Over the last few days and in the light of focus criticality with the Nikon D800, I’ve had a huge number of questions specifically regarding focus precision, and whether there’s anything we can do about lenses that are obviously off. Specifically, how does the AF fine tune setting work, and how does one calibrate their lenses?
1. Make sure your camera has AF fine tune. Any of the newer Nikons or Canons offer this feature, and perhaps other brands, too.
2. Find a suitably well-lit target that you and the camera can focus, and preferably one that’s flat but slightly off-plane to the camera, so it offers a variety of different focus distances. An ideal test chart is a world map on a wall, but with the camera to a slight angle to the wall (i.e. sensor plane is not perfectly parallel.)
3. Use a sturdy tripod, and set single AF. This is to rule out camera shake or false-AF movements from the camera attempting tracking.
4. Perform this test with the aperture set wide open. Stopped down, depth of field will hide focus errors.
5. If you have live view (I believe most cameras with AF fine tune do), then use magnified live view to achieve critical focus and take one shot. This will be your benchmark image – the sharpest that the lens can possibly deliver, when focused perfectly.
6. Switch back to single AF, and viewfinder mode.
7. Defocus the lens between attempts: this is important, otherwise the camera may be trying to move the focus gearing in too small an increment to overcome backlash.
8. Let the camera focus normally, and take a shot.
9. Compare the image from step 5 with the image from step 8 – if they’re the same at 100% size, then you’re fine – move on to the next lens.
10. If the images from 5 and 8 aren’t the same, then adjust the AF fine tune setting – say by 5 notches (if you have +/- 20 notches like the Nikons) and repeat 7, 8, and 9. If it’s better, keep going – do another 5-notch adjustment in the direction that created your improvement. If worse, then go back in the opposite direction. Reduce the adjustment amount changes and keep going. Basically, you want to iterate the process until you match your viewfinder-AF image with your live view focused image.
11. Save the adjustment value – if you can’t save camera settings to a card, you might want to write it down somewhere just in case.
12. Repeat for all of your other lenses.
A special note for zooms: most AF fine tune functions don’t let you input separate values for different focal lengths, so you’ll just have to try both ends of the zoom range and see which is worse; then run steps 1-12. Try to find a compromise setting that works acceptably well for all focal lengths in the range. Or, if you use one particular end of the zoom more than the other, you might want to weight your adjustment to this end of the range. MT