Auto Focus Microadjustments
Written by: Tiffany Joyce
I just spent a few hours over the weekend performing auto-focus micro-adjustments for three of my lenses. Not the MOST fun I’ve ever had with my camera, but a necessary activity for achieving correctly focused images.
Auto-focus micro-adjustments are sometimes necessary to calibrate the focus points of the lens/camera combination. Some lens/camera combinations tend to focus slightly in front of the focal point of the subject (“front-focus”), while others tend to focus more to the rear of the focal point of a subject (“back-focus”). When using auto-focus, the focus point should be tack-sharp exactly where it is placed. So if you determine that your lens/camera combination is consistently front- or back-focusing, perform an auto-focus micro-adjustment to correct this issue.
I used a Datacolor SpyderLensCal Lens Calibration System (hereafter referred to as the “target”), which I highly recommend for its ease of use and accurate results. I spent the most amount of time ensuring that the camera and target were lined up properly, which is very important to ensure accurate results. Here’s the setup that I recommend:
1. Put the camera on a tripod.
2. Place the target on its own stand or tripod, or on a flat level surface.
3. Separate the camera and target at a distance that is about mid-way along the lens’ focal range, or at about the distance you would normally shoot a subject when using the lens. Ensure the camera and the target are lined up horizontally and vertically.
4. Ensure the target is well-lit (as in bright sunlight or bright office lighting).
5. Set the camera on one-shot auto-focus and the drive to single shooting. Set the auto-focus point to single point AF and position the AF point directly in the center of the frame. When looking through the viewfinder, the auto-focus point should be positioned directly over the target’s focal point (on the LensCal, it’s the small square directly to the left of the ruler).
6. Use Program mode and set the ISO to 100.
7. Use a remote shutter release (or tethered shooting using the program’s shutter release). Mirror lockup is helpful as well, to ensure there is absolutely no vibration when the photo is taken.
It is extremely helpful to use tethered shooting while calibrating your lenses. Doing so will allow you to see the image results immediately on your computer and perform the necessary AF micro-adjustments on the camera without having to remove the memory card from the camera, download the photo onto your computer, open it in a photo editing program, check the focus alignment, put the memory card back in the camera, double-check the camera-to-target alignment to make sure nothing moved around, change the auto focus micro-adjustment setting, take another picture, and repeat that entire process for as many shots it takes to complete the calibration.
To ensure the target was level, I used the bubble level provided on the LensCal system. To ensure the camera was level, I used the bubble levels on the tripod, and I also used the very awesome, very cool electronic level built into my Canon EOS 7D (displayed when you press the “Info” button twice).
Once everything was all lined up and ready to go, I took the first shot with the micro-adjustment settings set to “0″ (as-is) to determine the extent of the focus issue. I was using the Canon EF-S 17-55mm f/2.8 IS USM, and this is the result of that first shot (zoomed in 100%):
As you can see (and as I suspected), this lens has a significant front-focusing issue. Front-focusing requires that I move the micro-adjustment value to the positive side of the +/- 20 scale (for rear-focusing you would use a negative adjustment). On the Canon 7D, access the Custom Function III-05 AF Microadjustments menu, then choose “Adjust by Lens”. A screen appears with the name of the lens currently being used, and the +/- 20 scale. Here is a comparison at the -5, 0, +5 and +10 adjustment points to give you an idea of how the adjustments effect the focus:
Top right = -5
Top left = +5
Bottom right = 0
Bottom left = +10
It might be a little difficult to tell with this screen shot, but the “0″ point was most in focus somewhere between the +5 and +10 calibration. I ended up setting the calibration at +7 for this lens. I performed the same process and set my Canon EF 50mm f1.4 USM, which was severely front-focusing, at +11. The Canon EF 28-135mm f/3.5-5.6 IS USM kit lens that came with the 7D was properly calibrated at 0. The Canon 7D has the ability to save the specific micro-adjustment settings for up to twenty different lenses. This process works similarly for other D-SLR’s which have auto-focus micro-adjustment capabilities (not an all-inclusive list, check your camera’s user manual):
Canon: 1DsMkIII, 1DMkIII, 1DMkIV, 5DMkII, 7D, 50D
Nikon: D3, D3x, D3s, D300, D300s, D700, D7000
Sony: A900, A850
Olympus: E-30, E-620
Pentax: K20D, K7D
I hope you find this article to be helpful in your own lens calibration efforts. Feel free to leave any questions, feedback or advice in the comments!
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