Fast Lenses - Performance when stopped down?

Discussion in '35mm Cameras' started by David Hunt, Mar 27, 2005.

  1. David Hunt

    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 27, 2005
    #1
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  2. It certainly should be - that is why we use "f stops". And, it's why
    exposure meters simply have scales showing just shutter speeds, and
    apertures. (and of course, film speeds which set the shutter speed/aperture
    combinations versus light levels)
    Occasionally the aperture blades may not stop down correctly and that can
    cause errors, but in general, the system works well.
     
    Malcolm Stewart, Mar 27, 2005
    #2
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  3. David Hunt

    Tony Guest

    Shutter speed will be exactly the same - performance will differ according
    to the strengths and weaknesses of teh lenses used.
     
    Tony, Mar 27, 2005
    #3
  4. David Hunt

    JR Guest


    Well The original poster was probably talking about image quality, not
    exposure....As you state the exposure will be the same, but he is asking
    will the 2 lenses have similar optical performance. That depends on the
    2 lenses in question. For example the Nikkor 50mm lense, the 1.4 and
    1.8....the 1.8 is said to be better slightly until f8 where they are
    pretty much equal, so unless you NEED to shoot at f/1.4 then the f/1.8
    lens will give you just as good, or better results. Also a lens is
    usually sharpest 2 to 3 steps below it's fastest f stop...so a lens that
    is at it's fastest at f/1.4 starts to get into its "sweet spot" at
    f/5.6, now a lens that is fastest at f/5.6 will not begin to get into
    it's sweet spot until f/11. But you probably were not comparing a f/1.4
    lens and a f/5.6 lens.

    JR
     
    JR, Mar 27, 2005
    #4
  5. No, the OP specifically asked, ".....will the shutter speeds be
    different on the two lenses for the same exposure?"

    .....and the answer is, no, they will not be different.
     
    William Graham, Mar 27, 2005
    #5
  6. David Hunt

    Bandicoot Guest

    t-stops, anyone?

    ;-)


    Peter
     
    Bandicoot, Mar 27, 2005
    #6
  7. David Hunt

    ian lincoln Guest

    AT the same focal length and the same aperture the exposure is the same.
    Even the focal length may not have that great an effect unless you were
    comparing extremes such as 28mm and 400mm. Basically cos the wider one
    would take in a greater angle of view to the point that the metering system
    is more likely to register an overall wider range of light conditions. To
    put it a simpler way a huge difference in focal length would result in
    different meter readings as would using a spot meter and a centre weighted
    system. All in all F stops are equal and are calculated as a fraction of
    one. So F5.6 is actually 1/5.6 of the area. The smaller the f number the
    bigger the aperture (hole).

    More importantly it is much more difficult to make an f2.8 lens than a 5.6.
    The wider the aperture the greater the area of glass being used so the more
    chance of a flaw showing up. 2.8 zooms are usually much larger and heavier.
     
    ian lincoln, Mar 27, 2005
    #7
  8. David Hunt

    Alan Browne Guest

    Reciprocity is obeyed. All lenses at f/n should let in the same amount
    of light, so shutter speed should be the same for the same exposure.
     
    Alan Browne, Mar 27, 2005
    #8
  9. Really ?
     
    Malcolm Stewart, Mar 27, 2005
    #9
  10. David Hunt

    Bandicoot Guest


    t/n, absolutely. f/n, approximately.

    f is a geometric measurement, so allows nothing for the quality of glass,
    or - usually more significantly - light lost through reflection from
    air-glass surfaces in the lens, which is in turn affected by how good the
    coatings are and by how many such surfaces there are (losses through
    glass-glass reflections are slight in comparison).

    So the f number is the focal length divided by the diameter of the entrance
    pupil (which is effectively the size that the aperture seems to be when you
    look into the front of the lens). This is easy to calculate, and will be
    constant for any lens of given design, regardless of quality variation - so
    lens makers like it.

    The t stop is a measure of the actual light transmission. That means it
    should be much more useful - but it is harder to measure and of course
    lens-makers didn't like it (though it is quite widespread in the cine
    industry) so it never really caught on in the stills world.

    Hence we are stuck with f stops which are _nearly_ the same thing as a
    measure of light transmission, but not quite. A complex zoom with the
    attendant large number of air-glass surfaces won't transmit as much light at
    a given f stop as a well coated simple design FFL lens. But the difference
    is too small to bother most people, in most situations, with most equipment,
    most of the time...


    Peter
     
    Bandicoot, Mar 27, 2005
    #10
  11. David Hunt

    Alan Browne Guest

    <snipped>

    For the purpose of the OP, the answer was more than sufficient. For
    that matter I doubt that the widest difference among my 6 lenses is more
    than 1/6 of a stop.

    <snipped>

    Cheers,
    Alan
     
    Alan Browne, Mar 27, 2005
    #11
  12. David Hunt

    ian lincoln Guest

    look at the original posters question. He knows absolutely nothing so i'm
    starting at scratch.
     
    ian lincoln, Mar 27, 2005
    #12
  13. An f5.6 lens has a diameter which is 1/5.6 of the focal length; so it's a
    linear ratio, and not area.
     
    Malcolm Stewart, Mar 28, 2005
    #13
  14. David Hunt

    Bandicoot Guest

    Oh, agreed - and the OP had that information, from you and others. Just
    wanted to be sure that simplification for one person's benefit didn't end up
    becoming gospel for everyone else. As we know, the web has a habit of
    that... ;-)


    Peter
     
    Bandicoot, Mar 28, 2005
    #14
  15. David Hunt

    Sander Vesik Guest

    Well, its not really area but focal length. The area farctions are a
    completely different thing.
     
    Sander Vesik, Mar 28, 2005
    #15
  16. David Hunt

    jones144 Guest

    Here is the original post, which I assume you have not seen.

    ---------------------
    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.
     
    jones144, Mar 29, 2005
    #16
  17. It really has nothing to do with "flaws showing up", if you mean
    inhomogeneities in the glass. It is entirely due to the fact that
    deviations from perfect optical behaviour (lateral and longitudinal
    chromatic aberrations, spherical aberration, coma, astigmatism,
    curvilinear distortion and curvature of field) are much more difficult
    to correct at wider apertures. This requires the use of many more lens
    elements, possibly including some with aspherical surfaces, and some
    highly specialised and expensive types of glass. The net result is much
    larger, heavier and more expensive, and (as previously notes) may not
    perform any better (or even as well) at smaller apertures.

    David
     
    David Littlewood, Mar 30, 2005
    #17
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