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Fast lens/Slow lens at same aperture. Same shutter speed?

Discussion in 'Lens Lust' started by hornagain, Sep 23, 2008.

  1. hornagain


    Jan 21, 2007
    Central US
    This following question has bugged me for years, and lately I have been thinking about it again as I ponder the possible purchase of a 70-200mm 2.8.

    I know that the calculated physical size of the aperture of a 'fast lens' at f8 is the same as that of a 'slow lens' at f8 for a given focal length, and thus the calculated proper exposure SHOULD result in the exact same exposure time.

    However, with all of the modern lens technology, all the various glass inside, all of the various coatings, etc, is the calculated exposure time really identical for both lenses at the same f-stop? I just don't see how this could be true given the very different construction. Yes, I could test this theory but I guess I am not *that* bugged by the question and am just seeking the wisdom of the board.

    In essence, I want to defy physics and insist that 'fast' lenses have faster shutter speeds at the same f-stop and focal length than cheaper or 'slow' lenses.

    Then of course, the follow-up question. Why don't manufacturers make high optical quality lenses that do not have those fast apertures? Smaller, lighter, less expensive, but the same or even better quality at f4 and above.
  2. pforsell


    Jan 15, 2008
    Same aperture = same exposure.

    The advantage of fast lenses is not only the wider apertures letting more light in but the greater control of depth of field.

    With a slowish lens that has maximum aperture of f/3.5 you cannot achive similar artistic freedom with the DOF than with a f/1.4 lens for example. The difference between f/3.5 and f/1.4 is almost 3 stops and could mean DOF of several feet compared to a few inches. This is important in portraiture, for example, when you need to throw the background out of focus.

    Furthermore, most lenses are sharpest when stopped down one or two stops. With the slowish lens that means shooting at something like f/8 for optimum sharpness and with the fast lens you could stop it to f/2.8 for optimum sharpness. The shutter speed advantage with the faster lens in low light is huge in this scenario.

    Usually manufacturers put their best effort in the fast lenses, because those will anyway be the most expensive. The fast pro lenses are the cream of the crop in sharpness, saturation, colors, build quality and resale value.

    But, to repeat the first sentence in my post, both at f/4 deliver equal exposure. The lenses and apertures are tuned so that same aperture delivers the same amount of light.
  3. fks


    Apr 30, 2005
    sf bay area
    aperture is the ratio of the focal length to the diameter of the entrance pupil. it's a purely geometric measurement, and doesn't take into account any losses from the light passing through so many different pieces of glass. so there is some truth to the claim that there are differences in the amount of light transmitted by different lenses at the same aperture.

    in actual use, the loss is nearly negligible for most lenses, especially with the current computer-designed optics.

    the movie industry uses the t-stop instead of the f-stop, where the actual amount of light transmitted is measured.

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