At what distance does a Canon 100-400mm lens reach it's maximum (infinity) focus range?

Question

The lens focuses from 1.8m to infinity. At what distance would you reach the infinity point? In other words at what distance would the lens stop changing the focus point and maintain its setting not matter how much further away an object, or point of focus was? And, beyond that point would it be wiser to set the focus to manual at infinity or continue to allow the autofocus to control the lens?

Answer 1

That's called the hyperfocal distance. Unfortunately, this distance is not fixed for a given lens. It depends on the specific focal length you select, the aperture, etc. Here's a few links if you want to delve into this subject matter:
http://www.dofmaster.com/dofjs.html
http://www.dofmaster.com/custom.html
http://johnhendry.com/gadget/dof.php

Answer 2

OMG has given you some good links on hyperfocal distance.

One thing that is important to remember about hyperfocal distance is that it gives you "acceptable" sharpness. For the best results, you may still want to set this manually by focusing on some object in the distance and just cranking the focus barrel until everything you like is in acceptable focus.

You are dealing with a variable focal length lens, so the hyperfocal length will change as you zoom your lens. How? The short answer is, the longer the focal length, the more you will be able to throw the background out of focus and the longer the focal length, the less wiggle room you will have when trying to use the hyperfocal distance to arrive at an acceptable degree of focus for much of your image.

For what it's worth, I see the last numbered distance on the focus scale is 20 meters (about 65 feet). After that, it goes to the infinity sign. Maybe this gives us a clue???