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Thread: DoF, Cropped Sensors - some explanations

  1. #1
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    Default DoF, Cropped Sensors - some explanations

    I recent wrote some material for a New to Photography section on another forum.

    In this first diagram is it easy to see why the focal length is always the focal length regardless of the sensor size.
    The so called 'multiplier' is only a crop. Lenses don't magically grow longer



    The next diagram explains why DoF changes with aperture. You don't really need to know why, just that it does.

    To visualise why aperture affects DoF we show how a point source of light projected
    through a large and small aperture lens has a smaller or larger distance of sharp focus.

    When light is projected in this fashion it is called the depth of focus and is the corollary of
    depth of field (imagine the light going the other way).

    In the diagram you can see the length of the depth of focus is longer for the small
    aperture lens.



    In this example the distance of sharp focus is defined by a tolerance of how far the red line
    is away from the centre line. This tolerance equates to a technical term called the circle of confusion.
    The circle of confusion is used by camera manufacturers in the design and
    development of cameras and lenses.

    I hope these two diagrams are helpful.
    Regards, Kym
    Honest & Direct CC Appreciated!
    http://www.users.on.net/~farnik/upload/pentax.png K-5, K-7, K20D, Glass bits up to 500mm, other stuff

  2. #2
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    Default Re: DoF, Cropped Sensors - some explanations

    Perhaps another thing to explain would be how teleconverters work and how they affect the DOF. For example does the 300/2.8 + 1.4 (=420mm f4) have a better (ie more diffuse) bokeh/background than the 400/4?
    I always find these things fascinating, the optics etc.

    I suppose that the same aperture principle applies to the human eye? As in, when it is dark, our pupils dilate, presumably creating a larger aperture which would then mean we would have a shorter DOF (?) Its a bit hard to test though.
    Tobias Hayashi
    Canberra, Australia
    www.flickr.com/photos/callocephalon

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    Default Re: DoF, Cropped Sensors - some explanations

    Interesting subject and helpful diagrams Kym Thanks for the quick write up

  4. #4
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    Thread Starter

    Default Re: DoF, Cropped Sensors - some explanations

    Quote Originally Posted by callocephalon
    Perhaps another thing to explain would be how teleconverters work and how they affect the DOF. For example does the 300/2.8 + 1.4 (=420mm f4) have a better (ie more diffuse) bokeh/background than the 400/4?
    I always find these things fascinating, the optics etc.

    I suppose that the same aperture principle applies to the human eye? As in, when it is dark, our pupils dilate, presumably creating a larger aperture which would then mean we would have a shorter DOF (?) Its a bit hard to test though.
    A TC does affect the FL (makes it optically longer) so when you add a 1.4x you lose a stop as well.
    The reason is that the existing lens aperture has not changed but as the focal length has been multiplied by 1.4 (remember the square root of 2 is 1.41) and the light area have been halved.
    And a 2x TC loses two stops (1/4 the light, square root of 4 is 2).

    The DoF when using a TC changes in the same way it does if you used the new FL (original lens x TC).
    The longer the FL the less DoF.

    Yes, The human eye loses DoF in the dark ! But our brains are smart and we adapt.
    Regards, Kym
    Honest & Direct CC Appreciated!
    http://www.users.on.net/~farnik/upload/pentax.png K-5, K-7, K20D, Glass bits up to 500mm, other stuff

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