Autofocus fine tuning is one of the most powerful, misapplied and frustrating things that you can do to try to improve your camera. It's powerful because it can make a mediocre slightly out-of-tune lens into a stellar in-tune lens performance-wise. It's often misapplied because too many people resort to it before first checking to see if they are using appropriate camera settings and/or have adequate hand-holding technique. It's frustrating because for the most part, the act of determining focus calibration is a trial-and-error process. Shoot. Examine. Adjust. Repeat... until you get it right.
However, if you understand the theory behind AF fine-tuning, then you can adjust your focus in one-shot without the repeated hit-and-miss attempts. The following is based off of a post from the redoubtable Marianne Oelund as posted on Jan 26, 2011. Note: The following can't be used with the Moiré Fringe method of focus calibration, as it requires the absolute distance in mm by which focus is deviating.
Nikon's AF fine tuning adjusts focus by allowing the user to manually apply a value (+/-20) to the camera's autofocus calculations, which is mechanically the same as shifting the optics of the lens slightly forward or backward along the optical path. The problem with this is that the documentation does does not define what the maximum value of "20" corresponds to, other than that is the most that you can adjust the focus forwards or backwards. The difficulty with this system is that you can tell whether you have front or back focus, and by how much distance... but the system gives you no indication of how much adjustment you have to apply. Hence, the tedious nature of trial-and-error AF adjustment.
Just as a reminder, the Nikon cameras that have autofocus fine-tuning capability are the D300s, D7000, D7100 and all of the FX cameras. If you have a D5200 body or lower... sorry, you'll have to have it sent in to be adjusted, or to try this if you are brave, don't mind that monkeying with the mirror stoppers actually mis-aligns outer vs central AF points, and don't care about voiding your camera's warranty. You've been warned.
First a bit of fact. The maximum amount of equivalent movement for the FX cameras is 0.10mm. For the DX cameras, it's 0.06mm. That is to say, for the same field of view, the adjustment factor applied on a DX camera will be the same as one applied on a FX camera. So, correspondingly, a +10 adjustment factor on a 24mm DX lens produces roughly the same amount of change that a +10 adjustment does on a FX camera with a 35mm lens. However, if you use the same FX lens (say, one of the 50mm's) on both a DX and FX body, then the field of view is different. It's the same lens, but you still need to use the different movement factors for each system.
- F = lens focal length
- D = subject distance where AF is actually focusing
- ∆ = 0.10mm for FX, or 0.06mm for DX
O1 = Far limit of AF fine-tuning
O2 = Near limit of AF fine-tuning
- All measurements in millimetres.
- Start off with AF fine-tune at 0.
- Measure D
- Calculate I1 = 1/(1/F - 1/D) - ∆
- Calculate I2 = I1 + 2*∆
- Calculate O1 = 1/(1/F - 1/I1)
- Calculate O2 = 1/(1/F - 1/I2)
- Subtract O2 from O1. This is the total adjustable range in mm.
- Let A = Divide (O1-O2) by 2.
- Your AF adjustment value is (Focus deviation in mm)/A * 20, where 20 is the maximum value that can be applied on any Nikon camera that has fine-tuning capability. This value is negative if your lens is back-focusing and you want to bring the focus point forward. Likewise, it is positive if you have front-focus and want to move the focus point back.
Of note is that this only works if the distance to your AF target falls between O1 and O2. If it falls either in front or behind of this range the amount of calibration needed exceeds what you can accomplish with the in-camera fine tuning function. Time to send it (along with your camera body) to the service depot.
FX Example (As originally provided by M. Oelund)
- 400mm lens
- D3s, D4 or D800
- Target is 7000mm away.
- Focus is 6mm behind when AF fine-tune set to 0
The calculations would then go like this. (This is a lot easier if you use an spreadsheet program like Excel, or if you have a scientific calculator that let you enter formulas in forward notation.) Note that the math isn't sensitive to the distance that your target is setup, but it is for the distance that your camera is back focusing. In other words, to determine the "D" term in the equation, you can ball park the distance that the real camera to target distance is, but you must be precise about the amount of focus deviation.
- I1 = 1/(1/400 - 1/7006) - 0.10 = 424.120
- I2 = 424.120 + 2*0.10 = 424.320
- O1 = 1/(1/400 - 1/424.120) = 7033mm
- O2 = 1/(1/400 - 1/424.320) = 6979mm
E.g., AF fine-tuning can move the focus point as close as 6979mm, or as far away as 7033mm. To adjust AF fine-tuning should be set to:
- 6mm/((7033mm-6979mm)/2) *20
.... which corresponds to a menu adjustment value of -4.
Here's an example with a DX setup in what is a fairly common shooting situation. "Normal" lens at close to mid-range subject distance with just enough back-focus to affect image quality but not enough to be catastrophic:
- Nikon D7100, D7000 or D300s
- Nikkor 35mm f/1.8G DX
- Target is 2000mm away
- Focus is 30mm behind when AF fine-tune set to 0
The corresponding calculations would be as such:
- I1 = 1/(1/35 - 1/2005) - 0.06 = 35.554
- I2 = 35.352 + 2*0.06 = 35.674
- O1 = 1/(1/35 - 1/35.352) = 2246mm
- O2 = 1/(1/35 - 1/35.472) = 1852mm
Therefore, the adjustment amount is
- (2246-1852)/2 = 197
From here, you can calculate the AF adjustment amount as
- (30/197) * 20
... which would then equate to an adjustment value of -3.
As a general rule of thumb, adjustment values of less than an absolute value of 2 tend to not be worth tinkering with. If everything else is perfect, then yes, even this small amount will improve image sharpness, but all else being equal, this is a level of adjustment that may or may not be reproducible in real life. In our example, the back focus is 30mm (a bit over 1 inch). There are a myriad of reasons why this might happen, including subject movement or the AF system naturally locking onto something with higher contrast behind your intended target. This is also why it is important to do do multiple sample shots to determine the D factor, as it is the most critical term in this calculation but is subject to the natural mechanical tolerances of the lens system.