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.

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Depth of field in movies

One of the best things about the EOS 5D Mark II movie function is that it shoots on the full-frame sensor and therefore you get really shallow depth of field. Great.....except when you want lots in focus.
As far as I can tell, photography is all about problem solving and so it was the other day. I was trying to take a short movie clip of myself (yes, I am that vain) and needed to make sure I was in focus. I had a couple of attempts and it wasn't working out as the depth of field was just too shallow - any time I moved, I moved out of the plane of focus and it was ruined. So how to get around it?
With no control over aperture, there is not much you can do, but one thing will help - your knowledge of depth of field in relation to focus distance.
Initially I was using a EF35mm f/1.4L USM lens and DofF was very shallow. I switched lenses to the EF24-105mm f/4L IS USM and moved the camera and tripod further back. This had two effects:
1. The lens only has an f/4 maximum aperture so instantly more depth of field as the lens couldn't open as wide
2. By increasing my camera to subject distance, I increased the depth of field

Bingo! Enough depth of field for me to move around while staying in the plane of focus.

It's that second one that people forget. While everyone is busy using aperture to control depth of field, they forget that actually the most over-riding function of depth of field is the focus distance, or camera to subject distance.
Get further away from your subject and wham! more depth of field. Conversely, if you want less depth of field, get closer in to your subject and suddenly you're in blur city.

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