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Thread: depth of field problem at f4... seeking advise

  1. #1
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    Default depth of field problem at f4... seeking advise

    i am still learning the capabilities of my 70-200 F4 L canon lens

    i shot quite alot at a games day recently...

    apparently, at f4, the depth of field is pretty shallow.
    for eexample, when i take 2 people facing each other (with a ball between them)

    i am about 6-8 meters away from them.. so one person is 30 cm closer to me than his/her game partner... (ie one person is 8 meters away from camera, other is 7.7 m away) ya know what i mean?

    yet only one person is focused and the other is off focus....

    i thought that at f4, the depth of field isnt that shallow. =(

    any explainations?


    side .... i cant seem to post a picture...
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    Regards, Mark Pang
    www.markpangphotography.com

  2. #2
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    Let me try to recall.
    The DOF is controlled by 3 things:
    - the focal length (u shooting at 70mm or 200mm?)
    - the distance (u mention 7.7m fm 1st subject and 8m fm 2nd subject)
    - the aperture (u mention f4)

    If u shoot at 200mm, at f4 I think yr DOF would be quite shallow....

  3. #3

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    Quote Originally Posted by haagen_dazs
    i am still learning the capabilities of my 70-200 F4 L canon lens

    i shot quite alot at a games day recently...

    apparently, at f4, the depth of field is pretty shallow.
    for eexample, when i take 2 people facing each other (with a ball between them)

    i am about 6-8 meters away from them.. so one person is 30 cm closer to me than his/her game partner... (ie one person is 8 meters away from camera, other is 7.7 m away) ya know what i mean?

    yet only one person is focused and the other is off focus....

    i thought that at f4, the depth of field isnt that shallow. =(

    any explainations?


    side .... i cant seem to post a picture...
    my posting rules says
    You may post new threads
    You may post replies
    You may not post attachments
    You may edit your posts

    I take it as you are 6 meters away from them, since you cannot really be for sure, based on most conservative calcuation :

    The DOF = Far Focus Limit - Near Focus Limit

    To calculate the Focus Limit we need to calcuate the Hyperfocal distance : -

    H = (L x L) / (f x d)
    Where:
    H = Hyperfocal Distance (in mm)
    L = lens focal length (200 mm)
    f = lens aperture f-stop
    d = diameter of circle of least confusion (in mm) for 35mm format d = 0.03

    H= (200 x 200) / (4 x 0.03) = 333,333.333333 mm ( 333.3333 m)

    Then using the Hyperfocal Distance, we calculate the Far / Near Focus Limit :

    Near Focus Limit

    NF = (H x D) / (H + (D - L))
    Where:
    NF = Near Focus Limit (mm)
    H = Hyperfocal Distance (in mm, from above equation.)
    D = lens focus distance (in mm = 6000)
    L = lens focal length (200 mm)

    NF = (333333.33333x6000) / (333333.33333+(6000 200))
    = 1999999999.99 / 339133.333333
    = 5897.38mm
    = 5.9m

    Far Focus Limit
    FF = (H x D) / (H - (D - L))
    Where:
    FF = Near Focus Limit (mm)
    H = Hyperfocal Distance (in mm, from above equation.)
    D = lens focus distance (in mm = 6000)
    L = lens focal length (200 mm)

    FF = (333333.333 x 6000) / (333333.333 (6000 200))
    = 1999999999.99 / 327133.333333
    = 6113.715
    = 6.114m

    Thus the Depth of Field = 6.114m 5.9m = 0.214m = 21.4cm ...... shallow enough? ;P ..... thus the second person out of focus is correct .... hehehehehe

    (Interlude break between my assignment ..... now back to my stupid assignment .... )
    Last edited by blurblock; 11th October 2004 at 02:09 PM.

  4. #4
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    Wow, blurblock is the man!
    (I forgot all these and handed back to teacher years ago, u refresh all my memory....)

  5. #5

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    eh...i am not a math person...but if u are further away from them....the DOF will be shallow if u use f4....i have gotten pretty shallow DOF at f8 in these circumstances..

  6. #6

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    Not a math person either ... but he ask for explaination, so give him the ultimate explanation lor ..... LOL .....

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    Heh Heh, good formula to plug into my palmtop tomorrow! Any difference in CoC for 1.6x crop factor DSLR?

    Cheers,

  8. #8

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    Quote Originally Posted by dkw
    Heh Heh, good formula to plug into my palmtop tomorrow! Any difference in CoC for 1.6x crop factor DSLR?

    Cheers,
    yes there is a difference. If I remembered it correctly, due to the crop factor, the CoC will be required to have a 1.6x crop factor too, thus it would be 0.625 x 0.03 = 0.01875 but we often use 0.02 (1.5x Crop) as a mean CoC. It is easier that way ... anyway, the two difference will be about negligible
    Last edited by blurblock; 12th October 2004 at 10:08 AM.

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    Quote Originally Posted by Witness
    eh...i am not a math person...but if u are further away from them....the DOF will be shallow if u use f4....i have gotten pretty shallow DOF at f8 in these circumstances..
    Using the DOF calculator as blurblock has provided, if you keep the focal length and aperture constant, the further away you are from the subject, the deeper your DOF.

    Useful link here-->http://www.dofmaster.com/dofjs.html

    Cheers,

  10. #10

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    Quote Originally Posted by dkw
    Using the DOF calculator as blurblock has provided, if you keep the focal length and aperture constant, the further away you are from the subject, the deeper your DOF.

    Useful link here-->http://www.dofmaster.com/dofjs.html

    Cheers,
    Same as my Answer .... just mine due to manual calculation is slightly more precise by a few millimeters .... LOL.....

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