Like many, I used to only use a lens chart to do AF Fine Tuning (AFFT) on my pro Nikon camera bodies. Based on my experience, I seem to be getting a hit-or-miss experience. I just simply cannot get a clear scientific right or wrong procedure using this method. The results I am getting was fuzzy: sometimes, it was spot on and sometimes, it was not correctly focused. The really confounding fact is that on the lens chart, it was clearly correct but when I started to shoot real life humans, the focus was off. It was lately that I found the method below to be more definite in getting the results I wanted. And, I am writing this especially for people who are having problems getting their AF tuned for the Nikkor 58mm/1.4G.
Subject : What is your subject matter?If only a lens chart is used to do AFFT, the resultant AFFT may be questionable. Why?
First, we are aiming the camera at a flat 2D lens chart. That’s a significantly easier focus scenario than real life. In real life, your subject may be shot at an angle with parts of it behind the focus point and some parts before the focus point. This scenario challenges the AF subsystem much more than a flat 2D lens chart.
Second, lens charts consists of black and white lines. That is the highest contrast one can ever obtain under the light condition you are using. It is the extreme and it is the easiest on the AF system to obtain focus. If your subject matter is also high contrast B&W lines, then a lens chart is fine. But, unfortunately, for most us, our subject matter is not solid B&W. It is usually shades of other colors and the focus point may consists of shades of much lighter colors and lower contrast. This will affect the AF system behavior. In general, real life scenes are that much more challenging than the high contrast B&W lines.
So, what do I do to solve this? I use a lens chart to bring the AFFT closer to its correct AF tuning vicinity but I will move on to further tune it using subjects that are closer to what I will be shooting in real life. Below, I list the steps I take after I have AFFT using a lens chart to verify and fine tune the last bit matching it to the real life subject I will be shooting.
How to check if AFFT is correct after tuning it using a lens chart?
- Open the lens to the widest aperture.
- Mount your camera on a tripod.
- Pick a subject closer to your real subject. E.g. if you are a wedding photographer planning to cover a wedding for a family consisting of mainly people with fairer skin tones, get a test subject who is typical of that ethnic group; preferably, with lighter color eyes so that it will really strain the AF system for the more challenging scenarios. (Recall that lower contrast, lighter color means harder to focus when light is poor.)
- Focus at your subject at an angle and pick an identifiable focus point on the subject. E.g. Sit your subject down, adjust the tripod height so that it is at eye level of the subject and place your camera so that you can shoot from an angle from the subject’s side.
- Make sure there are clearly identifiable parts of the subject directly after to the focus point. E.g. focus on the nearest eye but at the corner of the eye furthest away from you so that you have the bridge of the subject's nose as an identifiable point beyond the focus point.
- Make sure there are identifiable parts of the subject directly before the focus point; e.g. the corner of the same eye nearest to you.
- Shoot the image
- Zoom in the image on the LCD panel at the focus point
- Evaluate the vicinity of the focus point and see if the image displays the focus point as the most focused.
- If the focused point is the sharpest compared to the point before and after, then you are done. If not, your AFFT is still not quite correct. Determine if the sharpest point is before or after the focused point.
- Go back to your camera menu and adjust the AFFT accordingly by a couple of notches to compensate for the slight focusing error.
- Go to Step  and re-do all the steps until step  evaluates to true.