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Old Jan 12, 2011, 08:26 PM
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United States, MA, Walpole
Joined Dec 2003
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Some Lens Focusing Tips For the Discriminating User

I've gotten to the point where my final lens tweaking will be done during a bright sunny day. According to the weatherman, we can expect one here sometime about mid-March, so I'm going to post what I've found helpful for me to get that "best" focus. But I'll give fair warning you have to be anal about it to do this!

I think most of the camera videos I've seen posted here show the lens comes fairly well focused from the vendor, so unless you really don't like what you see, my best advice is not to bother with refocusing! I'll skip the part about removing the glue on the lens so it can turn and taping over all exposed circuit board components to avoid shorting and static discharge, and get right to the actual focusing nitty gritty.

I could NOT convince myself I had the best focus by viewing a web cam display on my PC because of too much motion while hand holding the camera, difficulty in turning the lens and not blocking the view at the same time, and no way to really compare one setting with the prior one at both far and near distances other than by memory. But YMMV, and this may be good enough for you. My goal was to optimize distant object focus (for AV purposes), while getting the depth of field as close in as possible before the distant focus deteriorated. And good focus across the whole video frame if possible.

I used my focus as received since it was pretty good and a good starting point, then I marked one of the "indent valleys" in the knurled ring on the lens barrel with paint (so it doesn't scrape off while focusing), with a matching line on the fixed base. There are twelve of these valleys, so they are 30 deg. apart, and I'll call the rotational angle between two valleys in the following procedure as one "notch".

Here's what I did:
  • Find a fixed outside scene with both far and near objects for comparison. I like bare tree limbs several hundred feet away as a minimum for distant objects, and just about anything for up close. Also have a spot where you can hold the camera steady and aim the camera the same during test video clips.
  • Take a short video, holding the camera steady on a distant object, a mid-range object, and a close-in (e.g. 8-10 ft. away) object. Each time you shoot a clip, save and rename it so you can tell the lens position from the name.
  • Rotate the lens one notch clockwise (CW) to move the focal point further away from the camera, and shoot a similar video.
  • Repeat the process with the lens rotated one notch counter clockwise (CCW) from the original to move the focal point closer to the camera..
  • Compare the CW video with the original, then the CCW locaton with the original. To do this open one rotation video and the original separately with the AviDemux editor, so you have two editor screens arranged side-by-side and can step through the clips frame-by-frame to get still images you can now very easily compare for focus on far, middle and near objects. Do the same comparison of the original position with the opposite lens rotation video. You should now be able to determine which direction of rotation should be fine tuned. If you can't, maybe the lens is focused really good in its original position and you can reposition it there are done!
  • If you can see a difference, repeat the process on that rotation side with a smaller rotational increments. I stepped through iterations of 1/4, 1/2, and 3/4 notch positions, comparing each with the prior (I told you I was anal!).
  • With this process, you should be able to find two lens positions where the focus appears about the same in both.
I finally found that I could see virtually no significant difference between the 1/4 and 1/2 notch CCW positions from the original location on my camera. It's interesting when you step through frame-by-frame to see how the H.264 compression treats the over all image, with a small area in the video appearing to go into and out of focus and back again from one frame to the next. So it becomes tough to decide which of the two clips is in better focus overall with these small lens rotational increments, but that just means you are pretty much at the sweet spot. I found three positions gave pretty much the same distant focus, with very slight differences in the close focus point. So my final lens resting spot will be based on which of three gives the better close focus. I'll finish that when a sunny days appears. I also saw that good focus in the lower left corner of my videos was almost impossible to achieve, so the cheap lens is showing it's faults there.

Good luck with your focusing efforts!
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Last edited by Tom Frank; May 04, 2011 at 09:08 PM.