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Thread: Another DOF question

  1. #21
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    Default Re: Another DOF question

    Quote Originally Posted by mfbatzap View Post
    ohh okok thanks!!
    i'll play around wif a prime lens and try get the right settings hahaha..

    how about zoom lens? if I need to zoom say in a concert badly lighted hall w/o tripod, I will need a larger aperture to compensate for the faster shutter speed... then how do i adjust the DOF? or in this case die die must pump up the ISO, reduce shutter speed and step up the aperture?
    in those situation, you will be struggling for longer focal length and also faster shutter speed that you wun even mind shooting wide open.

  2. #22
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    Default Re: Another DOF question

    Quote Originally Posted by ziploc View Post
    Just one minor clarification though. The DOF will only be affected by focal length if the object size at the focal point changes in the image. This is due to the magnification/reduction effect and hence the CoC comes into play. If on the other hand the object size is maintained by changing the lens to object distance, then DOF will remain constant regardless of the focal length.
    How do you maintained object size by changing lens to object distance without changing lens' focal length?

    Regards,
    Arto.
    Last edited by Artosoft; 4th April 2007 at 11:32 AM.

  3. #23
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    Default Re: Another DOF question

    I thought he meant that when you change the focal length, you change the distance to the object as well, so that the resulting object size remains unchanged.

  4. #24

    Default Re: Another DOF question

    Aperture, focal length, subject distance, sensor/film size, final print size, viewing distances,... all these affects the DOF of the photo.

    I have shared this a few times previously and have shown an example for print size and viewing distance.

    Here is a good read:
    http://www.cambridgeincolour.com/tut...h-of-field.htm

    Here is a DOF calculator that takes into consideration of print size and view distances:
    http://www.cambridgeincolour.com/tut...calculator.htm

    BC
    Last edited by Scaglietti; 4th April 2007 at 11:44 AM.

  5. #25
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    Default Re: Another DOF question

    Quote Originally Posted by cantaresg View Post
    I thought he meant that when you change the focal length, you change the distance to the object as well, so that the resulting object size remains unchanged.
    Ok, that's explain the need of changing focal lens as well.

    Regards,
    Arto.

  6. #26
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    Default Re: Another DOF question

    Quote Originally Posted by cantaresg View Post
    I thought he meant that when you change the focal length, you change the distance to the object as well, so that the resulting object size remains unchanged.
    Thanks cantaresg for helping to explain.

    What I meant was this:

    1. If the focal length is changed (e.g. by zooming in or out on a zoom lens), and the aperture and camera to object distance are unchanged, the DOF will be affected.
    2. If on the other hand the focal length is changed while the aperture is unchanged, and we move either front or back (hence chaning the camera to object distance) to maintain the same object size on the image, then the DOF will be unaffected.

  7. #27

    Default Re: Another DOF question

    Quote Originally Posted by ziploc View Post
    Thanks cantaresg for helping to explain.

    What I meant was this:

    ...
    2. If on the other hand the focal length is changed while the aperture is unchanged, and we move either front or back (hence chaning the camera to object distance) to maintain the same object size on the image, then the DOF will be unaffected.
    This is not true. If you change the subject distance, the perspective changes and the DOF changes even if you keep the subject size constant.

    BC
    Last edited by Scaglietti; 4th April 2007 at 05:57 PM.

  8. #28
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    Default Re: Another DOF question

    Quote Originally Posted by ziploc View Post
    2. If on the other hand the focal length is changed while the aperture is unchanged, and we move either front or back (hence changing the camera to object distance) to maintain the same object size on the image, then the DOF will be unaffected.
    Ideally yes.

    But in reality, no.

    Regards,
    Arto.

  9. #29
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    Default Re: Another DOF question

    Well, don't dismiss it so fast yet.

    Here are some pictures on luminous-landscape that illustrate this:
    http://www.luminous-landscape.com/tutorials/dof2.shtml

    And here is an article that discuss it, together with a DOF calculator that you can try it out:
    http://www.cambridgeincolour.com/tut...h-of-field.htm

    Cheers.

  10. #30

    Default Re: Another DOF question

    Quote Originally Posted by ziploc View Post
    Well, don't dismiss it so fast yet.

    Here are some pictures on luminous-landscape that illustrate this:
    http://www.luminous-landscape.com/tutorials/dof2.shtml

    And here is an article that discuss it, together with a DOF calculator that you can try it out:
    http://www.cambridgeincolour.com/tut...h-of-field.htm

    Cheers.
    This is one article I do not fully agree. He has introduce another factor here... ENLARGEMENT. Remember print size is part of the equation in calculating DOF.

    If keeping the print size the same, you will get different DOF. Using the DOF calculator you have linked, try this. (1.5x crop sensor size)

    100mm f/8 @ 10m ---> DOF = 3.594m
    50mm f/8 @ 5m ----> DOF = 3.966m
    24mm f/8 @ 2.4m ----> DOF = 7.369m
    17mm f/8 @ 1.7m ----> DOF = infinite (within hyperfocus distance)

    BC
    Last edited by Scaglietti; 4th April 2007 at 06:23 PM.

  11. #31
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    Default Re: Another DOF question

    Ok, I stand to be corrected. Don Fleming of DOFMaster had plotted DOF comparison charts for lenses of different focal length, and shown that the DOF is constant only under the circumstances that "the focus distance for the shortest lens is less than about 1/4 of the hyperfocal distance for that lens".

    http://www.dofmaster.com/dof_imagesize.html

    Cheers.

  12. #32
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    Default Re: Another DOF question

    correct me if i'm wrong..

    So if you zoom in (and your aperture value unchanged), your DOF gets more and more shallow? Similar to moving closer to the object?

  13. #33
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    Default Re: Another DOF question

    Yup that's correct, and beside of having shallow DOF, you get perspective compression, which is very useful in helping to eliminate background distraction.
    Last edited by ziploc; 5th April 2007 at 01:01 AM.

  14. #34
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    Default Re: Another DOF question

    Quote Originally Posted by mfbatzap View Post
    correct me if i'm wrong..

    So if you zoom in (and your aperture value unchanged), your DOF gets more and more shallow? Similar to moving closer to the object?
    Yes, similar. But remember, moving closer to object and zoom in will give different perfective, and also the DoF will not the same.

    Regards,
    Arto.

  15. #35
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    Default Re: Another DOF question

    Quote Originally Posted by Artosoft View Post
    Yes, similar. But remember, moving closer to object and zoom in will give different perfective, and also the DoF will not the same.

    Regards,
    Arto.
    i find DOF, hyper focal, focal length relationship very confusing. im using d200 and 85mm f1.8 last fashion playground at marina. i find my subject againts the background not separated enough. i use f2.8 constantly. do i need to stop down further to have a blurred background?

  16. #36
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    Default Re: Another DOF question

    Quote Originally Posted by peanutbeer View Post
    i find DOF, hyper focal, focal length relationship very confusing. im using d200 and 85mm f1.8 last fashion playground at marina. i find my subject againts the background not separated enough. i use f2.8 constantly. do i need to stop down further to have a blurred background?
    You should make aperture bigger (around f/2-f/2.8) and move closer to subject to have a blurred background.

    If possible, using 85mm f/1.4 .

    Regards,
    Arto.

  17. #37

    Default Re: Another DOF question

    Quote Originally Posted by peanutbeer View Post
    i find DOF, hyper focal, focal length relationship very confusing. im using d200 and 85mm f1.8 last fashion playground at marina. i find my subject againts the background not separated enough. i use f2.8 constantly. do i need to stop down further to have a blurred background?
    Remember that distance between you to the subject, and the subject to the background plays a part. You will want to be as close to the subject as possible and the subject as far from the background. Try to get a position that you can catch your subject away from the background. If it is not possible to change these distances, the only way is to open up (not stop down) the aperture. But there is only that you can do with wide aperture.

    BC
    Last edited by Scaglietti; 7th April 2007 at 09:42 AM.

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