Sigma 20,24,28/1.8 primes

Discussion in '35mm Cameras' started by brian, Apr 3, 2004.

  1. brian

    brian Guest

    True varifocals are essentially extinct now because virtually all
    designs incorporate one or more mechanical compensators to maintain
    focus. The days of optically compensated zooms with their inherent
    focus drift are long gone. However, in mechanically cammed zooms you
    can still get focus drift due to small manufacturing errors.
    Adjusting the endpoints is fairly easy, and is even done on very
    low-cost optics. However, you can still get drift between the
    endpoints. The only way to correct this in a purely mechanical zoom
    mechanism is to re-cut at least one of the cams, but to my knowledge
    this level of precision is only lavished on cine zooms. Smaller
    format digicams usually use stepper motors to control the zoom group
    motions, and here you can adjust out any focus drift using firmware.

    brian, Apr 3, 2004
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  2. brian

    Lewis Lang Guest

    Subject: Sigma 20,24,28/1.8 primes
    Hi Brian:

    Which zooom designs are true zooms throughout the zoom range? Is my Maxxum
    24-50mm/4 A lens a true zoom? Are the 28-70/2.8 and 24-70/2.8 and 16-23/2.8 and
    17-40/2.8 Canon EF L zooms true zooms throughout their range? Is their a source
    page for this type of data/info about a lens(es) that you know of?

    Lewis Lang, Apr 3, 2004
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  3. Varifocals are still quite common. In order to be a true zoom, a lens must
    be parfocal. One fairly obvious implication of that is that a true zoom lens
    must have the same near focus distance at all focal lengths. But there are
    many so-called "zooms" that have different near focus distances at different
    f.l.s. There have been some for autofocus SLRs, and they're very common in
    point-and-shoots (where there's little if any reason to have a true zoom).

    Neil Harrington, Apr 5, 2004
  4. brian

    brian Guest

    Naturally, it all depends on your definition. You have a point about
    parfocality, although your resulting "true zoom" definition is more
    strict than the one I normally use. I think of a varifocal lens as
    having a wandering focal plane position through-zoom when the object
    is at infinity. Of course, if you focus a zoom by moving the front
    group then you automatically maintain parfocality, and many systems
    still use this method.

    brian, Apr 5, 2004
  5. brian

    brian Guest

    Hi Lewis:
    Virtually all zooms are now designed to be true zooms (at least for
    infinity focus). However, manufacturing errors can easily cause a
    slight but noticeable drift in the infinity focus position. As Neil
    has pointed out, a number of these may not be parfocal through zoom,
    in which case you may have little or no drift at infinity, but will
    see it increase for close focusing distances.

    I've never seen any documentation about production lenses. Your
    24-50/4 is probably a positive-negative two-group design with front
    group focusing. If this is the case, then it will be a true zoom
    design both at infinity and for all other object distances. The other
    lenses you mention most likely have a
    negative-positive-negative-positive zoom group configuration, with
    more complex focusing arrangements. Bear in mind that this is just an
    educated guess. My 17-35/2.8 Nikkor has this latter configuration,
    and it does have a barely detectable focus drift in the middle of the
    zoom range as well as a slight loss of parfocality at extreme close
    focus. Both could be within manufacturing tolerances as far as I
    know; image quality leaves nothing to be desired, however.

    brian, Apr 7, 2004
  6. brian

    Lewis Lang Guest

    Hi Lewis:
    Hi Brian:

    Thanks for your comments. Doesn't the focus drift of the 17-35 Nikon cause
    slight errors which can become larger focusing errors when you blow up to large
    print sizes when the lens is used at or near wide open? How large are the
    prints you're making? Do you do manual focus touch ups on your focus with this
    lens after you autofocus with this lens because of the focus drift?

    Thanks and TIA
    Lewis Lang, Apr 7, 2004
  7. I meant parfocal within reasonable tolerances, of course. I have had (though
    not in recent years) zoom lenses with tolerances that were unreasonable.
    :) If the lens is designed to be a true zoom I'd accept it as such, even
    if its focus actually drifts slightly through the zoom range.

    That seems reasonable too. One interesting lens in this connection I think
    is Minolta's Maxxum 28-105mm f/3.5-4.5. In its original xi form (power zoom
    as well as power focus) it was a varifocal, and described as such in the
    literature. The xi series lenses evidently were troublesome mechanically,
    and the 28-105/3.5-4.5 was revised in the si (manual zoom) series, when it
    also became a true zoom. But it seems to be the same optical design as the
    original varifocal. So I presume from this that the same basic design can be
    made either way, varifocal or zoom.

    What I find intriguing are the little cheapo "zooms" in point-and-shoot
    cameras like Minolta's Freedom Zoom series. Minolta gets a remarkable
    37.5-160mm range out of a very small six-element lens (albeit at a rather
    grim f/5.4-12.4). It's hard to tell what such a lens is doing when you can't
    SEE it doing anything in the viewfinder, and besides it only does the actual
    focusing after the shutter release is fully pressed. But the fact that its
    near focus is 2 ft. at the short end and 3.1 ft. at the long end makes it
    clear that it's a varifocal, and there wouldn't seem to be any point to
    having a true zoom in such a camera anyway. I have a couple of these, and
    within their obvious limitations it's just amazing to me how well the darn
    things work (though for how long is a question, I suppose).

    Neil Harrington, Apr 7, 2004
  8. brian

    brian Guest

    Hi Lewis:
    I never rely on the focal plane to stay put throughout the zoom range
    because, as I've pointed out in other threads, only high-end cine lens
    manufacturers really get this right. It doesn't mean that still
    camera zooms are bad, just that none of us could afford custom cams
    for each lens. Autofocus makes focus drift even less of a problem
    than it is with pure manual zooms.

    brian, Apr 7, 2004
  9. brian

    Lewis Lang Guest

    Hi Brian:
    Hi Brian:

    Reason I ask you this is because my habit is to zoom in, autofocus, then zoom
    out again for framing - AF then is still aproblem if you don't AF again after
    the zoom in then zoom back out again, you'd have to AF each time you changed
    focal lengths and this is not nearly as accurate as having a true zoom still
    photography lens that would only need to be AFd once at longest focal length
    since the longer the focal length the zoom is AFd at the more chances of
    accuracy when you zoom out again for framing, ie. zooming into 105, AFing at
    105 then zooming back to 24mm is probably going to be alot more
    accurate/selective than just AFing at 24mm and hoping the camera's AF system is
    ultra accurate and/or ultra good at exact focusing. AF is quick, but its not so
    precise thatit can't use a little help from zooming in first, AFing, then
    zooming back out again ;-)
    Lewis Lang, Apr 8, 2004
  10. brian

    brian Guest

    Hi Lewis:
    I would definitely not trust focal plane stability through zoom more
    than autofocus accuracy until you've done some very careful testing.
    In fact I would expect AF to be more reliable. As I've pointed out,
    the focal plane stability is something that will vary significantly
    from lens to lens - much more so than aberration correction - and no
    consumer still camera lenses will be completely corrected for this.

    brian, Apr 8, 2004
  11. brian

    Lewis Lang Guest

    Subject: Re: Sigma 20,24,28/1.8 primes
    Hi Brian:

    Actually its not focal plane stability that concerns me most, but AF sensor
    accuracy and subject type I'm concerned with. Focal plane stability can be
    tested quickly and accurately in MF mode at different fl of the zoom. My real
    concern is with the AF sensor's largeness of coverage, its accuracy and

    The fact is that most subjects I shoot are 3D and when you focus on them with a
    24-50 (my zoom's focal length) at 24 (or somewhere else in the wide angle
    zone), the AF sensor will take in a much larger area, usually focusing on not
    the exact part of the subject I want but some other plane/angle within it. When
    I zoom out to 50mm, AF, and then zoom back again for framing, the sensor (with
    my careful aiming) is able to make slight AF tweaks, whereas if I just AFd in
    wide angle, when I double check focus by merely zooming in, the focus is
    minisculy off. This is not drift but the nature of the subject itself (3D with
    many different planes and/or receding from one plane to another) and the fact
    that at wide angle the AF sensor is simply covering slightly too much area. A
    common situation where this AF discrepancy due to the sensor covering too large
    an area is portraiture where a lens will most lekely AF on an eyebrow ridge or
    bridge of the nose instead of an iris or a pupil. At or near wide open
    apertures with little dof exact /precise placement of the AF sensor is amust
    for this kind of subject and is hard to achieve even by centering the AF sensor
    over the small part of the in-depth subject (eye area of the face) and
    recomposing. In this case it would behoove one to switch to manual focusing and
    focus on the exact part of the ye you want focused (pupil, iris and white,
    eyelash, whatever). However with less restrictive situations than close up (5
    feet or less) portraiture (especially at 85mm and longer zoom focal lengths)
    then having the ability to zoom in at the longest fl first, before AFing, with
    a skilled operator who knows not only exactly which part of the 3D subject to
    AF on but the 3D part within the 3D area of the subject to AF on, this would be
    the best route to go. Its the same principle as those who MF with eyepiece
    magnifiers. The more you magnify the area of the subject to be focused on the
    more focusing accuracy you have. By using the zoom as a built-in magnifier, but
    on the picture taking end as opposed to the eyepiece, then AFing, then zooming
    back out again for proper cropping/framing, this will ensure (given a true zoom
    has been tested to be parfocal all along its range) much better results than
    just AFing at whichever wide end focal length you desire without this "trick"
    (unless you are photographing flat and/or far away objects in which case the
    nature of the subject's flatness and/or depth of field will cover most slight
    AF sensor focusing errors).

    So given the above, I would expect AF sensors to be _less_ reliable, or at the
    very least, with a close by 3D subject, not nearly as accurate or repeatable
    without doing the zoom in, AF, Zoom out for cropping "trick" mentioned above.
    Try it yourself, even a perfectly parfocal zoom may/will focus at slightly
    different areas of the subject when focused at wide angle. When you focus at
    wide angle then zoom in, most likely you'll see a slight focus discrepancy that
    can be improved by either MF tweaking and/or the AF tweaking at the long end
    that should have been done in the first place A.K.A. "the trick" I've been
    mentioning. Even an expert pitcher, while always getting the ball he throws
    into the catcher's mitt, might strike different areas of the catcher's mit when
    the ball lands. Subject planes receding and/or receding at different angles
    and/or a number of subject planes at different depths w/i a specific area of
    the subject (as per given in my portrait example) would make the use of zoom
    in, AF, then zoom out as necessary as the old AF then recompose trick of a
    person who has only one central AF sensor.
    Lewis Lang, Apr 8, 2004
  12. brian

    Deathwalker Guest

    well i just skipped to the end of this unfeasibly large thread. and all i
    can say is "wot a load of bollocks!"

    zoom lense, multifocal vari wotsit titwank super aperture thingybob

    if its not a prime lense then you are going to have make a sacrifice of some
    kind somewhere and increase the complexity of the design. If a company
    claims to make zooms every bit as optically good and as fast as their primes
    then they are making 2nd rate primes.
    Deathwalker, Apr 8, 2004
  13. These are all prime lenses we have been talking about, whether FFL, zoom or
    Neil Harrington, Apr 8, 2004
  14. brian

    Deathwalker Guest

    ffl zoom or varifocal are not primes. see above.
    Deathwalker, Apr 9, 2004
  15. Apparently you don't even understand the terms you're using. Read the entire
    Neil Harrington, Apr 9, 2004
  16. brian

    TP Guest

    Don't you know which photo group he works for?

    If you do know, why do you appear so surprised?

    TP, Apr 11, 2004
  17. No, sorry, I don't know anything about him. Which photo group does he work

    Neil Harrington, Apr 12, 2004
  18. brian

    TP Guest

    TP, Apr 12, 2004
  19. Uh . . . Can I buy a vowel? :)
    Neil Harrington, Apr 12, 2004
  20. brian

    TP Guest

    "Consonant, please Carol!"
    TP, Apr 13, 2004
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