Choosing the right autofocus (AF) mode can be a real challenge. You could leave the driving to the camera and go with Auto-servo AF (AF-A) mode. If you go that route, don’t expect that dumb box of a camera to make the right choices. It will make choices but they’ll probably not be the same choices you would make.
Being the risk-taker that you are, I’m sure you spend most of your time shooting in either Single-servo (AF-S) or Continuous-servo (AF-C) mode. These allow you greater control and, when good choices are made, a higher success rate making keeper images. Among those choices, is deciding which one or more AF points to use. Do you use one, nine, twenty-one or all the AF points on your camera’s sensor? If just one, do you go with the center point or an outer point? If you choose a group of points, which group? Do you allow the camera to have a say in which AF points are used? So many choices.
Let’s assume you’ve chosen an AF mode, and selected the number and location of the AF points that will be used. The next challenge is to successfully place at least one AF point over your subject and acquire focus. When it all comes together, it’s a beautiful moment. The shutter clicks open and the image swiftly, silently, gets encoded as a collection of 1’s and 0’s on an SD card.
Later, when you look at the photograph in Lightroom and realize it’s still not in focus, that moment of joy becomes frustration. What happened? Why is the eye just ever so slightly soft?
Of all the factors than have the potential to cause an out-of-focus image, arguably the most pernicious is a camera/lens combo that is ever so slightly miscalibrated. Despite your mastery of the camera’s AF system, your successful effort to track the subject and the presence of mind to make an exposure at the decisive moment, that slight miscalibration wreaks havoc. Focus is not set on the eye beneath the AF point. Instead, focus is slightly in front of or just behind the eye. The result is an out of focus image that ends up being deleted rather than marked as a keeper.
Autofocus fine-tune is a tool offered by many professional and high end consumer cameras. It allows you to adjust where focus is set to compensate for a miscalibrated lens. How does it work?
In the above illustration, the focus plane of the camera is represented by the semitransparent, blue square overlay. While all photographs have at least a minimal depth of field, for simplicity, I’m illustrating the focus plane as a two-dimensional, flat zone. With large aperture, small focal ratio lenses being popular for portraiture, the shallow depths of field produced by such lenses leave little margin for error when it comes to achieving accurate focus. If focus is not set on the eye or within a few millimeters of the eye, the resulting image will look “soft” and out-of-focus. There will be portions of the subject’s face that look sharp and in-focus, but if the eyes look soft, the overall impression will be that the photo is soft.
A miscalibrated camera/lens combo may give every indication of making a properly focused exposure. However, despite the fact that the focus point may be directly over the subject’s eye, the camera will set focus slightly in front of or behind the eye. If you are shooting with a fast f-stop, that slight miscalibration can result in unacceptably soft images. Autofocus fine-tune allows you to compensate for this problem.
The above series of images illustrate how to use AF fine-tune to add an adjustment to compensate for a lens that consistently front-focuses or back-focuses when used with a specific camera body. AF fine-tune settings are not transferable. A setting on one camera may not be needed on a different but same model body. The setting is unique to that specific camera/lens combination.
Also, Nikon bodies do not allow you to define multiple settings for the same lens. For example, when working with a zoom lens, you are limited to one setting for that lens. If AF fine-tune is engaged, the adjustment will be applied regardless of the focal length used. I recommend you test a zoom lens at the focal length at which it will most likely be used.
The below series of images illustrate my approach to testing a lens to determine if an AF fine-tune adjustment is needed. Right click the below images to open a full-size JPEG in a new window.
The above series of images is a real world test under real world conditions. When shooting portraiture with the D610 and Tamron 24-70 f/2.8 Di VC USD, I typically shoot wide open with a mix of ambient light and flash at 1/200-second. If you’re going to test a lens to determine an appropriate AF fine-tune setting, test the lens under the same conditions in which it will most likely be used.
AF fine-tune is turned off for the first image in the series. The next ten images were taken with AF fine-tune turned on. A +2 adjustment is applied in the second image. Images three through six have adjustments of +4, +6, +8 and +10 applied, respectively. A -2 adjustment has been applied to image seven in the series. The next four images have adjustments of -4, -6, -8 and -10 applied, respectively. At each setting, I took five handheld exposures with vibration compensation (VC) engaged. The above series includes the second exposure in each five-exposure set.
Reviewing the exposures at 1:1 in Lightroom, all five exposures with AF fine-tune turned off were acceptably sharp at the focus point. Two of the five in that set were a bit shallow in focus, displaying minimal in-focus depth of field in front of the focus point. The set which most consistently produced sharp images with good depth of field both in front of and behind the focus point is the set with an adjustment of -6 applied.
Now, it gets complicated. Normally, I would choose the -6 setting for the Tamron 24-70mm f/2.8 VC and leave it at that. However, I also have a Tamron 70-200mm f/2.8 Di VC USD lens but the Nikon firmware does not distinguish between it and the 24-70mm f/2.8 VC. If I leave AF fine-tune turned on with a -6 setting for the 24-70mm f/2.8, the same adjustment will be applied when the 70-200mm f/2.8 is mounted on the D610. So, I’ve also tested the Tamron 70-200mm, using the same approach as with the shorter zoom.
The results of the testing with the Tamron 70-200mm f/2.8 VC were fairly straightforward. The best set of images was taken with AF fine-tune turned off. The set taken with an AF fine-tune adjustment of -6 were among the worst of the lot.
After testing both lenses, I’ve decided to store a -6 adjustment for the Tamron lenses but to leave AF fine-tune turned off. Both lenses make sharp, usable images without an AF fine-tune adjustment. If I remember to activate AF fine-tune when the 24-70 VC is mounted, so much the better.
Now, it’s time to get out and shoot.
Bill Ferris | November 2015