A method to Fine Tune your lens

Started Oct 17, 2012 | Discussions thread
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A method to Fine Tune your lens
Oct 17, 2012

Autofocus accuracy is more and more important if you want to exploit the full resolution of modern DSLR. With the release of the Nikon D800 and its resolution of 36Mp, the process of fine-tuning your lens for your body has become mandatory, especially fast primes with shallow depth of field.

The so called "left sensor problem" of the D800 has spread a lot in the forums and it seems to me that most methods available on the internet are unreliable if we want to have a better knowledge of this "issue".Therefore, I will explain you a method to fine tune your lens that I've found very reliable. In the process, it will help us to quantify the "D800 AF problem".
1) Build your custom LensAlign tool

A built version of our target.

AF Focus TargetYou can download a sheet to build your own lensalign tool here: There are 2 pdf files, one for creating a LensAlign with a "siemens star" every 1cm, and one with a "siemens star" every 2cm. I suggest you first try the one where stars are spaced every 2cm. To build the tool, all you need is a pair of scissors and some tape. The jpeg file available on the above website shows you a built version of the tool. All you need is to cut the target, fold it and keep the feet of the "siemens star" so that you can tape it along the line of the remaining piece of paper.
2) Setup your cameraWe are going to assess the focusing accuracy of the camera using the LCD. In order to make things easier, I suggest you bump up the sharpness of the camera to its maximum.  It is also useful to setup your camera so that when you press the middle of the pad, it switches to a 100% view of the focusing point used to take the picture (The D800 allows that, but the D600 does not seem to).Then put your camera at a distance of 40 times the focal length of your lens (If you do have a zoom, fix a focal length you first want to measure). For instance, if you want to fine-tune a 50mm lens, put your camera 2 meters away from the target. Of course, your camera needs to be on a tripod.Set your camera to AF-S single point. Set your AF fine tune to 0. Set your lens wide open.
3) Take picturesWe are then going to take 4 pictures of the target. You should first aim your center AF sensor to the focusing target (the horizontal stripes in the center), so that your focusing sensor is entirely in the black stripes rectangle. Move manually the focus wheel to infinity, focus, take a picture. Move manually the focus wheel to close focus, focus, take a picture. Repeat the last 2 steps.Now, look at the pictures at 100%. The "siemens star" on both side of the target will help you to find where the focus plane is. What we should aim for is a perfect symmetrical picture, where the center target in sharp, and the siemens star on either side have the same quality of focus. The first star on the left should be as sharp as the first star on the right. Same things for the second and third stars. The further you go away from the target, the blurrier it will be.First check the first and second pictures. If the focus point seems to move, you are likely to have a "precision" problem: under the same conditions, your camera does not focus at the same point. The same apply if you have a difference in between the third and the forth shot.  Usually, this problem is due to bad lightning condition. For instance, some artificial lights cause this problem.
4) Find your optimal AF Fine Tune for the central sensorStep 3 has been done with an AF Fine Tune of 0.If the lens is backfocusing, set it to -20 and repeat step 3. It should be front focusing now. If it does not, you are in trouble, and your body can't AF Fine Tune this lens with this sensor. In the good case where it front focuses, set your AF Fine Tune to -10 and repeat step 3. If it backfocuses, set it to -15 and repeat step 3, etc. Optimize your AF Fine tune for the central sensor. Don't expect to go below a precision of 2 AF fine Tune Units, it is almost impossible and useless.On a sheet of paper, write down: lens, focal length, focusing distance, AF sensor and the optimal AF Fine Tune you've just measured.
5) Repeat step 4 with the different sensors you want to tryUsually, it is a good practice to test central, extreme left and extreme right sensors.
6) Repeat step 5 with different focal length if you have a zoom
You will end up with a chart with different optimal AF Fine Tune, depending upon focal length and AF sensors. It will first drive you crazy.If you have a D800 and want to know if your body is acceptable (or "not affected", using a term that I don't really like as there are bodies more accurate than others, but there is no clear line in between "good" or "bad" bodies), look at the difference between the optimal AF Fine Tune numbers for outer sensors and the central sensor.- For f1.4 lens, this difference should be below 20- For f1.8 lens, this difference should be below 15- For f2.8 lens, this difference should be below 10These limits of "acceptable numbers" for a body have been given by Falk from Lumolabs. He studied various bodies and these numbers seems to be what one should expect from today's DSLR (wether we like it or not).The difference in between different focal length for a given sensor could also be huge. I don't have any fast zoom, therefore I have been unable to do any test, but I've seen people measuring a difference of 35 AF Fine Tune units in between the short and the long focal length of the AF-S 14-24. Wether or not it is acceptable (meaning that it's in the range of produced lenses), I have no idea.
7) Choose a number to put in your cameraIf your lens is a fixed lens, you should set camera for a number in between. For instance, for my AF-S 35mm f1.4 and my D800, I get: left (-15), center (-7), right (-7). I have decided to set my AF Fine Tune to -9. I am very close to the optimal value for the center and the right sensors and being 6 AF Fine Tune away from the optimal value for the right sensor still allows me to have a sharp focus with this sensor (but it is on the front side of the depth of field).If your lens is a zoom, try to find something that fits best your needs. There is no rule.
Then, go out and take some pictures. Especially, try to focus at infinity to check if everything is fine here.
Hope this helps. If you do the test, please report your numbers in this thread so we get an idea on how large is the discrepency of AF Fine Tune with respect to different AF sensors and focal length.
Nikon D600 Nikon D800
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