5D3 Lens drive when AF impossible

I have seen something recently relating to this. However I cannot find the article.

In the AF menu (AF4) and on page 99 of the manual this is discussed. Maybe (probably) I am being stupid. I don to understand this AND I am unable to change the settings on my camera.

Can some kind soul please give my some assistance on this subject.

I shoot ONLY sport and sports related stuff. Most of my work is done with a 400mm F2.8 II. Sometimes with a 1.4III as well. I use Servo AF .

Thanks in advance

What do you mean when you say you can't change it? You should be able to set it to ON or OFF, whichever you like. If the setting doesn't register, then there's something wrong with the camera. There's no mode (well, except green box modes and similar) which will prevent you from changing this parameter.

Or are you asking about the purpose of the setting itself? I'm not really sure from your post. Come back!

Default mode = "ON". If the camera AF system sees no contrast or too little light to focus, the AF system will rack the lens motor from end to end while looking to see if the situation gets better. Sometimes between the ends of the range it will find something to latch onto. Usually it just hunts around uselessly.

Optional mode = "OFF". If the camera can't find a contrast edge to focus on, it just leaves the AF motor sitting. What it does after that depends on another custom setting, whether the shutter release is AF priority or Shutter priority. In the former case the camera will not take a picture. In the latter case it will take an out of focus picture.

Thank you gentlemen for your replies. I am sorry if I did not phrase my question precisely enough.

I think I now understand what changing this setting will achieve.

I found the original thread I read, there are some interesting comments.

http://www.fredmiranda​.com/forum/topic/13282​15

Just want to add that the definition of "sees no contrast" includes when there indeed is contrast where the AF point is aimed, but the lens is set to a distance so far away from the correct one that the existing contrast is outside the metering range. This happens more frequently with long-range telephoto lenses, since they have a shallow depth of field by definition. If such a lens is set at infinity, it may not be able to meter a contrast that's closer than 10 meters, even if that contrast is good.

Such a case qualifies as a situation where the camera will use the setting for Lens drive when AF is impossible to determine what to do.