Question about micro focus adjust procedure

Started May 18, 2012 | Discussions thread
Jerry-astro Forum Pro • Posts: 10,806
Re: Thanks to those that replied....


As long as you're focusing on a flat target, you should be fine. The ruler can help you see where you're off and approximately by how much. I wouldn't get too hung up on the 50x distance. I ran into similar problems with my 100-400L and ended up trying a couple of different distances all closer than the recommended 50x just to see if results were consistent. Once I zeroed in on the correct MFA setting, I also took a few shots at 100mm and 200mm just to see if the images were still in focus. Everything looked good and the lens has performed like magic ever since. If only the photographer (me) could perform as consistently.

TimR32225 wrote:

Thank you all for your suggestions.

I spent some time today and made microfocus adjustments to about a half dozen lenses. I found some of the accepted methods pretty tedious because of the suggested shooting distance of 50 times the focal length of each lens. For a 400mm lens, this works out to around 65 feet. At that distance, the target is so small the even when viewed at 100% magnification on the computer, or at full zoom on the camera LCD, it's difficult to tell if one image is much sharper than another one, even if they are shot at 3 or 4 clicks apart on the micro focus adjustment scale.

I finally ended up resorting to the old method of using a yardstick wth a strip of plastic taped to it in a perpendicular orientation. I placed the yardstick angled away from me at around a 20% angle from horizontal. So from around 20 feet away, I was shooting down the length of the yardstick but focusing on the narrow strip of plastic I had attached to it, rather than the yardstick itself.

I used a black cable tie for the target, and it extended about 3 or 4 inches to the left and right of the yardstick. I placed a light colored tarp on the ground so the camera would easily focus on the black cable tie and get a good focus lock.

Making several test shots without any micro adjustment, I determined that my 100-400, 300 f2.8, and 500 f4 were all focusing behind the target rather than on it. With the narrow plastic strip taped at the 20 inch mark on the yardstick, all three lenses were focusing an inch or two behind the 20 inch line.

All three of these lenses needed either -5 or -4 adjustment, but several other lenses needed adjustments in the opposite direction. I am not confident that I got the adjustments exactly right, but I think they will be better than before. I will have to do some real shooting at longer distances to see if these lenses require further adjustment.
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