400mm f5.6L: 3.5m or 8.5m?

[jim monroe]

After reading this forum for the last 6 months I knew eventually I would be getting some L lenses and the 400mm f5.6L is my first. I have read a fair amount about this lense and if anyone is interested in a review I receommend the article The Forgotten 400 at Luminuous Landscape website.
What I haven't read a thing about anywhere is an option I noted when getting the lense and reading the instruction manual. It has two settings one 3.5m to infinity and the other I think 8.5m to infinity (I don't have the lense in front of me right now but the exact value is not critical).
Obviously with the setting of 3.5m rather than the 8.5m one can focus on closer objects. My question is: why not just set the lense at 3.5m to infinity which gives the larger range or does when focusing on a far away object one really pick up some extra sharpness, etc., by switching to the 8.5m setting?
Anyone with any experience along these lines?

[PacAce]

After reading this forum for the last 6 months I knew eventually I would be getting some L lenses and the 400mm f5.6L is my first. I have read a fair amount about this lense and if anyone is interested in a review I receommend the article The Forgotten 400 at Luminuous Landscape website.
What I haven't read a thing about anywhere is an option I noted when getting the lense and reading the instruction manual. It has two settings one 3.5m to infinity and the other I think 8.5m to infinity (I don't have the lense in front of me right now but the exact value is not critical).
Obviously with the setting of 3.5m rather than the 8.5m one can focus on closer objects. My question is: why not just set the lense at 3.5m to infinity which gives the larger range or does when focusing on a far away object one really pick up some extra sharpness, etc., by switching to the 8.5m setting?
Anyone with any experience along these lines?

I don't have the 400L but I do have the 100-400L which as a similar switch. Basically, if you do a lot of shooting of subjects that are at a distance, say for example flying birds, then it really is helpful to have the switch set to 8.5m. This way, if the camera loses the lock on a bird, for example, and needs to re-aquire the lock, it'll have a smaller focusing range to go through which makes it quicker. In other words, when the camera loses a focus lock, it will travel the entire focusing range to try to get the focus lock again. If you had the lens set to 3.5m, and the lens was set to infinity when it lost focus lock, the camera will focus all the way down to 3.5 and then back up to infinity again in the attempt to regain focus lock.

[CyberDyneSystems]

Indeed,. many of Canon's (and other brands) higher end telephoto lenses have these switches on them.

The actual name of the switch tells the whole story.. they are called "Focus limiting switches"

And as PacAce says,. in a long telephoto,. by "limiting" the close focus distance,. AutoFocus speeds can be improved dramatically. Especially in an instance when something would cause the AF to "hunt".

The only downside to these switches seems to be the frequncy with which they get turned on or off when taking the lens in and out of your camera bag!

So check those swtches before you shoot! :)