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Soft focus on Nikon D300s-using 70-200 2.8 zoomed at higher focal lengths

SusanSusan Member
edited March 2012 in Nikon
I am having issues using my Nikon 70-200 2.8 lens - especially when zooming at higher focal lengths.
I have my camera set to manual mode... AF-S and single point AF on the back of the camera..
I have used focus lock (shutter half down) and re-compose and wondered whether this was my issue.... maybe when I recompose I am out of the range for the face????
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  • http://www.digitalbirdphotography.com/6.7.html

    image

    One of the dangers of focus-and-recompose. Left: the camera is (auto) focused on
    the bird, defining a focus plane that contains the subject. Right: after recomposing (rotating
    the camera through a small angle), the focus plane no longer contains the subject, so the
    bird may be slightly out-of-focus. If the DOF is large enough, relative to the angle of rotation,
    the bird will still look sharp.
  • http://www.mhohner.de/essays/recompose.php

    Many AF cameras contain not just a single AF sensor but several of them spread across the frame. However, the central sensor often is of higher sensitivity, larger or in other ways special. So many users of these cameras do not select one of the other sensors when the main subject is not in the center of the frame. Instead they use the following method:
    1.Select central sensor
    2.Focus with the central sensor pointed at main subject
    3.Recompose with AF locked
    4.Release

    The result is often that the main subject is slightly out of focus, and the blame is put on the camera. However, this focus issue is a direct result of recomposing and is not the camera's fault. Here's an illustration of the situation:

    image

    The camera is at the base of the diagram. It's first focused on point A. After focus is acquired, everything on the plane of focus that touches point A is in focus (blue line on the right). Then the photographer recomposes - with locked focus - so that the camera points to point B. The focus distance does not change because AF is locked. After that, everything on the plane of focus touching point B appears sharp (blue line on the left). But now point A is no longer on the same plane of focus. The plane of focus that would be correct for A is closer to the camera (solid black line). The amount of misfocus is d (red line), and it becomes larger when you recompose farther. It looks like the camera has a backfocus issue. It also looks like with wide-angle lenses the problem is bigger than with tele-photo lenses. However, this kind of focus error is actually a user error, not a camera problem, and wide-angle lenses just give the user more opportunities to make this mistake than tele-photo lenses (because you can recompose more before the subject slips out of the frame).

    There also is a secondary problem when recomposing strongly. The cameras mentioned above often also base exposure and flash exposure on the selected AF sensor, assuming it points at the main subject. So the metering cell(s) close to the selected AF sensor are given priority over the other cells. But when you recompose, the selected sensor no longer points at the main subject, and you may end up with incorrect (flash) exposure.

    The lesson to be learned here is to always select the AF sensor closest to the main subject, even if this sensor has a weaker performance than the central sensor. Recomposing by a significant amount should be avoided as much as possible, for accurate focus and correct exposure.
  • Susan, some good info on rotational POV problems with AF, but the problem could be the result of a myriad of issues ....

    The first thing you need to determine is that you don't have a malfunction with the equipment you are using. Try mounting the camera on a tripod and shooting something at the zoom range and focal range similar to where you are having problems. Use a target that has fine detail. By using a tripod, you eliminate the possibility that your soft focus isn't actually coming from camera shake ... remember that a 70-200 on the D300s is actually capturing at the equivalent of a 300mm lens ... you didn't mention the shutter speed used, if it is below 1/300 sec. and you aren't bracing your grip on the combo ... that could be your problem ... If in this test, the camera/lens combo is offering tack sharp results, then your problem is likely technique related.

    Secondly ... learn to use the back button for AF activation (there is a custom menu setting for this) instead of the half-press shutter release to lock focus ... simply use your thumb to invoke AF, remove your thumb and the focus is locked ... thereby removing the possibility that something is going a rye with locking focus ... After over 20 years of using the shutter release for activating AF, I made the move to the back AF button and have to say that separating the tasks, has improved my AF capabilities dramatically ... It was quite awkward at first, but I wouldn't go back now.

    Thirdly ... if you find out you have a bit of AF focus softness at certain zoom ranges, you could have the camera and/or lens checked for calibration ... or you can actually dial in AF focus using a setting on the D300s ... this takes a bit of trial and error to set up, but once you set it, it works quite well. It also is lens independent ... the adjustment you make will only be applied to that specific lens when it is attached. Your other lens will not be affected by that adjustment.
  • Hello Susan,

    what everyone is saying about focus and recompose is true but if ur zomed in to 200mm it shouldn't make to much of a difference, i had the same issue as u, see my thread http://neilvn.com/forum/discussion/512/sent-it-in-twice-but-it-still-back-focuses#Item_10 on my d7000 with my 70-200 after much testing i set my cameras micro adjustment to -15 for that lens,

    just went out shooting yesterday and it seems pretty sharp now this image is taken at 1250 of a sec f2.8 image
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