I don't understand why this should be the case with lenses?

Discussion in 'Digital Photography' started by Mr.Will, Jul 26, 2003.

  1. Mr.Will

    Mr.Will Guest

    I have two lenses, one is a 75-300f4/f5.6 and a 35-110 f 4-f5.6
    FOr some reason I need less light to get a decent exposure and colour
    saturation with the 75-300 lens.
    Yet both are f 4/5.6 - it doesnt seem to make sense to me.
    Also with my prime f 1.8, if I shoot at f 4 it seems once again to require
    less light for a decent exposure.
    Perhaps this is due to it being prime and not zoom - Ive heard prime is
    simply better due it being optimised for a particular job.

    Still it seems odd to me that f4 doesnt always mean f 4.
    Can anyone explain this to me?
    Mr.Will, Jul 26, 2003
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  2. Mr.Will

    Leon Guest

    Lets eliminate perhaps the obvious... Do you have filters attached to both
    lenses and if so are they exactly the same filter?
    Then, are you using the proper size filter on each lens so as not to be
    restricting light from one over the other?

    Are your findings the same with no filters at all?

    Are you shooting both lenses at 75 and 25 respectively to take advantage of
    maximum aperture on both lenses when doing the comparison? Does either lens
    have a hood that could be restricting light?
    Leon, Jul 26, 2003
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  3. Mr.Will

    Mark M Guest

    Hoods don't restrict the light in a sense that would be significant here.
    He didn't include enough details about exposure for anyone to answer his
    question. His camera body may be compensating differently, but we wouldn't
    know. Also... If he's using print film, then all bets are off--since the
    processor of his film has FAR more control over exposure than he (the
    photog) did.
    Mark M, Jul 26, 2003
  4. Look up `t numbers' in a good photo reference. specifically, f4 does
    not mean t4...

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    Paul Repacholi, Jul 26, 2003
  5. Mr.Will

    Charlie D Guest

    Aren't "t" numbers brought into play at when lenses are focused closer,
    usually much closer than infinity?
    I don't feel like looking it up. ;)
    Charlie D, Jul 26, 2003
  6. Mr.Will

    Charlie D Guest

    ok, thanks.
    Charlie D, Jul 26, 2003
  7. Mr.Will

    Mxsmanic Guest

    A t-number is the true aperture of a lens including transmission and
    other losses. It's measured by actually measuring the light passing
    through the lens at a given nominal aperture setting.

    T-numbers are useful in cinematopgraphy for very precisely matching
    exposure from shot to shot and from lens to lens, so that difference
    scenes do not change in brightness when multiple shots of the same scene
    are edited together. Two cameras with two lenses set to t/8 will
    produce exactly the same exposure on film; if they were just set to f/8,
    tiny differences in their true aperture might cause brightness to change
    visibly when cutting from one camera to the other in a scene.

    In still photography, this obviously is far less of a problem, so
    t-numbers are not normally used. The difference between t- and
    f-numbers is too small to have an influence on correct exposure.
    Mxsmanic, Jul 26, 2003
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