I know there are a lot of MA procedures out there so I wanted to share with everyone what I am doing and hoping I can improve on it based on what others learned and experienced. I fiddled with a few procedures several months ago but it wasn’t until now that I had to actually make an adjustment. My new 24-105 required an adjustment of +9 which seems like an awful lot! (I bought this lens for Christmas, my first copy had what appeared to be an air bubble in one of the elements, and now my replacement is way off focus. Seems like Canon QC took a dive this year with all the disasters, but that’s another topic for another time.)
I tried 3 other procedures before I found this one:http://www.dgrin.com/showthread.php?t=116078
Credit goes to Scott Quier, thanks for sharing Scott if you are out there!
For the most part I follow Scott’s procedure with a couple of adjustments. I use a basketball for the subject on the first pass instead of a tree trunk and I use a chart with lines and text for the final pass.
I’ll summarize the procedure here:
1. Manual exposure, RAW, widest f stop, locked down on tripod using RC6 remote. Level camera with7D’s leveler, lens parallel (square) with target
2. Take 9 shots beginning with -20 MA in 5 step increments thru +20. I use a basketball as my subject and focus on specific point.
3. Remove camera (or flash card) without moving tripod
4. Download to PC, leave RAW as is, use DPP to export to jpg (this allows rapid cycling to find sharpest image)
5. Use ACDSee (cause it is easier) to crop center of images (same crop values for all images)
6. Using ACDSee, cycle thru images to find sharpest one.
7. When I find sharpest image, I identify what the MA value is.
8. Using the value in step 7, I take 7 more shots adding 3 to each side. For example if my sharpest image in step 7 was +10, then I will take test shots +7 thru +13 in one step increments. I use a test chart as my subject
9. Repeat steps 3 - 7 to find my sharpest image.
Why a basketball for the first pass? I focus about 45 degrees off the face of the ball on specific point like the tip of a letter. This allows me to see any front and back focus. Also, a BB has great texture, detail, and lettering making it easy to focus your eyes on the tiniest detail
Why a flat test chart for the second pass? Now that I am very close, I don’t want any curvature to throw me off. I want to focus on fine lines and text
Questions I have
1. What is the best distance to perform this test? I did my test at 16’
2. What focal range is best on a zoom lens? I did my test at 105mm (on crop body BTW). Is it best to test at mid zoom?
3 Best f stop wide open or the optimum aperture for that lens +/- f8?
Thanks for the feedback!