D50, 50mm 1.8, AF Points and Focus

Discussion in 'Nikon' started by rob_piontek, Apr 7, 2008.

  1. I have long thought that my D50 and 50mm 1.8 do not autofocus very well
    together. Normally I'm using the single point AF mode, put it on my subject,
    focus, reframe, take the picutre. The faces of my subjects, where I put the AF
    point, often seem softer than they should be. It's not really bad, but
    consitently bad enough to make me think that somethings not quite right.
    Especially when occasionally the stars align and I get a really sharp image with
    this lens. I know that there are many things that go into making a sharp image,
    i.e. wide open most lenses aren't that great, especially the closer you get to
    the edges, very small depth of field wide open for an f1.8 lens, not shooting on
    a tripod, etc.... so it's always just been a suspicion.

    Last night I finally did some tests to try to figure this out. I didn't
    completely convince myself, but what seems to be the case is that the center AF
    point works better than the others. When I use the other AF points, the focus
    point the camera chooses seems to be a bit farther away than it should be. It
    seemed to be pretty consistent. I'm curious if anyone else has noticed similar
    issues.
     
  2. Yep, normal. Center AF point is more sensitive than the other points. Its also known as a cross-type sensor, it can detect vertical and horizontal lines, and the others are vertical-type sensors which detect horizontal lines.(yeah, horizontal)
     
  3. I'm curious if anyone else has noticed similar issues.
    Yes, but only in the worst-case situations. When I am in very close and wide-open, I get the same results that you have experienced. I don't know if this is from a conflict in the accuracy of the AF points, or it is from the lens itself, which may have some field curvature (while close-focused and wide-open), but I have been able to see this effect while I was testing my 50mm f/1.8 AF on a tripod with a multiple distance subjects.
    From my test, the center is sharper than the edges when trusting the AF points to confirm focus.
    In the real world, at moderate distances, hand-held, 3D sunjects, this is mostly a non-issue. I have many good shots with the lens used wide-open for off-center composed subjects.
    00P4l4-42764084.JPG
     
  4. ... And center point. The CD case sticker is very sharp, and can be read with ease when viewed at 100% magnification.
    00P4l8-42764184.JPG
     
  5. I notice that too with my D50, The outside points hunt more so than the center one. It's a
    bummer, but not much can be done.
     
  6. <<I'm using the single point AF mode, put it on my subject, focus, reframe, take the picutre.>>

    Possibly your problem may be due to focus and then reframe. Reframing can place the focus point at a significantly different distance from the frame center.
     
  7. Albert - nice test! The center CD definitely looks sharper, but it should be easier to focus on because there is more depth of field at the larger distance, right? Also, the right hand CD seems to be on an angle, so it's not clear to me what should actually be in focus.

    Mike - that's definitely true, but the camera - subject distance does not change, so the only issue would be whether the lens gets significantly softer as you move away from the center. Isn't this a standard technique of focusing?
     
  8. ...should be easier to focus on because there is more depth of field at the larger distance, right?
    Well, not posted was the shot of the rear/left most CD which was also not as sharp as the center one. I tested all three points across the frame, but only posted the two above to make my point without clogging the thread with photos.
    Also, the right hand CD seems to be on an angle,...
    The CD case may appear to be angled, but when I set up the test, I made sure that all of the case fronts were at the same angle, square to the film plane. You may be looking at the spine of the case which is angled, since the "V" shape of the slightly open case allows the case to stand. The front of all of the cases are all square to the camera.
    Again, this is all basically overthinking the potential (or limitations)of the lens. In the real world, we don't usually set up this perfect positioning of a subject. The 50mm f/1.8 is a fine lens for real subjects in an environment.
     
  9. Lenses are sharper in the center than nearer the borders. Take a look at the test results on Photozone for this lens as an illustration, especially wide open. Perhaps what you're seeing is this loss of sharpness rather than incorrect focus? Just a thought.
     
  10. I have this same setup.. D50 and a 50mm 1.8. I have to say this lens is the best one I own. The pictures are a little soft, but not out of focus soft. See below. I just attribute this to the higher ISO I tend to use and that I always have this thing on 1.8. I wonder if yours might be defective, honestly, but do first try focusing with the center focus spot and reframing -- that's what I always do. I've been satisfied.
    [​IMG]
     

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