A lot of folks emailed me asking me about an easy way to AF fine tune their Nikon lenses. So here I have described a rather simple way of AF fine tuning to get the best performance out of Nikon lenses.
AF fine tune is needed only by those camera systems that use a phase detect AF system. Phase detect systems work by superimposing 2 images of the subject from different positions and then finding the difference to adjust focus. Now the problem occurs when there is always a difference between these 2 images and no amount of focusing in or out really brings these in perfect alignment. This usually occurs due misaligned focus sensors and the issues that one commonly sees resulting out of this is front focus or back focus.
Till some years ago, when there was no way of adjusting lenses for these obvious front and back focus issues, the only way to resolve this was to send the camera body and lens to the camera service center to do a calibration.
But now, Nikon has started including the AF fine tune feature on its enthusiast and high end professional bodies. I think all Nikon cameras since D7000 have this feature except the D5xxx and D3xxx bodies. Canon too has this feature and calls it AF micro adjustment.
All Nikon DSLRs have a green dot, which is now a white dot in newer Nikon DSLRs, that is used to confirm focus. Traditionally this has been used to confirm focus while using Manual Focus (MF) lenses. I have used this extensively to confirm focus while using MF on my macro lenses. This little green dot can now be used for Auto Focus (AF) fine tuning your lenses on your Nikon bodies, besides its traditional use to confirm focus for MF lenses.
There are a lot of third party software and tools that you can buy to AF fine tune your lens to your camera, but IMHO these are quite unnecessary if you can use the AF confirmation dot to good use and do a careful AF fine tune for each of your lenses. It is a good idea to re-tune all your important lenses before a major photo shoot or at least once in a few months.
2 widely used third party tools/software for AF fine tune are Reikan FoCal and LensAlign manual calibration system. Ofcourse, both these systems are very good if used correctly and are quite accurate and save you much time. But these obviously cost money and so if you are a cheapskate like me and don’t mind doing a bit of hard work, then the focus dot confirmation method is a pretty good alternative.
The first question that crops up in your mind is whether or not your lens needs an AF fine tune. If you think your lens is working perfectly fine and right up to your expectations in all conditions you probably don’t need it. But I was surprised that all my lenses needed some sort of adjustment even though I always thought that they were performing well and as expected.
This AF Fine Tune option is available under the settings menu (The spanner menu option). Unlike the Canon system, that allows you multiple settings to be stored for one lens, which I think is quite useful, especially when you are fine tuning zoom lenses and can thus save multiple values for different focal lengths, Nikon allows you to store just 1 value. So my advise is to use the most used focal length of that zoom lens and save the value for that focal length. You can store AF fine tune values for up to 12 lenses on your Nikon DSLR. Remember that different bodies will need different AF fine tune values with a particular lens. So perform this tuning for all your bodies.
The benefits of AF fine tune are usually realized when you fine tune fast lenses and telephotos that have narrow DOF considerations.
Following are the steps to be followed to AF fine tune your lens
- Ensure that you are doing this process with the lens aperture wide open as DOF will keep increasing with every stopped down aperture and can result in erroneous AF tune values.
- Choose a high contrast target, ideally a resolution chart. If you do not have a proper resolution chart then even a newspaper having lot words in different font sizes will do. Clip this newspaper on a flat wall perpendicular to the lens. Ensure that the lens is parallel to the ground.
- The lens should be mounted on firm tripod with the ball head locked.
- The distance to the target should be roughly about a few feet more than the minimum focusing distance (MFD) of the lens or at a distance that you usually use this lens from your subject. Care should be taken to ensure that your subject is not at or beyond the infinity focus of your lens.
- Start the Live View (LV) on your camera and focus on the subject. LV uses contrast detect AF, so is not effected by problems that are seen on phase detect systems. Its usually a lot more accurate than phase detect.
- The green dot in the VF should be lit up on focus conformation using LV AF. This is the reference focus that the phase detect system should match to ensure that it works correctly.
- Ensure that the subject and the tripod and the camera do not move after this point.
- Now shut off the LV. Switch the AF switch on the camera or lens to MF. Ensure that your camera is in MF mode with no disturbance or movement at all to the camera, tripod or the target.
- Press the AF on or half press the shutter if that is used for AF.
- See the dot in the VF
- If there is a constant green dot as long as the AF button is pressed, you know that phase detect AF system is working correctly and as expected. If the green dot is intermittent and a small arrow keeps showing up intermittently as well, it means that you need a compensation of the AF fine tune value in the direction of the arrow. If instead of the intermittent dot you see a constant arrow, then surely you lens needs a massive correction in the direction of the arrow.
- Now if you do need a correction as determined from the above step, increase or decrease the AF fine tune value beyond default value as needed and perform the same test again. This should be repeated till you get a constant green dot.
- Now that you have got a constant green dot, it does not mean that the AF fine tune is perfect already. It may be at the border of perfection and you need the average value of the entire range of values for which the dot is constant to set your final AF fine tune value to.
- Lets say that you get a constant dot at AF fine tune value 0. Now keep reducing the value -1, -2, -3 and so on till the arrow and dot start showing intermittently. This is your border negative value. Repeat the same steps in the other direction and keep increasing the value of AF fine tune till you get the same intermittent arrow and dot. This is now your border positive value. Now take the average of these 2 border values and that is your final AF fine tune value. For e.g. If -9 is your negative border value and +4 is your positive border value, then the average of these 2 i.e. -3 is your AF fine tune value of your lens.
Try this method for all your lenses and let me know if this worked for you 🙂 and let me know if you have any questions.