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p.1 #1 · p.1 #1 · Micro Focus Adjustment Technique



I have developed a method for performing micro-focus adjustment that is both fast, accurate and does not cost anything. I thought I would share it here.



This method requires a tripod, a flat focus target, and an environment with constant lighting. A computer is not required. I can calibrate a given lens in less than 5 minutes with dead-on and reproducible results.



This methods uses JPEG file size (bigger is sharper) to determine optimal micro focus adjustment. A sharply focused image has more high frequency content than a poorly focused image. JPEG compression attempts to preserve high frequency image information and for any given quality setting will produce larger files for images with greater detail. In order to determine optimal micro focus adjustment you simply step through the adjustments and capture a JPEG. You then review the images in-camera and select the one with the largest file size and set the MFA to the value used for that shot. This is easily done using image review with the info screen selected that displays both file size and MFA adjustment value.



To produce accurate and reproducible results, however, a few critical setup details must be observed. The basic goal is to take images in which the only variable is the MFA setting.



Camera must be on a tripod. Use MLU and 2 second shutter delay.

Target illumination should be constant.

Use manual exposure with widest aperature for the lens.

Set ISO to 100.

Use any white balance other than automatic.

Use JPEG large and fine.

Use One Shot Autofocus



The MOST important part of the setup is the target and its orientation to the camera. Failure to observe the three first items below will produce inaccurate results.



The target must be large enough to fill the entire field of view.

The target must be flat.

The target must be perpendicular to the axis of the lens.

The target image itself is not that critical. It should be black and white with a pattern at the center suitable for autofocus to acquire lock and a bunch of detail elsewhere. A white wall "decorated" with black tape would be fine.



I use a three passes to arrive at my final MFA settings.



First pass: -15,-10,-5,0,5,10,15 (increments of five)



Second pass: Increments of two centered on the previous high file size

(example if high file size was on 5, then retake at -1,1,3,5,7,9,11)



Third pass: Increments of one centered on the previous high file size

(example if high file size was on 7, then retake at 4,5,6,7,8,9,10)



Set MFA to the setting used for the highest file size on the last pass. Done.



I manually defocus the lens between each test shot to ensure that the autofocus has to actually move the focus elements each time. I also put the MFA adjustment menu into My Menu for faster adjusting and I also delete all images between each pass so as not to get confused. I want my lenses accurate at the distances I most often use them so that is how I determine how far away to set the target. If you prefer to use a multiple of focal length that will work as well.



If your setup is correct you will see highly repeatable results which will be confirmed by pixel peeping back at your computer if you so desire.



Enjoy.













