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focus and your eyes

Discussion in 'General Discussion' started by gmaker1, Oct 11, 2005.

  1. Question:

    When using auto focus lenses, should not the image be in focus even if your eye does not see it that way? What would cause auto focus to be off?

  2. Baywing


    Feb 22, 2005
    CT USA
    Not knowing which camera you are using, the generic answer is that the AF adjustment in the camera may be off and it needs re-adjusting.
    One factor that enters in as where the AF bracket in the viewfinder is pointed and which one is active. Another thought, if yor camera has closest subject priority engaged, the closest object will be the focused one, regardless of where you are pointed. Maybe some details of the exact situation you are talking about will help.
  3. MontyDog


    Jan 30, 2005
    #1064 - You have an error in your SQL syntax;
  4. cwilt


    Apr 24, 2005
    Denver, CO
    Sometimes you can trust AF, and sometimes not. I would suggest first making sure you know where the AF sensors are in relation to the AF brackets in the viewfinder. Some of them on my D2x are not aligned, but it didn't take long to adapt to thier placement.

    on edit:
    Two people more knowlegable than I replied before I could hit the submit button. :smile:
    Last edited by a moderator: Oct 11, 2005
  5. OK guys, let me see if I can be more specific. I have posted last week two imges of swans: https://www.nikoncafe.com/vforums/showthread.php?t=55301

    The image of the full body looks soft and a bit out of focus compared to the second one. Any ideas as to why, or maybe it is just me. I use a D2h.

    Last edited by a moderator: Mar 15, 2017
  6. MontyDog


    Jan 30, 2005
    #1064 - You have an error in your SQL syntax;
  7. Paul:

    I am asking. thanks.

  8. MontyDog


    Jan 30, 2005
    #1064 - You have an error in your SQL syntax;
  9. nfoto

    nfoto Guest

    The brutal truth is that the screens of the DSLRs are very poor for doing manual focusing. It is all too easy to misalign the dioptre adjust of the finder so your eyes focus either below or above the correct plane and thus you will come to an erroneous focus point. In general, if used correctly, most AF systems can achieve at least as good or more frequently more accurate focus than the user can in manual focusing mode.

    That being said, I'm a little exasperated about all the reports of "back" or "front" focusing lenses and cameras. Personally I consider this to be blown out of proportions and that user errors are the main culprit. Or should I rather say, that the user is unable to interpret the function of the AF system and solicit its positive response instead of abusing it.

    For example, if you place the AF function on the release and run the camera in AF-S mode, the poor AF sensor has just a single, tiny chance of getting the correct focus, and it's all to easy for the system to overshoot the target if the situation is the normal, viz. something small in the for- or middle ground, with a remote background, and the focus "target" doesn't extend sufficiently to completely cover the AF sensor area (which itself extends outside the marked region of the finder). So, the target is now 3-dimensional, with depth, and the poor camera has the ungrateful task of maximising contrast within a subject region having three dimensions. It usually ends up distributing the sharpness correctly according to its own criteria, but not to the user who has processed the envisioned scene by his/her own biological and emotional circuitry, and want the face sharp, not the bush in the middle-ground despite the latter is the section of the AF sensor with a maximum of contrast. Or, the AF could latch onto the proper subject, but the user is a little shaky and when the release is pushed further down, the AF sensor loses track of its former target and in a split second, the camera decides to adjust focus again, but the result is not what you want, you wish to have the bird sharp, not the white-water some distance behind it.

    The 17-55 DX has been a current popular lens to bash for a number of focusing issues. My sample is perfect. It focuses precisely where I want to, the way I have set up my D2X cameras. However, by changing a few setup parameters for my D2X, and being sloppy and careless when I shoot with it, it's a piece of cake to provoke an "infinity" issue or "front focus" or "back focus". Does this mean the lens, or the AF system of the camera, is flawed? Certainly not.

    In a statistical sense, a person with his head into the fire, and his feet in a bucket filled with ice, has an average temperature and thus should be feeling comfortable. The AF systems of the camera are like this. They maximise given criteria to arrive at the proper setting of the lens. We must understand those criteria, and how the automated systems manipulate the data. The way we set up our cameras has a major bearing on how successful AF will be. Thom Hogan recently published a very informative paper on AF on his web site, more people should read this.
  10. MontyDog


    Jan 30, 2005
    #1064 - You have an error in your SQL syntax;
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