9 May 2009

Focus distance limit switches on lenses

Some of the Canon lenses feature a little slide switch to let you restrict the range of focus distances the lens will operate over, but why? 

With the longer focal length lenses there's often quite narrow depth of field and also quite a large range of movement for the optics to cover from the closest focus distance out to infinity. If your subjects easily fall in to the range covered by a lens restricted limit switches then you get a much faster focus shift from minimum to maximum subject distance as the focus assembly only moves a small set of it's possible range. There are quite a lot of lenses in the range with focus limit switches including the EF100mm f/2.8 MACRO USM, EF135mm f/2L USM, EF70-200mm f/4L IS USM. In fact most L series lenses with focal lengths longer than 100mm have focus limit switches. In the case of a mega lens like the Ef800mm f/5.6L IS USM there's three positions for the limiter including the full 6 metre close focus to infinity range!

Coming back to something more normal the popular EF70-200mm f/4L IS USM has focus limit ranges 1.2m to infinity and 3m to infinity. So if you are shooting say motorsport and are over 3m from the track boundary choose the restricted range of focus, the lens will move from the new 3m minimum focus to infinity much faster than it would from 1.2m to infinity. This makes the AF seem more snappy when you pick up a camera and the initial focus is way off. There's just less time spent hunting for the subject while the lens racks in and out from minimum to maximum focus distances.


1 comment:

Henry Mounser said...

A great post, I've learnt something very useful today. Thanks.