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ND filters + lock focus

edited March 17 Posted in » Nikon D3100 Forum
Hey Moose and Bruto. I am sold on the Manfrotto xume suite and I am going to order it in the next few days. I'm starting with a B +W 10-stop filter for shooting long exposures of waterfalls.

I'm having trouble though locking the focus on my D3100 for long exposures. I am referring to the manual on pages 37-39. I just want to lock focus not exposure and I'm not sure how that's going to work in bulb mode. For now I have been manually switching off the Auto Focus and VR by hand but I keep on bumping the camera just enough to lose focus. Any suggestions?

Comments

  • edited March 18
    You should turn off VR before you focus, and that switch will then not be a source of vibration. Once the shutter opens, both exposure and focus will be locked, so you need worry only about what happens before you shoot. If your camera is set to AFS, it should stay locked after initially hitting focus, if you don't release the shutter.

    If you're using a remote, you will probably have either to turn AF off, or to use the exposure lock (EDIT TO ADD...I mean the AF/AE lock button, to lock focus) button. On a long exposure, there should not be any issue with jiggling the button on release if you're careful. A little initial jiggle will not make much difference in a long exposure, and your depth of field should be enough not to worry about focus shift.

    One other way of doing this, other than manually turning off AF, would be to go to back button focus. In this case, you'd go to the "buttons" setting in the setup menu, and set the AE/AF lock button to "AF ON." When this setting is enabled, the shutter button will no longer work the AF, and AF will be performed by pressing the back button. Once you let go of the back button, focus is locked wherever you left it.

    As I recall, the D3100 does not accommodate the infrared remote control, so the odd behavior of the D3200 is not likely to be a factor, but on the D3200 the infrared remote disables back button focus entirely. I don't know whether this is also the case for the plug-in remote the D3100 requires, but you may have to experiment, because if the same thing happens, you'll have no option but to turn off AF or hold the focus lock button. On the D3100 (EDIT TO ADD- I mean the D3200), as soon as you push the IR remote button, it tries to refocus, and of course with a dark filter, it will fail. The only way to prevent this is to turn off AF.

    Again not quite sure how the D3100 behaves, but the D3200 also does not have a true "time" exposure function, and that means that for longer exposures than 30 seconds, the shutter button must be held down throughout, which is a likely source of vibration. The way around this is to use the remote, which opens the shutter on the first push, and closes it on the second - a true "time" setting that works well. But since this also messes up the back button AF, you must turn off the AF before you shoot. You'll have to experiment a little if you use a remote, as I'm not quite sure how it will behave. I'm guessing that the same time exposure is available with the D3100 remote, but not sure how it handles back button focus.

    If you're using a tripod, and manual focus, one option might be to switch to Live View. In LV, it's sometimes easier to nail manual focus, as you can magnify the viewfinder a bit. If you can turn off AF and manually focus before shooting, you should be fine.

    There is another undocumented anomaly in the D3200, which can be useful or annoying depending on your circumstances. In Live View the usual "focus priority" in AF is modified. As you undoubtedly know, in viewfinder mode, when using AF, if focus is not found, the shutter will not fire. In Live View, this is modified. It will seek focus, but if it does not lock, it will fire the shutter anyway after a second or so. That can be handy, but it also means that in the dark you may think the AF found focus when it did not. You can test this in the D3100. Put it in Live View, AFS, and try shooting with the lens cap on. The D3200 will fire in LV, but never in Viewfinder mode.

    If you're using a tripod you should be all right with a little jiggle. Just let the camera settle a moment before hitting the shutter. If you're using an exposure under 30 seconds, just set the camera up, focus, and use the self timer. The delay will be enough to let it settle. The self timer works fine with back button focus as well. On the D3200, the only limitation here is that the self timer does not work at all in Live View and AFF auto focus. It starts but never fires. But it works fine in AFS, and in all viewfinder modes.

    If you're setting the camera on a railing or a rock, I strongly suggest you get at least a cheap table tripod, or perhaps a Gorillapod, and that will solve any problems of accidentally jiggling out of focus if you're careful. Promaster makes a neat gadget that works as either a table tripod or a clamp, so you can set it on a surface or clamp it to a railing, branch, etc. Very handy for traveling.
  • edited March 19
    As always, great info! Thanks!
  • edited March 19
    Thank you!
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