1. This site uses cookies. By continuing to use this site, you are agreeing to our use of cookies. Learn More.

I can't get into the 43-46mm range with my DA 18-55mm zoom lens

Discussion in 'Pentax Chat' started by psj23, Aug 16, 2013.

  1. psj23

    psj23 Well-Known Member

    I wasn't sure if I should post this in the Lenses Forum, but since it has to do with a Pentax I took a chance and posted here. Well, this is the problem I have:

    I was just toying around with my new Pentax DA 18-55mm f3.5,5.6 (weather-resistant) lens. I haven't had a chance to give it a "test" since getting it a couple weeks ago, so I went out to the backyard and went snap-happy to make sure the mechanics feel OK across the zoom range. Then I noticed that, no matter how finely I try, I cannot get the lens into its 43mm->46mm focal range; it skips from 42mm to 47mm given the info. I get upon reviewing the pics. I have tried everything to get into that range but the result is always the same. I have no other zoom to compare it with so I was wondering if this is normal behaviour for this particular model. I'm pretty close to asking for an exchange.
     
  2. Bob Maddison

    Bob Maddison Well-Known Member

    Re: I can't get into the 43-46mm range with my DA 18-55mm zoom l

    Although I am not familiar with your camera / lens I would suggest that this is normal behaviour. Just think about it, the information sent from the lens to the camera is limited, and focal lengths of a zoom lens are nominal anyway. You must also remember that when any lens with internal focusing (and this includes all zoom lenses) the focal length is changed. Therefore the inforamtion given to the camera is 'stepped'. The FLs you are seeing are the nearest practicable values.

    All that really matters is that the lens physically focuses smoothly across its full range. In fact, what is more important is that you are able to frame the scene in the way you want regardless of FL and focal distance. With an optical viewfinder, accurate framing is rare and it usually shows a slightly narrower view than the fianl image thus giving you the option of a little cropping. An optical view finder and live view is usually accurate as it represents the scene as seen by the camera's sensor.
     
  3. psj23

    psj23 Well-Known Member

    Re: I can't get into the 43-46mm range with my DA 18-55mm zoom l

    I really should have been more specific: The lens does not report focal lengths in that range. Yes, it does AF in that range. Sorry about that.
    Also, you say zoom FLs are nominal. Why is that?
     
  4. Bob Maddison

    Bob Maddison Well-Known Member

    Re: I can't get into the 43-46mm range with my DA 18-55mm zoom l

    Most lenses only have their nominal FL at infinity. Many zoom lenses are only true zooms at infinity: as soon as you shift the focal length, there is a small change in focus which is compensated automatically by AF. I have a couple of lenses that have a very obvious shift of focus if I focus long, lock focus and then take a photo at a short FL.

    Even a prime lens isn't immune from this. for instance a 50mm lens might have an actual FL between 48 and 52 mm although most are better than this. As a physicist (retired) I grew up with the concept that nothing is perfectly accurate and that we must live with some errors, great or small, in any measured or stated parameter. In most cases this is good enough, but for some scientific work it is important to know just how accurate a measurement really is.

    As a simple example, I have a 170 - 500 mm lens. When I set the FL scale to say 250mm, I cannot read that scale to better than perhaps +/- 20mm. Nevertheless the FL recorded by the camera might be more precise and is probably better than +/- 10mm and will probably not differentiate between say 195 and 205 mm. Even then, that FL is probably only about right at infinity and could change by a few mm when focused nearer. It simply doesn't matter.

    Any lens could be made to display its FL in the EXIF file to perhaps +/- 1 mm. However, it would cost a LOT, and to no real purpose.
     
  5. psj23

    psj23 Well-Known Member

    Re: I can't get into the 43-46mm range with my DA 18-55mm zoom l

    Well stated & clear. Thank you!
     
  6. Bob Maddison

    Bob Maddison Well-Known Member

    Re: I can't get into the 43-46mm range with my DA 18-55mm zoom l

    If you want to experiment and see just how your camera records the lens FL there is an excellent little (free) program "ExposurePlot". Just take a long series of pics moving the lens zoom control a small amount between shots; the smaller the better. Now upload those photos to your computer and into their own folder. Open ExposurePlot and open that folder. The program will show you the lens apertures, shutter speeds, ISOs and, importantly, the FLs used. The results are in the form of a histogram and you can see just how many steps are used in recording the FLs. Ideally this should be equal to the number of pics taken, but it will probably be a lot less if your FL steps were very small.

    If you used a fixed shutter speed and auto exposure, assuming the light was constant, it will also show you how the lens aperture varied as the lens was zoomed: unless you are using an expensive zoom lens with a constant maximum aperture!
     

Share This Page