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Thread: Depth of Field (DOF)

  1. #1
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    Depth of Field (DOF)

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    hey..

    just wondering why the DOF is so different on different cameras..
    some of my tetra shots taken with f8 have suchs a deep DOF.. while say.. take for example some of the fry shots taken by benetay have a lower DOF even though he use F11 or F12.. whys that?? is there any other way to lower DOF besides F-stop? shutter speed?

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    DOF is determined by the aperture and focal length of the lens, and also the distance from the subject. the nearer the subject, the shallower the DOF for the same given aperture and focal length.
    why I don't do garden hybrids and aquarium strains: natural species is a history of Nature, while hybrids are just the whims of Man.
    hexazona crumenatum Galleria Botanica

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    oh.. cool.. thanks.

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    Which cameras are you comparing?

    The DOF is also affected by sensor size.

    Factors affecting DOF:
    1. Sensor size
    2. Focal length of lens
    3. Distance from the subject
    4. Aperture

    BC

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    how does sensor size affect DOF?
    why I don't do garden hybrids and aquarium strains: natural species is a history of Nature, while hybrids are just the whims of Man.
    hexazona crumenatum Galleria Botanica

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    Quote Originally Posted by hwchoy
    how does sensor size affect DOF?
    Larger sensor ---> shallower DOF

    That's why it is more difficult or almost impossible to get a portrait shot with a nice out-of-focus background with a compact or prosumer camera.

    BC

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    I mean how does a large sensor causes a shallow DOF? as far as I know DOF is an optical property of the lens. compact cameras have large DOF due to their extremely short optical focal lengths (these guys have 3-5mm focal length lenses).
    why I don't do garden hybrids and aquarium strains: natural species is a history of Nature, while hybrids are just the whims of Man.
    hexazona crumenatum Galleria Botanica

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    Quote Originally Posted by hwchoy
    I mean how does a large sensor causes a shallow DOF? as far as I know DOF is an optical property of the lens. compact cameras have large DOF due to their extremely short optical focal lengths (these guys have 3-5mm focal length lenses).
    Wah... you really want me to explain... a damn bloody long story...

    It has to do with the perception of our eyes, the circle of confusion (COF). In the actual image, only a single plane will be exactly in focus. Because of the limited resolution of our eyes, our eyes will perceive a those areas that are slightly out-of-focus as being "in focus". Thats the reason for the depth-of-field, a range of distance where objects appear "in focus" to our eyes. Think projection TV, when you are too close your eyes can see that the image sort of blurr, the image will only appear sharp when you are a distance away.

    With this principle, you can better understand why some factors affect the DOF. Strictly speaking, even print size and viewing distance matter.

    Try reading this first:
    http://www.luminous-landscape.com/tu...ries/dof.shtml

    BC

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    I think you have misunderstood what they are trying to say. I need to run so I just quickly quote two points from luminous landscape.

    "The answer is, no. There is no difference whosesoever. DOF doesn't care about the recording media type or size, though a lower COF is used for medium and large format, since the amount of magnification to make a decent sized print is much less than for 35mm."

    and

    "Why therefore do digital cameras have greater Depth of Field? The reason for this is that the imaging chips on most consumer digitals is very small, around the size of ones smallest finger nail. This means that a normal lens for a format that small is as short as 15mm. A 15mm lens at f/5.6 has Depth Of Field from about 2.5 feet to infinity. Not too much opportunity for selective focus, is there?"

    what it means is that digicams have large DOF not because their sensor is small, but rather the optical system has very short focal length because of the small sensor.

    we can discuss some more I fly back this evening…

    I am actually trying to figure out hyper focusing.
    why I don't do garden hybrids and aquarium strains: natural species is a history of Nature, while hybrids are just the whims of Man.
    hexazona crumenatum Galleria Botanica

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    Given the same lens the DOF is different on different sensor size. That is the reason why the DOF table for 35mm cannot be applied on DX format(Nikon). There is another set of DOF table for DX sensor size which is different from the 35mm format.

    Hyperfocal point is the minimum distance that you focus on where objects behind the focal point will fall within the DOF. That means if you focus at the subject at the hyperfocal point or beyond, all of the background will appear in focus.

    BC

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    Can you guys speak english please..
    Something about the water & the fishes that calms me down.

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    Case study:

    Compare: Nikon F6 (35mm film), Nikon D200, Nikon Coolpix 8800 (Sorry I Nikon user... )

    Sensor size:
    F6 - 36mm x 24mm
    D200 - 23.6mm x 15.8mm
    G6 - 8.8mm x 6.6mm

    Case 1
    Fixed actual focal length - 28mm
    Subject at 1m
    Aperture: f/2.8

    F6:
    Equivalent focal length - 28mm
    Depth of field: 0.897m - 1.129m (Total: 0.232m)

    D200:
    Equivalent focal length - 42mm
    Depth of field: 0.930m - 1.081m (Total: 0.152m)

    CP 8000:
    Equilvalent focal length - 110mm
    Depth of field: 0.972m - 1.029m (Total: 0.057m)

    (note: This may not be meaningful because the equivalent focal length is different and composition of picture is different.)


    Case 2
    Fixed angle of view: equivalent focal length - 150mm
    Aperture: f/5.6
    Subject at 1m

    F6:
    Actual focal length - 150mm
    Depth of field: 0.993m - 1.007m (Total: 0.014m)

    D200:
    Actual focal length - 100mm
    Depth of field: 0.989m - 1.011m (Total: 0.022m)

    CP 8800:
    Actual focal length - ~35mm
    Depth of field: 0.965m - 1.038m (Total: 0.073m)

    (Note: in this case the photo composition is similar with same aperture size.)



    In either cases, the DOF is different for different sensor size. This is because the projection of circle of confusion on different size frame. A lens with a certain focal length and aperture will project the same circle of confusion however in relation with different sensor size. And this caused the resultant differences in DOF. Maybe you can draw some diagrams yourself and make some calculations and you will understand why.


    With the same angle of view & aperture, a large sensor size also will have bigger "blurrness" in the out-of-focus area. That's why the bokeh on the 5D looks nicer than compared to the G6 for example, even though both background may be outside the DOF.


    BC

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    I found a better article for DOF:

    http://www.photo.net/learn/optics/dofdigital/

    BC

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    That's the way that I've understood all the hype between FF vs. cropped sensor as well - sensor size does matter due to FF having a narrower DOF.
    Eric

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    Quote Originally Posted by bclee
    Hyperfocal point is the minimum distance that you focus on where objects behind the focal point will fall within the DOF. That means if you focus at the subject at the hyperfocal point or beyond, all of the background will appear in focus.

    BC

    yup I am trying to find this point for my 100mm Macro.
    why I don't do garden hybrids and aquarium strains: natural species is a history of Nature, while hybrids are just the whims of Man.
    hexazona crumenatum Galleria Botanica

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    ah I actually came across that photo.net article before, but did not find the time to read through the calculation. looks like just the think to read while I am stuck in BKK tomorrow.
    why I don't do garden hybrids and aquarium strains: natural species is a history of Nature, while hybrids are just the whims of Man.
    hexazona crumenatum Galleria Botanica

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    Quote Originally Posted by hwchoy
    yup I am trying to find this point for my 100mm Macro.
    There are a few DOF calculator on the web. The are some variations due to assumptions about print size and view distance, but they should generally give you the idea.

    I usually use the on on Dpreview.com:
    http://www.dpreview.com/learn/?/Glos...f_Field_01.htm

    It will give the hyperfocal distance for the respective aperture for your 100mm macro.

    BC

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