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noobish question: fixed-aperture (zoom) lenses

Discussion in 'Open Discussion' started by sprinke, Aug 23, 2011.

  1. sprinke

    sprinke Mu-43 All-Pro

    Apr 5, 2011
    Pasadena, CA
    I've been puzzling over the concept of fixed-aperture zoom lenses. Am I correct in saying that those lenses cannot be stopped down at all, and therefore you cannot adjust the depth of field? And you would have to control exposure through shutter speed and ISO only?

    I'm wondering what the applications of these lenses would be. Are they particularly suited for certain kinds of photography?

    Just curious. Thanks.
  2. songs2001

    songs2001 Mu-43 Top Veteran

    Jul 8, 2011
    no it just means the maximum aperture is the same throughout the zoom range. for example a 70-200 2.8 will have a max of 2.8 whether at 70, 100 or 200, you are also able to stop down the lens as well.
  3. Bokeaji

    Bokeaji Gonzo's Dad O.*

    Aug 6, 2011
    Austin, TX
    It's the largest aperature you can use for it's entire zoom range
    Say it's an f4 zoom 20-100. You could be as wide open as f4 at 20 and all the way to 200.
    You can also stop down to whatever else you'd like. F5.6/8/16/whatever
    You just can't go to f2.8/2 etc

    Sent from my iPad using Mu-43 App

    You beat me! I was one finger iPad replying. No fair! ;) 
  4. WT21

    WT21 Super Moderator Subscribing Member

    Feb 19, 2010
    The advantage to fixed aperture zooms is that you can take a meter reading and set your exposure for your subject, then regardless of how you zoom in, your shutter speed isn't changing.

    For instance, let's say you have a subject in them middle of the room who is well lit, and he (or she) is in a dance competition. They are moving all around the room. The BACKGROUND behind the subject may be varying -- dark and light. You could decide you like ISO 800 1/250 second and aperture 4.0 for your shots. Zooming in and out would draw in more or less of the background, which might change the OVERALL exposure of the scene, but not the exposure for the SUBJECT, who is in the middle, and well lit. If your zoom is a constant f4, then you can be confident you can stay on ISO800 (to avoid more noise) and 1/250 to get good motion capture.

    If, however, your zoom collapses from f/3.5 on the wide end to f/5.6 on the long end, then when you zoom in, your subject will be too dark at ISO800 and 1/250 a second. So that means you have to let your camera set the exposure. If your camera is setting the exposure, it may be seeing more of the varying background (light and dark) and alternatingly blowing out or darkening your subject, because the camera takes an overall reading. It doesn't really know what your subject is (you can try and use spot metering, of course, but that has other issues, too, especially if the action is fast moving).

    A related example would be if there are many dancers and you are taking pictures of everyone. Some dancers have dark dresses and some light. The lighting conditions in the room aren't changing, but if you are getting in many dancers with mixed dresses you might get one meter reading, vs. zooming in close on one that has a dark dress. With a fixed aperture, your exposure doesn't have to change (remember, in this case the lighting isn't changing), and you'll get the correct exposure. With a variable aperture, the camera has to be left to choose the exposure, and if you zoom in on a dark dress, the camera will try to push the dark dress to middle grey, and create a poor exposure that you'll have to alter in post.

    So, for good pro shots in a place with fairly constant lighting, a constant aperture is very handy. You can take exposure adjustments completely out of the equation. For most amateurs it's not that important, but could still be handy.

    BUT the thing most people like is that, generally, constant aperture in a lens is correlated with relatively bright lenses. No one makes a constant f/6.3 zoom (well, not anymore anyway). So, constant apertures (like the 7-14) are a good indicator of the manufacturer putting in some quality to the lens.

    Wrote this quickly. Hope it makes sense.
    • Like Like x 1
  5. sprinke

    sprinke Mu-43 All-Pro

    Apr 5, 2011
    Pasadena, CA
    Thanks all. Obviously I had it all wrong. Those lenses sound pretty cool. Do they tend to be more expensive?
  6. Luke

    Luke Mu-43 Hall of Famer

    Jul 30, 2010
    Milwaukee, WI
    the confusion comes in when people read fixed aperture zoom and think that the aperture can not be changed. It just means the the maximum aperture is constant throughout the zoom range. Most inexpensive zooms get "darker" as you zoom in with a higher maximum f stop.
  7. Luke

    Luke Mu-43 Hall of Famer

    Jul 30, 2010
    Milwaukee, WI
    we just crossed posts..... their designs are more complex to do them well and involve more glass. They are heavier, almost always better quality, almost always "brighter" (and almost always more expensive).

    But if you don't mind manually focusing (which I think you don't) there are tons older zooms with constant apertures that you could mount on your m4/3 cam.
  8. nickthetasmaniac

    nickthetasmaniac Mu-43 All-Pro

    Jan 11, 2011
    Yes - significantly :smile:
  9. Most fixed maximum aperture zooms tend to be fast, high-grade lenses. The cost of the lens is not entiry attributable to the fixed aperture, but it is a feature that is often only found on lenses that would have been expensive regardless.
  10. ~tc~

    ~tc~ Mu-43 Hall of Famer

    Oct 22, 2010
    Houston, TX
    X2 - as you can see from the post above, they are targeted at very serious photographers.

    Note that they are almost always choked on the wide end. For example, on the Panny 7-14 f/4, the diameter of the lens opening at 14 mm f/4 is the same as 7 mm f/2. This has the effect of stopping down the lens, and you get the usual IQ benefit.
  11. I have an old Tokina 28-70 zoom where you can see this happen. At the maximum aperture (f/4) the iris is circular at 70mm and hexagonal at 28mm. I don't quite know how it is handled on more modern lenses where you don't see any effect quite so dramatic as this.
  12. Ned

    Ned Mu-43 Legend

    Jul 18, 2010
    Alberta, Canada
    Just as a side note, there are actually lenses which have a fixed aperture the way you were thinking. They're called Mirror Lenses. :) 
  13. Howi

    Howi Mu-43 Veteran

    Feb 23, 2011
    beat me too it Ned, must learn to read faster......
  14. GaryAyala

    GaryAyala Mu-43 Legend

    Jan 2, 2011
    LensBaby's and Holga's also have fixed apertures (and my Canon 50mm f/1.8 after it bounced off the pavement).
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