DIY Focus Fine-tuning on a Nikon D70

I've chanced upon some excellent articles on the websites of Eric Boutilier-Brown and Leon Goodman, in particular, their guides on Do-It-Yourself Auto-Focus fine-tuning on the Nikon D70. This is especially useful for IR-modified cameras.

The technique would probably work on other SLR's with some minor variations, unless the mechanism is totally different. Not for the faint-hearted or those with jittery hands.

For those who feel rather MacGyver-ish, I've reproduced the procedure (and illustrations) below from Eric's site. Proceed at your own risk:-

1. Preparations

Set up camera on tripod and aim at a brick wall 20 meters away. Use a long, large aperture lens, such as a 50mm F1.4.

Take reference image to compare with adjustment (to see if you are getting better or getting worse).

2. Mirror Lock-up

To hold up the mirror when adjusting the AF-mirror adjust screw. You can tape it, but then the risk is the mirror becomes sticky and invites dust / other problems. Eric uses latex gloves, to avoid dust and debris from getting in, apart from the expected fingerprints.

3. Inserting the Allen key

With the mirror out of the way, you will see the two black adjustment screws on the right side of the mirror box. The one in the back, just in front of the shutter is the AF-mirror adjust screw, which needs to be modified for normal AF focus.

With great care, insert a 2mm hex key into the mirror box and set into the AF-Mirror adjust screw. It is very important that the hex key is inserted very carefully so the shutter, which is very close to the AF-mirror adjust screw, is not damaged.

4. Focus Adjustments
Placing the hex key in the screw is a little tricky, but once seated, it moves quite freely.

Rotating the AF-mirror adjust screw in a clockwise directions shifts the camera's focus point closer, the opposite direction shifts it farther. A bit of trial and error here until you nail the focus. Important to check with the the earlier reference image. Make sure the tripod does not move.

And we're done. Personally, I would not currently ever dream of doing this, but who knows what lies in the future?

Links to the original resources:-
UPDATE [5 June 2010 ] : The address from Eric Boutilier-Brown's site has changed from "" to "". Thanks to Robert Sun for pointing this out!


Robert Sun said…
This is a informative article with good source.
One minor typo you might want to correct.
The article from Eric's site could not be found.
Marvin Chong said…
Hi Robert,

Thanks for pointing this out. I've corrected the link to the D70 modification page, and also the link to Eric's main page.

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