Nikon autofocus tuning on cameras

Started Jan 14, 2011 | Discussions thread
Marianne Oelund
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How to calculate AF fine-tune range and setting
In reply to haroon08, Jan 26, 2011

haroon08 wrote:

Hey guys. Can someone please explain to me how the autofocus tuning works and how do you know if you need it? (meaning how to know when to go (+xx) or (-xx)). Hope this makes sense. Thanks!

AF fine tuning works by adding an offset (proportional to the fine-tuning value) to the nominal image displacement detected by the AF sensor. This is functionally equivalent to moving the AF optics slightly along the optical axis.

In the D3/D3s, the maximum amount of equivalent movement is plus or minus 0.10mm (for the D7000, the figure is 0.06mm). Using these values, we can calculate the AF adjustment range, and use that to estimate what fine-tuning setting to use. This eliminates the time-consuming trial-and-error process of adjusting, re-checking, adjusting again, etc.

If you would like to use this approach, you will probably want to set up a spreadsheet or program to run the calculations. Here is the procedure, assuming you start with AF fine-tune set to 0:

Note regarding units: Millimeters are assumed for both subject distance and lens focal length

F = lens focal length
D = subject distance where AF is actually focusing
Delta = 0.10mm for D3/D3s, or 0.06mm for D7000

1. Calculate I1 = 1/(1/F - 1/D) - Delta
2. Calculate I2 = I1 + 2*Delta
3. Calculate O1 = 1/(1/F - 1/I1)
4. Calculate O2 = 1/(1/F - 1/I2)

Now, O1 and O2 are the limits of where the AF point can be moved to, if you use the +20 or -20 fine-tune settings, respectively. If your intended focus point (target position) is between O1 and O2, then you will be able to fine-tune AF for this situation. However, if the target you focused on is in front of O2 or behind O1, then the focus error is not within the range of AF fine tuning.

Here is an example which will hopefully make the process clear ( or, if you have a DOF calculator, skip to the bottom paragraph ):

You have a 400mm lens on a D3s, and focused on a target which is 7000mm away. With AF fine-tune set to 0, the actual point of focus ended up 6mm behind the target, i.e., at 7006mm. [Recommendation: Repeat the focusing operation multiple times, in both directions , then take the average result.]
Then calculate:
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) = 7033
O2 = 1/(1/400 - 1/424.320) = 6979

This means we can use AF fine-tuning to move the focus point as close as 6979mm, or as far away as 7033mm. With our target at 7000mm, it's safely within adjustment range. With a fine-tune setting of -20, the focus point would move to 6979mm, which is 27mm closer than it is now. Since we only need to move the focus point by 6mm, we set AF fine-tuning to -20*6/27, or -4.

Note that you do not really need to measure your subject distance precisely; you may assume any figure that's reasonably close. It's only important to have the error value (6mm in the above example) accurate.

Alternative Calculation

If you have a DOF calculator, you can use it to find O1 and O2. For a D3/D3s, use a lens aperture of f/5.6 and a COC diameter of 0.017mm. For a D7000, use a lens aperture of f/5.6 and a COC diameter of 0.010mm. (If you were wondering, yes, this means the AF fine tuning range corresponds to the DOF of the lens when set to f/5.6, and you can probably use this for other Nikon camera models.)

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