AF Micro-Adjustment tool

[Bart_van_der_Wolf]

Hi folks,

Having acquired my 1Ds3 (wow, what an amazing tool), and having spend some time getting used to the controls and menu layout, I decided to calibrate my lenses using the AF micro-adjustment feature. I can tell you, it makes a huge difference in focus accuracy and repeatability.

I've made a simple tool to assist in the procedure, and you can download it here. It should be displayed on an LCD screen a 100% zoom setting (so at native size).

It works by exploiting the interference patterns or moiré between the R/G/B LCD elements and the camera's LCD elements when directly viewed with Life View. With good optics and perfect focus, the moiré is maximized. Depending on shooting distance, choose the Life View zoom setting (full, 5x 10x) that shows it best. When the situation permits, you will see larger R/G/B aliasing of the LCD elements. You'll also notice how narrow the field of focus is, especially with lenses that react strongly to small focus ring rotations.

This is my first version of the target, therefore I may update it with some other features, also depending on feedback. So let me know if it works for you, or what might help to make it better.

Bart

[Doug Kerr]

Hi, Bart,

That sounds like a very interesting approach.

Originally Posted by Bart_van_der_Wolf

It works by exploiting the interference patterns or moiré between the R/G/B LCD elements and the camera's LCD elements when directly viewed with Life View.

I'm having trouble visualizing the principle here. Are you saying that the moiré phenomenon lets the viewer better discern the sharpness of the image of the "focus target", or what? I'd enjoy being able to grasp a little better what chain of effects you are exploiting here. It sounds fascinating.

Thanks.

[Bart_van_der_Wolf]

Originally Posted by Doug_Kerr

I'm having trouble visualizing the principle here. Are you saying that the moiré phenomenon lets the viewer better discern the sharpness of the image of the "focus target", or what?

Hi Doug,

What happens as one manually adjusts the focus is that at the exact optimal focus setting the background will change from uniform gray into larger (colored) aliased dots when viewed on the camera's LCD. The circles and cross hair will allow to acquire AF easily, and when calibration is optimal, moiré will be maximized on the camera's LCD.

A procedure that works for me:
- I switch to LifeView, which in its current implementation will only allow manual focus.
- Optimize manual focus by searching for maximum aliasing. This will only occur a best focus (and assuming a decent enough lens is used), otherwise the defocus will act as a low-pass filter and prevent the aliasing.
- Switch off LifeView, and watch the lens barrel's focus indicator for the next step.
- Use AutoFocus (single AF spot) to focus on the (laptop) LCD screen, and watch the direction of adjustment. That will show whether the current AF calibration setting will front or back-focus.
- Apply an adjustment via the camera menu, and repeat the procedure. Once the adjustment is optimal, there will be no difference between manual and auto-focus.

The flat computer screen will prevent misinterpretation of the focus distance, because the AF system cannot react to phase effects from subjects at other distances. That makes it quite easy to get repeatable results. And because the computer LCD emits light, it can be easily done indoors, at common shooting distances for the lens to be calibrated for.

For super tele lenses it is probably easier (for distance reasons) to use a Zoneplate type of chart outdoors.

Bart

[Doug Kerr]

Hi, Bart,

Originally Posted by Bart_van_der_Wolf

What happens as one manually adjusts the focus is that at the exact optimal focus setting the background will change from uniform gray into larger (colored) aliased dots when viewed on the camera's LCD. The circles and cross hair will allow to acquire AF easily, and when calibration is optimal, moiré will be maximized on the camera's LCD.

How neat!

. . . otherwise the defocus will act as a low-pass filter and prevent the aliasing.

Of course! What the moiré does is allow us to visualize the higher-frequency components in the image (just as many so-called "contrast detection" AF systems actually do), and of course they increase as the "low pass" effect of the defocus spread function declines.

This of course makes perfect sense as moiré is a visible manifestation of aliasing, which occurs from high-frequency components beyond the Nyquist limit for the system. Less HF content - less moiré.

The flat computer screen will prevent misinterpretation of the focus distance, because the AF system cannot react to phase effects from subjects at other distances. That makes it quite easy to get repeatable results. And because the computer LCD emits light, it can be easily done indoors, at common shooting distances for the lens to be calibrated for.

Very clever!

Brilliant, my friend.

Best regards,

Doug

[Ben Rubinstein]

Sounds incredible, all I need now is for my 5D's to have had this feature! :-)

[Nicolas Claris]

ABSOLUTELY brilliant Bart!

I sure will give it a try tomorrow…
For zoom lens like the Canon 24-70 what lenght would yoy choose for calibrating? 50 mm?

[Bart_van_der_Wolf]

Originally Posted by Nicolas Claris

ABSOLUTELY brilliant Bart!

I sure will give it a try tomorrow…
For zoom lens like the Canon 24-70 what lenght would yoy choose for calibrating? 50 mm?

Canon suggests to use the longest focal length of the zoom range, 70mm in your example. Of course you can check afterwards how well that works for the shorter ones, but in my experience they are harder to calibrate because the DOF is already a lot more. The DOF of the shorter focal lengths will probably exceed the adjustment range anyway.

Afterall, what the AF micro adjustment does is add an offset to the lens calibration data to change the average focus position within the DOF limits of the widest aperture. That is done more easily at longer focal lengths.

Bart

[Nicolas Claris]

Thanks Bart
I'll post my findings tomorrow…

BTW, nice Santa this year, hé!

[Bart_van_der_Wolf]

Originally Posted by Nicolas Claris

BTW, nice Santa this year, hé!

Yes, Santa Mark III !

Bart

[Nicolas Claris]

Once you realize that the tests should be run wide open, it becomes really easy…

So, thanks Bart (and Canon…)

My Sigma 12-24 comes back to life… micro adjustement settings = +15 !!! (and I thought it was a good copy!)
Canon 24-70 = +3
Canon 70-200 ƒ2.8 = 0
haven't check the 500 yet… nor the 6mm