7D AF in AI SERVO (photos included)

Started Feb 4, 2013 | Discussions thread
OP Ondrej Zeman Junior Member • Posts: 32
Re: 7D AF in AI SERVO (photos included)

TTMartin wrote:

I checked my settings, and they matched yours. (assuming track sensitivity 0 meant centered). Like I said, I had no problems tracking Indy cars with the camera. That was using a 70-200 f/2.8 lens. I was using a smaller aperture as I wanted the whole car in focus.

Once it picks up the movement of the car the sensitivity shouldn't matter. But, it might help it detect the movement to start with.

Since you said there were no wow moments from the videos, I'm assuming that you are using AI Servo and not AI Focus mode.

Back button auto focus will NOT be faster and more accurate. It is simply a user interface change that some people are more comfortable with.

You said you had calibrated the lens. What was the method used? What is the MFA setting of the lens?

Yes, of course I am using AI SERVO for the incoming car.

When I did the test in my first post, the MFA was set to -1 (moved towards the camera). The thing is, as I already mentioned, it is hard to find the correct value for MFA. Here is why:

To find out what MFA value I should use, I had my camera on tripod and took photos of a car that was ~50 meters away (as I read somewhere that the distance for calibration should be same as I will most often shoot at). This way I came up with value of -1. BUT when I took a photo of something else (or even of the same car few minutes later), the focus would be different with every photo! Sometimes OK, sometimes FF, sometimes BF.

Today I recalibrated the lens using the rule that it should be 50 times the focal length, so it was just about 6.5 meters which lead to value of -4. Mind you, I already had -4 for months before doing the previous test and still had inconsistent results of focus.

So after I put in the value of -4, I took 3 photos of 4 different scenes (objects in different distances). Camera was on tripod, used wired shutter, ONE SHOT, spot AF and central AF point selected.

SCENE 1 (AF point was over the license plate of the Audi)
- http://don.vn.cz/lenstest/mfa/IMG_7117.jpg OK
- http://don.vn.cz/lenstest/mfa/IMG_7118.jpg NOT OK - focus changed, why?
- http://don.vn.cz/lenstest/mfa/IMG_7119.jpg KIND OF OK

SCENE 2 (AF point was over the grille of the silver Skoda Fabia - the car with "Akuna" on it's side)
- http://don.vn.cz/lenstest/mfa/IMG_7120.jpg OK
- http://don.vn.cz/lenstest/mfa/IMG_7121.jpg OK
- http://don.vn.cz/lenstest/mfa/IMG_7122.jpg BAD - the focus changed, why?

SCENE 3 (AF point was at the bottomleft corner of the window, on the flower)
- http://don.vn.cz/lenstest/mfa/IMG_7123.jpg OK
- http://don.vn.cz/lenstest/mfa/IMG_7124.jpg OK
- http://don.vn.cz/lenstest/mfa/IMG_7124.jpg GOES BLURRY SLIGHTLY

SCENE 4 (AF point was where the chimney touches the roof of the house)
- http://don.vn.cz/lenstest/mfa/IMG_7127.jpg OK
- http://don.vn.cz/lenstest/mfa/IMG_7128.jpg BAD, the focus changed, why?
- http://don.vn.cz/lenstest/mfa/IMG_7129.jpg OK

All these photos were shot in RAW and converted to JPG without any adjustments. Again, f2.0, ISO 100, 135/f2 lens.

Why, when nothing in the scene, nor on the camera changes, does it focus to different place every time? Is this normal?

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