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Thread: is an f/2.8 on a digital cam equivalent to a smaller aperture in a dslr?

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    Default is an f/2.8 on a digital cam equivalent to a smaller aperture in a dslr?

    Hi, read a book saying that f/2.8 is somewhat equivalent to f/11 or f/12 on a dslr. is this true? how to calculate the difference, and why do the makers not make the aperture reading the same on both kinds of cameras?

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    Quote Originally Posted by asturias105
    Hi, read a book saying that f/2.8 is somewhat equivalent to f/11 or f/12 on a dslr. is this true? how to calculate the difference, and why do the makers not make the aperture reading the same on both kinds of cameras?
    the DOF is equivalent to around F/11 - F/14
    F/2.8 of light and brightness but F/11 of DOF effect

  3. #3

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    No, f/2.8 is the same on digicam or DSLR. But the depth of field of a digicam is much larger than that of a DSLR, so the DOF of a digicam at f/2.8 might be equivalent to f/11 on a DSLR. The exact aperture it is equivalent to depends on the ratio of the sensor sizes.

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    the aperture IS correct in relation to the sensor size/image circle its trying to project, the DOF is different because of the relationship of sensor size to absolute focal length. its simply a matter of physics.

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    the aperture number is merely a ratio of the focal length of the lens.

    how to compare apples with oranges?

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    f/2.8 on a digicam with a lens of 7mm focal length (let's say 35mm equivalent) @ 7mm (widest) gives an aperture of just 2.5mm.

    f/2.8 on a 35mm camera gives an aperture of 12.5mm.

    As you can see, opening is smaller.

    Not sure if the logic is right, but now if you take a 2.5mm aperture (opening) and apply it to a 35mm camera with a 35mm lens, you get an f-number of 35mm/2.5mm = f/14.

    Regards
    CK

  7. #7

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    ok let me get this correct.

    so f/2.8 on a digicam and a dslr is the same aperture size except that the DOF is different due to the sensor size and focal length of the lens?

    ckiang: are u suggesting the aperture size is actually different?(2.5mm and 12.5mm)

    or is it the f number is the same for both, but the aperture size is actually different and DOF is therefore different? Add to it diff sensor size and diff focal length?

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    Quote Originally Posted by vinvin
    ok let me get this correct.

    so f/2.8 on a digicam and a dslr is the same aperture size except that the DOF is different due to the sensor size and focal length of the lens?

    ckiang: are u suggesting the aperture size is actually different?(2.5mm and 12.5mm)

    or is it the f number is the same for both, but the aperture size is actually different and DOF is therefore different? Add to it diff sensor size and diff focal length?
    do note that "f number" is not aperture size. "f number" is actually a ratio.

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    I raised a similar question some time back and it was a good discussion with
    many educational feedback. See this thread:
    http://forums.clubsnap.org/showthread.php?t=101263

    Cheers,
    L.Lee

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    Quote Originally Posted by vinvin
    ok let me get this correct.

    so f/2.8 on a digicam and a dslr is the same aperture size except that the DOF is different due to the sensor size and focal length of the lens??
    Nope. The actual physical size of the aperture is different. Please note f/2.8 is a RATIO, not an actual size. Think about it, my cell-phone camera also has a f/2.8 lens, but the entire phone itself is not much bigger than the rear opening on a 70-200/2.8 lens, how can the actual aperture size be the same?

    Quote Originally Posted by vinvin
    ckiang: are u suggesting the aperture size is actually different?(2.5mm and 12.5mm)?
    Yes, it is correct.

    Quote Originally Posted by vinvin
    or is it the f number is the same for both, but the aperture size is actually different and DOF is therefore different? Add to it diff sensor size and diff focal length?
    The DOF is as you have suggested. Due to the smaller sensor size and the relation of focal length and the distance from lens to sensor, a f/2.8 lens made for a smaller sensor will give you much more DOF than one made for a larger sensor.

    How about DOF effect of a standard 35mm lens on a cropped sensor? Hah! Another kettle of fish.......

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    Quote Originally Posted by dkw
    How about DOF effect of a standard 35mm lens on a cropped sensor? Hah! Another kettle of fish.......
    Yeah, a very highly complex and highly debated one.

    Regards
    CK

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