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Thread: need some help in DOF( deth of field )

  1. #21
    Senior Member ZerocoolAstra's Avatar
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    Default Re: need some help in DOF( deth of field )

    Quote Originally Posted by wayneloh2609 View Post
    i am using a canon 50mm f1.8 lens . i love the effect when i am taking close up on object or faces with the background blurr . but when i try to take a full body of a person the background seems to be clear too . any setting i can do to make the background blurr ?


    hope some kind hearted pro could give me some help thanks alot
    You didn't mention what camera you are using, but i'll assume it's one with a cropped sensor.
    By trying to get a full-body portrait, what you're doing is stepping back, thus increasing Distance A (with reference to Sulhan's diagram) but with no change to distance B.
    With a 50mm lens on a cropped body, I'd assume you'll be quite far away from the subject. Thus the depth of field has effectively increased, rendering the background sharper.

    but i guess you already knew the answer....
    Exploring! :)

  2. #22
    Senior Member ZerocoolAstra's Avatar
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    Default Re: need some help in DOF( deth of field )

    I'm curious about something:

    If I have a wide-angle prime (eg. 24mm f/2.8) and a normal prime (eg. 50mm f/1.8),
    which one will give me nicer bokeh for the same full-body shot, for example?

    Of course both will need to be at f/2.8 for comparison sake
    I don't have the wide-angle prime, so I can't do a comparison.
    Exploring! :)

  3. #23
    Senior Member darrrrrrrrrr's Avatar
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    Default Re: need some help in DOF( deth of field )

    Quote Originally Posted by ZerocoolAstra View Post
    I'm curious about something:

    If I have a wide-angle prime (eg. 24mm f/2.8) and a normal prime (eg. 50mm f/1.8),
    which one will give me nicer bokeh for the same full-body shot, for example?

    Of course both will need to be at f/2.8 for comparison sake
    I don't have the wide-angle prime, so I can't do a comparison.
    just ask someone with a constant aperture zoom to compare the various focal lengths?

    anyway i've read here and here that the DOF remains the same for both cases, but i think longer focal lengths will give a nicer perspective for portraits, eliminate background clutter and generate more absolute blur.

  4. #24
    Senior Member ZerocoolAstra's Avatar
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    Default Re: need some help in DOF( deth of field )

    oh ya... Forgot about constant aperture zoom...!
    Exploring! :)

  5. #25
    Senior Member ZerocoolAstra's Avatar
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    Default Re: need some help in DOF( deth of field )

    Quote Originally Posted by darrrrrrrrrr
    just ask someone with a constant aperture zoom to compare the various focal lengths?

    anyway i've read here and here that the DOF remains the same for both cases, but i think longer focal lengths will give a nicer perspective for portraits, eliminate background clutter and generate more absolute blur.
    thanks for the link!
    The perspective change is quite dramatic from long focal length to short, thus giving the impression that the depth of field changes. The background gets thrown much further back with a wide-angle lens, thus making it seem to be in focus.
    Exploring! :)

  6. #26
    Deregistered shaoken's Avatar
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    Default Re: need some help in DOF( deth of field )

    For those F1.8 lens on a body with sa crop factor, you the bokeh will be stronger as compared to what you will get if the lens is mounted onto a full-frame body.

    By using a zoom lens will also help to achieve the effect you want.

  7. #27

    Default Re: need some help in DOF( deth of field )

    Quote Originally Posted by shaoken View Post
    For those F1.8 lens on a body with sa crop factor, you the bokeh will be stronger as compared to what you will get if the lens is mounted onto a full-frame body.

    By using a zoom lens will also help to achieve the effect you want.
    The bokeh will be stronger for a subject at the same distance. Which means that a properly composed subject on a FF -> cropped subject on APS-C.

  8. #28
    Member valentinik's Avatar
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    Default Re: need some help in DOF( deth of field )

    For discussion sake,
    I find that whenever I shoot a subject, and my camera focus scale is around the Infinity area, it is impossible to achieve the background blur no matter what the f-stop. The fact is that once you step back to fit the whole person into the frame, the focusing reached infinity.

    Plainly put if the lens is the same throughout;
    A FF cam is better that cropped cos you don't have to step back so far.

    What say you guys?
    Valentinik
    Canon 5DMK2 | 24-105 f/4L | 70-200 f/2.8L IS | 100 f/2.8L IS Macro

  9. #29

    Default Re: need some help in DOF( deth of field )

    Quote Originally Posted by wayneloh2609 View Post
    i am using a canon 50mm f1.8 lens . i love the effect when i am taking close up on object or faces with the background blurr . but when i try to take a full body of a person the background seems to be clear too . any setting i can do to make the background blurr ?


    hope some kind hearted pro could give me some help thanks alot
    Try here and you will understand the problem you are facing.

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

  10. #30
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    Default Re: need some help in DOF( deth of field )

    Quote Originally Posted by surrephoto View Post
    Roughly, the exact mathematics are confusing, because to get the same perspective on FF/Crop, the focal length is different and affects the overall intensity and quality of the bokeh and DOF.

    For example;

    50 mm @ f1.8 on FF = 31 mm @ f1.1 on 1.6x crop.

    50 mm @ f1.8 on 1.6x crop = 80 mm @ f2.9 on FF

    This is thus the "Full-frame Advantage".
    Where did you get the numbers for the f/stop values and what are they based on?

  11. #31

    Default Re: need some help in DOF( deth of field )

    Quote Originally Posted by calebk View Post
    Where did you get the numbers for the f/stop values and what are they based on?
    http://www.cambridgeincolour.com/tut...ensor-size.htm

    "Depth of Field Equivalents"

  12. #32
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    Default Re: need some help in DOF( deth of field )

    DoF depends on focal length, aperture & subject distance (and CoC which is outside the scope of this discussion).

    For the same focal length, aperture & subject distance, the DoF is the same, regardless of whether it is 1.6 crop or not (the physical focal length of lens is unchanged on crop camera, even thou the angle of view is different). But on the crop body the angle of view will be different. Note that the article mentioned "when filling the frame with a subject of the same size and distance", in which case to get the same subject size the subject distance must increase for the crop sensor, hence the DoF increases. It is not due to the crop but because of the increase in subject distance to get the same subject size on frame.



    Quote Originally Posted by surrephoto View Post

  13. #33

    Default Re: need some help in DOF( deth of field )

    Quote Originally Posted by Limsgp View Post
    DoF depends on focal length, aperture & subject distance (and CoC which is outside the scope of this discussion).

    For the same focal length, aperture & subject distance, the DoF is the same, regardless of whether it is 1.6 crop or not (the physical focal length of lens is unchanged on crop camera, even thou the angle of view is different). But on the crop body the angle of view will be different. Note that the article mentioned "when filling the frame with a subject of the same size and distance", in which case to get the same subject size the subject distance must increase for the crop sensor, hence the DoF increases. It is not due to the crop but because of the increase in subject distance to get the same subject size on frame.
    Precisely. At the same focal length (FF vs Crop), the DOF is the same. However your image is cropped on the crop-factor body, causing you to need to stand further back to re-frame the image, causing a wider DOF...

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