal length of 7-21 mm (equivalent to 35-105 in 35mm). Even at f/2.8 at the longest, your background will never be nicely blurred out.


  8. #8
    Moderator ziploc's Avatar
    Join Date
    Jan 2002


    I've read somewhere that

    Actual aperture size = focal length / f-stop value

    hence the notation f/2.8, f/5.6 etc. Comparing a lens with, say 15mm and a lens with 150mm, with the same f-stop value, the 15mm will have a much smaller actual aperture opening which would result in deeper DOF.

    In the case of digital camera, eg. the G1, since the lens is 7-21mm, at 21mm (105mm equivalent at 35mm) its aperture opening is still much smaller then its 35mm equivalent at the same f-stop, resulting in the deeper DOF.

  9. #9

  10. #10


    wow thanks a lot for the info guys

    so a 50mm prime lens can achieve shorter DOF than a 35mm general..rite? also a faster lens will give shorter DOF at its max aperture as compare to a slower one at the same focal length....rite?




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