shutterhack

This guide is a cut-n-paste from Bill's discussion on nikonians.org

I'll probably take some heat for saying this but...

AF errors original in the body and not with the lens.
AF, like metering, always occurs with the lens wide open so that's why aperture doesn't matter.
The decreased Depth Of Field (DOF) of the wider aperture is why AF errors are more apparent with fast lens.
Therefore I think you may have an AF issue with your D50 body.

There are many ways to test this; probably more than 50% of them are invalid.
All the tests that are valid have one thing in common; you AF on a target that is perpendicular to the camera, not at any angle.

Here are two variations:

1) On a tripod, with a focusing rail if available; AF on a target at the minimum aperture of the lens and then set the body to MF.
Take an image at the AF position.
Then move the entire camera away from the target a small amount and take another image.
Do it several times and then repeat moving the camera forward several times.
How much to move and how many images is not an exact science.
But with a target about 400-500mm away I'd say increments of about 5mm or 1/4 inch and about 5 images on either side of the AF position is a good place to start.
Examine the images, if they get sharper as you move the camera away or toward the target then AF accuracy is not perfect.
No camera is perfect, so the question is how bad?

For an example of what you might see look at A Technique for Measuring Relative Sharpness (2005-08-04 86/7071) on my web site.
The second chart in the initial post shows this nicely.

2) A variation on the "ruler shot".
Use a "lazy susan" or turntable and place a ruler so the face of the ruler runs through the center of the turntable.
AF at the minimum aperture with the ruler perpendicular to the camera and set the body to MF.
Rotate the turntable about 45 degrees and take your shot.
You should not be too close to the target with a wide angle lens because this will distort your perception of the result; but a gross AF will clearly show.

If your body needs AF adjustment it is best done my Nikon.
But if you're gutsy and the D50 is like the D70 then you can take a look at this AF Sensor Nerdvana Achieved

Good luck!!!
Bill

Visit me at: http://home.comcast.net/~NikonD70

"AF Sensor Nerdvana Achieved"

PREFACE

I will preface this post by saying that in taking over 24,000 pictures with my D70 I have never had problem with an image that I attributed to an AF Sensor adjustment issue. However, in more exacting tests of image sharpness; I have known for some time that my D70 has been back-focusing slightly. So, as winter sets in; I have decided to adjust my AF accuracy. I'll outline what I did but this procedure clearly is not for everyone.

BACKGROUND

AF accuracy depends in large part on adjusting the AF mirror. The mirror needs to be adjusted so the distance from mirror to AF Sensor and the distance from mirror to CCD Sensor match.
The adjustment is performed by turning an elliptical peg that the mirror rests against in the D70 body. There are two pegs, the one closer to the CCD sensor adjusts the AF mirror, the other controls the Viewfinder mirror. The peg accepts a 2mm hex wrench.

The Viewfinder and AF mirror adjustments interact. If I were adjusting both I would do the Viewfinder first; but my Viewfinder seemed fine so I went straight to the AF mirror adjustment.

Obviously, to perform this adjustment you need a good way to judge sharpness. I use custom written software to analyze the raw linear data in NEF files to get an "objective" measure of sharpness.

See the cached Nikonian thread on my web site entitled A Technique for Measuring Relative Sharpness (2005-08-04 86/7071) (particularly post#7) for details.
(The average and standard deviation in Photoshop histograms could be used to approximate my technique.)

PROCEDURE

My D70 was mounted on an Adorama focusing rail (really a focusing stage) on a tripod.
My test target was alternating white and black lines on a vertically positioned LCD monitor.
(My sharpness measure requires a repeating high contrast image.)
I used a 50mm f/1.8 AF Nikkor at f/1.8 at a distance of approximately 500mm from the target.

To collect one set of data I would

1. AF with a half-press of the shutter
2. Switch the body from AF to M focusing
3. Move the body forward 3mm
4. Take 7 shots moving the body backward 1mm after each shot
5. Switch the body back to AF from M focusing

The result is 7 images (from -3 to +3) centered about the AF point.

I would compute sharpness for the 7 images and plot the result.
The desired result is a U-shaped curve with 0 as the sharpest point.
Lines that slope up are front-focus; lines that slope down are back-focus.
Turn the peg CCW to fix front-focus; turn it CW to fix back-focus.

I get access to the peg the same way as I would to do a sensor cleaning.
I lock the mirror up and I take the battery out.

RESULT

After a number of attempts (10!) I have reached AF Sensor nerdvana.

Here is the chart of my attempts:
http://home.comcast.net/~NikonD70/gif/AFAdjustTry.gif

My first adjustment was in the wrong direction and then I moved slowly toward my goal.
This adjustment is quite sensitive and I think would be much more challenging without an objective way to measure sharpness.
Note that the final set of 7 images are visually identical when viewed at 100%

Here's a sharpness chart for the final cam position from -5mm to 5mm:
http://home.comcast.net/~NikonD70/gif/AFAdjustCenter.gif

To state the obvious, I adjusted for the center AF Sensor; and the others are just along for the ride. If the AF Sensors are not all in the same plane then the off-center sensors will be out of adjustment. In that case I would imagine the camera would need Nikon service rather than self adjustment.

However, in my case things look pretty good. Here are sharpness profiles for all 5 AF sensors:
http://home.comcast.net/~NikonD70/gif/AFAdjustAll.gif

And considering that my LCD monitor might not be perfectly square to the camera I think this results is excellent.

(Looks to me to be more square left/right than up/down.)

Bill

Visit me at: http://home.comcast.net/~NikonD70

Labels: Landscape, Nikon D40, Perhentian 2007, Photography, Sigma 30mm f/1.4 EX DC HSM

Posted by Mohd Fadzly Abdul Mubin on Friday, June 29, 2007 at 7:53 AM | Permalink