Lenses with fixed aperture

Discussion in 'Digital Cameras' started by Derek Fountain, Jan 12, 2005.

  1. I'm shortly going to upgrade from a compact P&S (Canon S40) to a Canon DSLR
    and have been pondering lenses. I find a lot of lenses which have a fixed
    aperture, such as the well regarded 17-40mm f4 L lens. I'd have thought
    that fixed aperture would be a bad thing...

    One of the things about a compact camera is that changing aperture from one
    end of the range (f2.8 for the S40) to the other (f8 for the S40) really
    doesn't do a great deal in many cases. I was rather looking forward to
    working, experimenting and learning with a system that allows a good range
    of aperture adjustment. But now, in my quest for a small number (like 1 or
    2) of quality lenses as a starting point, I find myself homing in on lenses
    with fixed aperture.

    Am I right to be concerned about this, or is having a single wide lens fixed
    at f4 a good thing for reasons I don't understand?
    Derek Fountain, Jan 12, 2005
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  2. Derek Fountain

    chidalgo Guest

    17-40 f4L is a fixed-aperture lens?!?!?!?!?!!?

    Read a book or something on SLR & lenses. The f4 indicated is the
    MAXIMUM aperture you can obtain for that lens. But it's not always
    fixed. The only lenses that i know with fixed aperture are those in
    cheap point&shoot cameras, and mirror lenses.

    By the way, the "L" series are the pro lens series on Canon.
    chidalgo, Jan 12, 2005
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  3. I am, and I'm asking questions when confused. Thanks for the pointer though.
    Derek Fountain, Jan 12, 2005
  4. Derek Fountain

    Skip M Guest

    Fixed aperture, in this case, refers to the fact that the aperture does not
    change when you zoom the lens, i.e., it will stay at f4, if you set it
    there, whether you are at 17mm or 40mm, or somewhere in between. Of course,
    you can set the aperture at anything from f4 to f22 (I believe) and have it
    stay there. A wide aperture lessens your depth of field, so that
    backgrounds can be pleasantly out of focus, diminishing distracting details
    behind your subject. So, the 24-70mm f2.8L can, but doesn't have to, stay
    at a maximum aperture of f2.8 throughout its zoom range, but the 28-135
    f3.5-5.6 IS will be at a maximum of f3.5 at 28mm, f4 by 50mm, f4.5 at 70mm,
    and f5.6 at 100mm. So you can keep the background more out of focus at the
    long end with the 24-70 than you can the 28-135.
    I know it's confusing, I have a "fixed aperture" 400mm (old Canon) FD mount
    Spiratone "Baseball Bat" telephoto that is permanently set at f8.
    By the way, small sensors like in the S40 give the lenses a greater depth of
    field, so apertures don't make as much difference as they do with the larger
    sensors of DSLRs, or so it seems.
    Skip M, Jan 12, 2005
  5. Derek Fountain

    Mark² Guest

    For a lens that is (for example) a 24-70mm 2.8 L, the "2.8" simply indicates
    that the maximum aperture of 2.8 is available throughout the zoom
    range...which is highly preferable to most consumer zoom lenses that end up
    giving you a reduced max aperture as you zoom toward the tele end.
    Remember, this ONLY refers to maximum aperture, and does NOT indicate a
    limitation of adjusting to smaller apertures. On lesser lenses, you'll see
    a range of apertures given, but that only indicates how much the **maximum**
    aperture changes throughout the zoom range. Most zooms allow larger
    apertures at the wide end of the zoom than at the telephoto end. You can
    always reduce the aperture to whatever the limitations of the lens is
    (usually anywhere from f22-f32, or perhaps f64, up to whatever it's max ap

    The only modern lenses (that I'm aware of) that have a TRULY "fixed"
    apertures (CANNOT be changed) are mirror lenses--which are NOT a good choice
    for most people, due to their strange rendition of background out-of-focus
    elements, and highlights.
    Mark², Jan 12, 2005
  6. The fixed aperture at f4.0 simply means that the aperture is f4.0 at
    17mm as well as 40mm. I do not know of a lens with that short a focal
    length that is that slow. But I do not know everything.
    The 17-40 fixed at f4.0 will also allow 5.6, 8.0 and more than likely f
    11, and mabey f16.
    Jack-of-the-Dust, Jan 12, 2005
  7. Derek Fountain

    Darrell Guest

    The higher priced lenses like the Canon L 17~40 is not a fixed aperture
    lens, it's a constant aperture lens. This means it stays the same through
    the entire zoom range. Example when wide open at f:4 it stays f:4 all the
    way from 17 to 40mm. If I set it at f:8 it stays a constant f:8 through the
    entire zoom range. A less expensive lens can be f:3.5-5.6 so at 17 it's
    f:3.5 but drops to f:5.6 at the 40mm end. The TTL light meter will
    compensate for this, but if I switch to manual, like when I use a studio
    flash I would have to correct for the offset as I zoom.

    I hope I have explained this clearly, if not ask and I can try to simplify
    it more.

    Darrell Larose
    Ottawa, Ontario
    Darrell, Jan 12, 2005
  8. This sentence straightens out my confusion. :eek:} Thanks to you and the others
    who pointed out the (now obvious to me) concept.
    Derek Fountain, Jan 12, 2005
  9. Derek Fountain

    Skip M Guest

    You're welcome!
    Skip M, Jan 12, 2005
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