Fast Lenses - Value when stopped down?

Discussion in 'Photography' started by rob, Mar 26, 2005.

  1. rob

    rob Guest

    In message <[email protected]> - "Steve Gavette"
    :>
    :>
    :>:>> Hi All,
    :>>
    :>> I understand the value of a fast lens in that more light is let in when
    :>the
    :>> lens is wide open. However, is this the same when the lens is stopped
    :>down?
    :>> i.e. if I have a 200mm f2.8 and a 200mm f4 and shoot a scene with both
    :>> lenses at f5.6 do I get the same result or will the shutter speeds be
    :>> different on the two lenses for the same exposure?
    :>>
    :>> Thanks!
    :>> David.
    :>>
    :>The 2 lenses at the same f stop will produce similar results with same
    :>shutter speeds. One advantage to a faster lens however, is that lenses tend
    :>to become sharper (to a point) as the aperture is stopped down. In your
    :>example, the 2.8 lens *may* be sharper stopped down to f4 than the f4 wide
    :>open.
    :>
    :>

    Plus a f2.8 lens is able to AF faster/better in low light conditions than a f4
    lens (assuming the camera can take advantage of it).

    Rob
    www.rcp.ca
     
    rob, Mar 26, 2005
    #1
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  2. rob

    David Hunt Guest

    Hi All,

    I understand the value of a fast lens in that more light is let in when the
    lens is wide open. However, is this the same when the lens is stopped down?
    i.e. if I have a 200mm f2.8 and a 200mm f4 and shoot a scene with both
    lenses at f5.6 do I get the same result or will the shutter speeds be
    different on the two lenses for the same exposure?

    Thanks!
    David.
     
    David Hunt, Mar 26, 2005
    #2
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  3. rob

    UC Guest

    F-numbers are the measure of light transmittance (measured
    geometrically) and are the same for all lenses and focal lengths. In
    actual practice, some slight differences in transmittance do occur, but
    these are generally insignificant except in motion picture (MP) work,
    because of the wide-range zoom lenses that are used in cinema work. For
    MP work, lenses marked in 'T'-stops are used. For these lenses, the
    actual transmittance is measured and the lenses marked accordingly.
     
    UC, Mar 27, 2005
    #3
  4. The 2 lenses at the same f stop will produce similar results with same
    shutter speeds. One advantage to a faster lens however, is that lenses tend
    to become sharper (to a point) as the aperture is stopped down. In your
    example, the 2.8 lens *may* be sharper stopped down to f4 than the f4 wide
    open.
     
    Steve Gavette, Mar 27, 2005
    #4
  5. rob

    Alex Guest

    The shutter speeds will be the same but the results may not.

    A 200mm f2.8 lens might at f5.6 be sharper because it's been "stopped
    down" 2 stops where the f4 lens is only stopped down 1 stop. Also, the
    fact is that a 200mm f2.8 lens is aimed at the more advanced part of
    the market (advanced amateur to pro) while the f4 version would, more
    likely, be aimed at "consumers".
     
    Alex, Mar 27, 2005
    #5
  6. rob

    UC Guest

    Not necessarily. It's easier to make a good slower lens than a good
    faster lens. The slower lens wide open may be better than the faster
    one at the same stop.
     
    UC, Mar 27, 2005
    #6
  7. rob

    rufref Guest

    Why spend the extra money for a "fast" lens? If you are not a pro you
    probably don't need to and you can use a tripod, or any steadying device,
    and slower shutter speed to get the same results, and maybe a better depth
    of field.

    Tom
     
    rufref, Mar 27, 2005
    #7
  8. Of course it's harder to make faster lenses, that's why they cost more. Of
    course what you say is possible, particularly if you are talking about
    lenses from 2 different manufacturers. But with 2 lenses from the same
    maker, it's not likely. According to MTF graphs, the Canon 85mm 1.2 *is*
    sharper than the 1.8. The 200mm 1.8 *is* sharper than the 2.8. The same
    likely holds true for other manufaturers as well. Who would buy the more
    expensive lenses if they weren't better?
     
    Steve Gavette, Mar 27, 2005
    #8
  9. Better how? Beyond needing less available light, I'm not convinced the
    differences at the same F--stop are significant. There are likely
    exceptions both ways.
     
    Oliver Costich, Mar 27, 2005
    #9
  10. I know that there are exceptions. The Nikon 50mm 1.8 has a slight edge over
    the 1.4 at all apertures to f8. www.photodo.com has MTF graphs for older
    lenses, you can see for yourself. In general, a faster lens tends to be
    sharper than a comparable slower one, particularly at smaller apertures.
    Again, I am comparing lenses from the same manufaturer, i.e. the Nikon 300mm
    f4 vs. the f2.8
     
    Steve Gavette, Mar 27, 2005
    #10
  11. rob

    dj_nme Guest

    For SLR camera lenses in my opinion, better in several ways.
    Firstly, the larger apeture (low f-stop) lenses allow the autofocus (if
    available on the lens/body combination) to act more quickly, as the lens
    only stops down when the shutter fires.
    Secondly, it is easier to manualy focus (if you want to) in dimmer light.
    Thirdly, when stopped down 2 or 3 f-stops for maximum sharpness the
    autofocus is quicker and manual focus is easier than cheaper, smaller
    apeture (higher f-stop) lenses.

    It is realy about sharpness and speed of focussing.
     
    dj_nme, Mar 27, 2005
    #11
  12. rob

    retoohs Guest

     
    retoohs, Mar 27, 2005
    #12
  13. rob

    Guest Guest

     
    Guest, Mar 27, 2005
    #13
  14. rob

    Alex Guest

    And faster auto focusing.
     
    Alex, Mar 27, 2005
    #14
  15. rob

    UC Guest

    The speed factor. In most cases, the slower lenses are better. Not
    always, but usually.

    Some companies produce different lines of lenses for different
    purposes, as typfied by Canon with the 'L' series lenses. Others, such
    as Leica, produce lenses all to the same standard, differing merely in
    speed. In the case of Leica lenses, at least, the lens quality at each
    speed is about as good as can be had at that particular time. Usually,
    but not always, the slower lenses are better on an absolute scale. In
    some cases, however, the slower lenses are older designs, and the
    latest fast lenses can sometimes surpass them.

    Leica has made several 180mm lenses for the reflex system, ranging in
    speed from f/4 to f/2. The earliest was a 2,8 lens designed in 1965. It
    was rather large and heavy. An f/4 lens appeared in 1976, which was
    considerably smaller and lighter. When Leica introduced a smaller,
    lighter 2,8 in 1980, the f/4 was dropped, as there was little point to
    continuing the lens when the new 2,8 was not much heavier or larger. A
    few years later an f/2 APO-Summicron was developed, which is
    extraordinary in quality, and was even better than the 2,8. Since it is
    quite expensive, however, Leica introduced an APO version of the 2,8
    lens. The two APO lenses are similar in quality.
     
    UC, Mar 27, 2005
    #15
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