Hi,
I recently "calibrated" all of my lenses using the microadjustment feature on the custom function set. (Canon 1Ds mark III)
It worked beautifully. Only ONE lens was spot on. (my 17-40L, tack sharp even at f4). And another (the old 50mm macro was close). All others I had to use +8 to +10 for best results. I used a variety of focal distances, focal lengths (on the zooms) and subjects to make the adjustment. I used a tripod and a remote realease for consistent results. I also took photos around the house to check focus more casually. I used all lenses wide open.
I noticed some lenses are quite inconsistent in their AF specially the 50mm f1,4 and the Sigma 12-24mm. All other lenses, once adjustments were made were consistently spot on.
The adjustments made a HUGE improvement in AF performance even at infinity on distant subjects. It was quite evident with the 85mm f1.2L. Without the adjustment I focused on the landscape outside my apartment balcony and the image was out of focus noticably even though there was good light and good contrast. After the adjustment the same landscape was tack sharp whe i zoomed in the image.
As you decrease focal lengths (and or subject distance) and use smaller (numerically higher) appertures the increase in image sharpness is more subtle but still there.
I performed the main tests for adjustments indoors at 3 distances, about 8m, 3m and 1m or less (using close to minimum focus on each lens). I made sure the camera was level and sensor parallel to target. At close distances the subject was at an angle to more precisely see the degree of fornt or backfocus present. I checked back and forth between zero adjustment and images with the lens adjusted. I used the camera on JPG L with sharpening set to 6.
I used the following procedure:
First AF (center point only, single shot), check focus distance on lens, turn on Live View, focus manually, check distance, notice difference, make microadjustment and use AF, adjust setting until the focus distance is identical to the best MF result using the AF. It takes a few tries but it isnt too bad.
I found out that with most lenses you can calibrate focusing on a high contrast subject at short distances, make an adjustment and the adjustment is valid at all focusing distances. For some reason with the 50mm f1.4 and Sigma 12-24 I adjusted at one distance, moved to another and the adjustement had to be changed a bit for best result. I then returned to the initial distance and a slightly different adjustment was necesary than the first try at that same distance. I setled on a happy medium.
I knew my 70-200mm f2.8L IS was a tad off from experience in the field and I had compensated a bit by sometimes focusing a tad closer and or using f4-f8 instead of f2.8, specially at 200mm. My 24-70mm 2.8 was pretty bad at 70mm requiring +10. Now its quite good at 70mm f2.8. I noticed that if the lens is microadjusted at any focal length it is spot on at all zoom settings. It is easier though and recommended to do the adjustment at the longest focal length. It was anoying and I did have to trash some images and others were not as sharp as they could be. With the 100mm macro i had not notice since i use that lens most of the time at f11 with strobes. Same with the 50mm macro. With the 85mm it was frustrating specially at f1.2 on the mark III, it worked well on the mark II however. Focus was always off and quite noticable. My 17-40L was always sharp and the test confirmed what i knew from my experience using it.
(I tested the Sigma 12-24, Canon 70-200f2.8L IS, 24-70f2.8L, 17-40f4L, 50mm macro, 100mm macro, 85mm f1.2L, 50mm f1.4.)
Take the time and try to do this with your camera and lenses. It is well worth it. You will most likely get sharper, most consistent results. I found out the 1Ds Mark III was quite consistent and once adjusted AF worked very very precisely like it should even in low light.
