Today’s Question: My Canon 100-400mm lens lets me choose between 1.8m to infinity and 6.5m to infinity [for autofocus]. Would you explain the conditions under which you would select one or the other? Most people I know seem to leave it permanently on the 1.8m option.
Tim’s Quick Answer: These ranges relate to the distances the camera’s autofocus system will search when attempting to achieve focus. Selecting the narrower (and more distant) range can enable focus to be achieved more quickly, and can therefore be helpful as long as you don’t need to set the focus closer to your position than the selected range provides.
More Detail: When you establish focus for a scene, you are essentially setting the distance at which you can expect sharp focus for the lens. A given lens will have a close focus limit, representing the closest distance from the lens that focus can be achieved. In the example included in today’s question, the closest focus point for the lens is 1.8 meters (a little less than six feet). So, when you activate autofocus, the distance at which focus can be established could be anywhere between 1.8 meters and infinity.
Naturally, the greater the possible focus distance range the more time might be required to find the right point at which to focus. Many lenses therefore provide an option to restrict the autofocus to search across a shorter total distance. For the lens noted in today’s question, you can opt to limit the autofocus search so that the closest distance the lens will attempt to achieve focus is 6.5 meters (a little more than 21 feet).
If you know you won’t need to focus any closer than the starting point for the narrower range, you can select that range on the lens. In this example, by switching to the more distant range the lens won’t try to achieve focus for the range between 1.8 meters and 6.5 meters, even though the lens is capable of focusing within that range.
So, if you are confident you won’t need to focus closer that the more distant range will allow, you can select the more distant range to help ensure you’ll be able to establish focus as quickly as possible, all other things being equal. That is because you are restricting the range the lens will search for focus, so less time will be necessary to search the full range in order to establish focus.