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    zoom lens

    variable aperture zoom lenses.

    eg 70-300 f4.5-5.6

    when taking the same subject, framed to be the same size, will the DOF be the same at both ends wide open?

    I ask because I assume the variable aperture is actually a fixed physical size, and it is due to the extension of the lens that the effective aperture is smaller due to that movement. Since the effective aperture is relative to the zooming, then would the DOF also be changing at the same rate?
    Last edited by reaction; 02-11-2011 at 10:00pm.

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    Dof is a factor of aperture, focal length and distance to subject.
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    I did a test recently with an angle of view calculator and a DOF calculator. I found that a 50mm lens at f/2.8 and a 200mm lens at f/2.8, each at a distance to subject such that the same angle of view was achieved, both had exactly the same DOF.

    I was interested in seeing if there was any advantage in using a longer focal length to getting a wider DOF around subject distance, while still blowing the background out of focus - this does not appear to be the case.

    With this in mind, one would expect to be able to achieve a shallower DOF at 70mm and f/4.5, than at 300mm and f/5.6, if each shot was taken so that the subject was framed the same size.

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    Yip, you will notice though that although dof is the same, the 200mm image flattens the subject foreground and background and renders a pleasing effect accordingly

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    the crop factor of the sensor is also a factor in DOF. So all up you have crop factor, aperture, focal length and distance to subject all affecting the depth of field in any photo.

    So given the OP's need to have the the shot framed to the SAME SIZE in the viewfinder (on the sensor), the distance to subject would vary between 70mm and 300mm, thus the example below:

    Crop sensor factor 0.65
    Aperture used f4.0
    @70mm - distance to subject 5 metres
    DOF would be 1.92m (from 4.22m from lens through to 6.14m from lens}

    Same again, but distance to subject is now 20 metres** (to get same/similar field of view in the viewfinder)
    focal length is now 300mm
    DOF would be 1.62m (from 19.22m to 20.84m)

    Now say you had a full frame camera (thus the sensor crop size comes in)
    to get the same field of view, you would need to change distance again, thus the DOF would change(again) also.

    There are some great DOF calculators out there (esp good on smart phones, available for both iPhone and Android phones). Or you can play on the internet : http://www.dofmaster.com/dofjs.html

    Pick your camera, put in the settings and it will give you a DOF, front to back, result.

    **I chose 20 metres as being close enough, based on 70/300 being approx 1/4 thus 5 metres x 4 is 20 metres, for distance to subject to ensure a similar field of view in the viewfinder/on the sensor
    Last edited by ricktas; 03-11-2011 at 7:18am.
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    I've played with DOF calculators but never knew how to calculate distance to be framed to the same size. Is it just a simple straight division?

    Also, in Lucas' example, would in both items the sharpness drop off at the same rate? ie something 10m behind the subject but still in frame would look equally blurred?

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    Quote Originally Posted by reaction View Post
    variable aperture zoom lenses. eg 70-300 f4.5-5.6 when taking the same subject, framed to be the same size, will the DOF be the same at both ends wide open?
    No.

    The two images will not have the same DoF.

    The image taken at FL=300mm will have the greater DoF as it will be shot at F/5.6. (provided the lens is used on the same camera format, for each shot)



    Quote Originally Posted by reaction View Post
    I ask because I assume the variable aperture is actually a fixed physical size, and it is due to the extension of the lens that the effective aperture is smaller due to that movement. Since the effective aperture is relative to the zooming, then would the DOF also be changing at the same rate?
    I believe I understand the rationale for the question.
    And I believe I understand that the question is predicted on the Axiom of Depth of Field and that is why you specifically stated the FRAMING of the Subject would be the same in each image.

    With respect to the maximum aperture being a fixed physical size, that is correct.

    But the effective aperture changes, because that fixed size (think of it as “x”mm diameter) will make the “f-stop” different, depending upon the Focal Length of the lens . . .

    Because the f-stop is the RATIO of the focal length to the aperture – so obviously as the lens gets a bigger Focal Length, the F-stop (the ratio) must become a bigger number.

    Does that answer your question?

    WW
    Last edited by William W; 03-11-2011 at 10:27am.

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    Quote Originally Posted by reaction View Post
    I've played with DOF calculators but never knew how to calculate distance to be framed to the same size. Is it just a simple straight division?
    Yes.
    It is a simple arithmetic ratio – division or multiplication only, within any same camera format.

    For example: with a 135 format camera such as a Canon 5D - the (linear) FoVv (Field of View Vertical) is 5ft with a 50mm lens at an SD (Shooting Distance) of about 10ft 6 inches.
    If you want the same FoVv, with a 100mm lens, then the SD will be 21feet
    And with a 150mm lens, the required SD will be 31ft 6 inches.


    Quote Originally Posted by reaction View Post
    Also, in Lucas' example, would in both items the sharpness drop off at the same rate? ie something 10m behind the subject but still in frame would look equally blurred?
    Yes.
    The images’ DoF will “drop of at the same rate”

    ###

    No.
    The blur will very possibly not look the same, even if the same subject is photographed in the same position and in the same lighting conditions.

    This is because the Perspective** is different and the resultant background (in blur) will have a different spatial relationship to the main subject within the final image.

    WW

    ** The Perspective changed when the SD changed.
    Perspective is dependent upon the Camera's Distance and Angle, to the Subject.
    Last edited by William W; 03-11-2011 at 10:26am.

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    Quote Originally Posted by William W View Post
    But the effective aperture changes, because that fixed size (think of it as “x”mm diameter) will make the “f-stop” different, depending upon the Focal Length of the lens . . .

    Because the f-stop is the RATIO of the focal length to the aperture – so obviously as the lens gets a bigger Focal Length, the F-stop (the ratio) must become a bigger number.

    WW
    I was hoping that since the DOF and effective aperture were both a result of the Focal Length x aperture size, they would change at perhaps an equal rate or there may be some math ratio that can be calculated against them.

    Of course there is the other variable that is the changing Shooting Distance to keep FoV constant, but that is also related to change in Focal Length.

    But I see now that 70-300 can be 4-5.6 or 4.5-5.6 so it mustn't be the same

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    Quote Originally Posted by William W View Post

    This is because the Perspective** is different and the resultant background (in blur) will have a different spatial relationship to the main subject within the final image.

    WW

    What I mean is, ignoring the perspective, say it's a photo of some text and say it's right in the middle of the frame. Would the text be equally unreadable in both cases? I'm not sure how or if it is possible to judge that...

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    Quote Originally Posted by reaction View Post
    What I mean is, ignoring the perspective, say it's a photo of some text and say it's right in the middle of the frame. Would the text be equally unreadable in both cases? I'm not sure how or if it is possible to judge that...
    Sorry I don't understand the question.

    But - do you mean something like if we took a picture of a row of birds, on an angle of about 45 degrees . . . ? ? ?



    Would the same birds, seem to be the same "amount" out of focus in each shot . . .

    The answer is "yes".


    WW
    Last edited by William W; 04-11-2011 at 4:39pm. Reason: korrected spellung

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    Quote Originally Posted by reaction View Post
    I was hoping that since the DOF and effective aperture were both a result of the Focal Length x aperture size, they would change at perhaps an equal rate or there may be some math ratio that can be calculated against them.
    No.

    The Focal Length is irrelevant for the DoF in your scenario.

    Your question specifically stated that the Subject was to be framed the same in each shot.

    The only two relevant factors affecting the DoF in this case are:
    • the Aperture used and
    • maintaining the same camera format (using the same sensor size camera).


    WW

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