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Thread: understanding f-number

  1. #1
    David J. Heinrich
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    understanding f-number

    Can someone please help stupid me understand this. I've heard it said that in terms of brightness, f/4 at a given focal length is the same whether on an SLR, MF, or LF. However, I'm not quite sure I understand that.

    I understand that the same amount of light is collected on a given aperture size. However, for an f/4 lens on an SLR, that light is being put on a 35mm frame. For a LF, it is being put on a 4x5 or 8x10 piece of film. It would seem to me that in terms of "brightness", it certainly isn't the same, even at a given f-number.

    Can someone please explain this to me.

  2. #2
    Jim Graves Jim Graves's Avatar
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    Re: understanding f-number

    f-stop = focal length / lens diameter ...

    So, a 150 mmm lens (common on a 4x5 camera) would have to have 3X the lens diameter as a 50 mm lens (common on a 35 mm camera) to pass the same amount of light. That is why most LF lenses have higher minimum f-stops than 35 mm cameras.

    There are some other factors such as lens designs, glass used, element shapes, etc. that also factor in ... but that is the basic concept, all things being equal.

  3. #3

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    Re: understanding f-number

    It sounds like you are thinking, quite correctly, that, say, a 300mm f 5.6 normal for a 8x10 for instance, throws more light, in total, than the 50mm normal at f 5.6 for a 35mm camera. the 300 5.6 does throw more light.

    The thing that makes it all work out so that your metering is accurate for all formats, with no correction for format, is that the longer lens is placed further away from the film and spreads all it's light over a much larger area. It works out that the amount of light in any unit of area at the plane of focus is the same for any lens of a given f stop. The extra total light gathering power of a big longer lens is exactly offset by the greater distance from lens to film, and thus, greater area the light is covering.

    Best,

    C
    Last edited by CG; 28-Dec-2008 at 21:47. Reason: clarity

  4. #4

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    Re: understanding f-number

    Quote Originally Posted by Jim Graves View Post
    f-stop = focal length / lens diameter ...
    That's like saying mm = distance. Aperture = focal length / diameter.

    The f-stop is a calibrated unit of measurement that allows to have a consistent measurement of how much light a given aperture puts on the film. The idea is that any lens set to a given f/stop will always put the same amount of light on the film (or sensor), yielding the same exposure, no matter the focal length, which is why the f/stop is expressed independently of focal length or film/sensor format.

  5. #5
    All metric sizes to 24x30 Ole Tjugen's Avatar
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    Re: understanding f-number

    Not quite, Rakesh, the f-number is dimensionless.

    The F in f-stop is actually the focal length, and the "diameter" it should be divided by is the diameter of the entrance pupil of the lens at a given aperture.

    So a 360mm lens (F=360) with a 10mm aperture (entrance pupil) would be set at f:36.

    Note that the colon in the notation is the European equivalent of a "/", so what it says is that the entrance pupil is 1/36th of the focal length. And since both the focal length and the aperture size are given in the same units of measurement, the f-number is a dimensionless ratio.

  6. #6

    Re: understanding f-number

    Have a read of this it helped my understanding of fstop and shutter speed
    http://www.fredparker.com/ultexp1.htm

    bob

  7. #7

    Re: understanding f-number

    This has been explained well here - a given aperture will throw the same amount of light on a given piece of film/sensor, regardless of the focal length. As the lens gets longer, more light is lost in the journey through the lens to the film plane, so the physical aperture (opening) of the lens has to get larger to compensate.

    I have a quick and dirty aperture area calculator on my website, for those who are interested:

    Aperture area calculator

    There are also a few other useful essays and tutorial videos on my photography tutorials page.

    Cheers,
    Peter

  8. #8

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    Re: understanding f-number

    Peter,

    Rather than saying "As the lens gets longer, more light is lost in the journey through the lens to the film plane", you will have more complete understanding you consider that as the lens gets longer the image magnification increases and so the light collected from a given area of the subject is spread over a larger corresponding film area.

  9. #9

    Re: understanding f-number

    Both are correct, really. It's the inverse square law at work, along with the spreading of the image over a larger area. The larger film area only works intuitively if you're talking about going from a smaller to a larger format, but the effect is the same with different lenses on the same format.

    Peter

  10. #10

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    Re: understanding f-number

    But the inverse square law is identically "the spreading of the image over a larger area."

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