AF System. How It Works. Back/Front Fucus Issues Analysing

Started Feb 24, 2009 | Discussions thread
yepun Junior Member • Posts: 49
Re: this part is correct

Thanks for your text it is really good!

Its RKM wrote:

Therefore, the
camera MUST be making at least a second measurement of focus before
lighting the AF confirm lamp.

There I would go further. It's not just a second look, it's a fairly high frequency the AF sensor is read. I guess something between 3Hz and 20Hz. If you do your suggested experiment, an entry level camera will travel much further till it recognizes "ups, wrong direction, turn around" than a high end 1D camera. There the AF sensor has a much faster readout.

This leads to the ultimate question, how can then BF/FF exist? I'll bring some light in here. I repair lenses daily and often these older lenses (300/2.8 400/2.8 500/4.5 ...) where Canon has no spare parts any more and I have to repair the old AF unit. So I know what is going on there.

We have to start at another point. What means "in focus" on the image sensor? This is the layer where most of the light rays entering the front lens focus and come together. The important word in that sentence is MOST. Light rays coming from the outer region of the lens often focus in a slightly different layer than the rest. This is one of the reasons why lenses get better when stopped down. Now lets go to how a phase difference AF works. It looks through the right and left side of the front lens and does triangulation. You are getting the point? The AF uses for its measurement parts of the lens which focus in a different layer than the light from the whole front element forming the image on the sensor. Hmmm, bad thing, how can we get rid of that - We need a lens calibration.

When the camera sends the command I want you to focus, the lens sends back same parameters and one of them is called "best focus compensation". This value tells the camera when my AF sensor says 0.045 units FF, the real image on my image sensor is in focus. During lens calibration these values are programmed into the eprom of the lens. Older lenses send two best focus compensation values. One is for the 2.8 AF and one is for the 5.6 AF. Because they look through different parts of the lens the need different compensation values. That are the lenses which are often adjusted by soldering bridges on the electronic circuit. If the bridge is on or off, the lens sends different values. New lenses which are adjusted by computer get the values directly written into the eprom. They even send 4 best focus compensation values. Two for 1D AF systems and and two for the rest of the cameras. This is necessary because the 1D AF system is different and looks through different parts of the lens than the rest of AF systems.

So the answer is quite easy. The lens sends not the correct compensation value, the camera will focus the lens to the wrong position -> Wrong focus independent of OneShot or Servo, the camera can't know it better.

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