|Click here for full size image. D7100, 55-200 f/4-5.6 VR (5370x1200 px. JPG, 1 Mb.)|
Now that even the most modest Nikons feature 24 Mp. sensors, optical quality and lens performance (let alone photog skills) are increasingly coming under pressure.
It is no coincidence that many of the latest Nikon mid-range and top-of-the line cameras include a feature called “AF Fine Tuning” which allows users to adjust the auto focus of their sample of a particular lens to their particular camera.
On Flickr some users are commenting on dialing in as much as +/- 20 AFFT correction on their lenses, which, to me, seems excessive.
Although it is reasonable to assume that lenses might be off a couple or even a bunch of millimeters between one sample and another, claiming that they are 5 or more centimeters wrong seems like a lot.
The large majority of users also say that their lenses back-focus, which is curious because, according to my tests, the rule of thirds applies here: one third of my lenses back focuses, one third front focuses – mostly my old AF-D and MF (AI, AI-S) lenses – while another third focuses spot-on, most notably my third party samples.
Note that MF lenses cannot be corrected, and that correct focus was acquired using the focus confirmation dot in the view-finder, which could be the reason they are a tad off.
On their European website Nikon offer a procedure for AF fine tuning, which could be called the “Ruler Test”
It consists in mounting a ruler in a 45º angle next to a stationary object that serves as a central focusing point – a book – and focusing on this center point.
However, in my experience, this test is extremely difficult to interpret, because of the gradual transitions between the exact focusing point and the out of focus that can – supposedly – be observed on the ruler.
Thus, I have devised a different test in the form of a “trapped” model, which consists in more pronounced focusing steps of 6 mm each, which is the thickness of the domino stones this test is called after.