Fine Tuning Autofocus on a Nikon D300
As a follow up to the lens tests that I recently talked about, I thought it would be worth looking at the Aotofocus Fine Tune Function found on some Nikon Dslr’s.
The D300 (and I’m sure this applies also to the D3 and D700, and possibly other cameras), has a setting for “fine tuning the autofocus”. You may well ask the obvious question: “why would you need to fine tune the AF?”. The answer is that the camera can go out of calibration. Its much the same reasoning as why you may have to adjust your watch from time to time (pardon the pun!).
You probably don’t need to calibrate the Autofocus, but if you are getting consistently out of focus pictures – like you focus on somebodys eyes, and find the ears are sharp and the eyes are soft, then this may be because the AF is out. You will notice this effect more at wide apertures (F2.8 being a “wide” aperture, f16 being a “small aperture”), because your picture will have a “shallow” depth of field. If you consistently shoot at small apertures then the depth of field increases, and more of the image will be sharp.
So, how do you go about fine tuning the AF? This is quite a subtle process, and you will ideally need to set your camera up using a tripod, and also maybe a cable release – or if you don’t have one of those, use the self timer. The other thing you will need to do, ideally, is to “tether” the camera to your computer. You can do this using a USB cable and software such as Nikon Capture, Aperture, Capture One, and probably Lightroom. I will use Aperture as the software for this piece as that is what I use 99.9% of the time.
Path to find tether (as shown using Aperture on a mac)
Tethering allows you to capture images straight into the computer, which has the advantage of being able to allow you to scrutinise your tests at high resolution (which you will need to do for accuracy, view full screen at 100%). You need to take the test images using the widest aperture on the lens – this will minimise your depth of field and give you far more accurate results!
Next you need to find a test target. I think something like a newspaper is pretty good – you have high contrast, and good edges on the typescript, which allows you to see focus.
Using a newspaper as a test target
There are also test charts available on the internet which may be helpful. The one I have used, which also works very well, can be found here – its on page 18 of the pdf document, there are also full instructions and other lens testing things in the document.
On the D300 you can find the AF fine tune setting in the menu using this path: Menu>Setup Menu>AF fine tune. You will need to have a lens on the camera and it AF fine tune will need to be switched ON. The camera will remember the lens you are using and apply the fine tune when that lens is put on the camera.
You must remeber that all you are doing is changing the Autofocus of the camera – by using the chart all this does is tell the AF where to set itself. If, as I described in previous posts relating to the 70-300mm lens, you think there is a problem with the back focus of the lens/camera, this will not necessarily resolve that problem. Back focus is a different beast, and relates to the adjustment of the camera “flange to focal plane” distance (the flange being the plate the lens butts up to, and the focal plane being the plane where the CCD sensor (or film) sits on a camera). If you are using a number of lenses and have AF calibrated them all and your pictures are still a bit soft, then this could well be a back focus problem, and the best thing to do if this is the case is to send the camera back to Nikon, or a cheaper and very good alternative repair specilaist in London that I would highly recommend is Fixation
