Explanation of distance switch on 100-400 lens.

Started Dec 30, 2012 | Discussions thread
Narcosynthesis Senior Member • Posts: 1,828
Re: Explanation of distance switch on 100-400 lens.

If you imagine the range of focus the camera can use, from close focus all the way out to infinite - when you focus it, it checks whether the image under the focus point is sharp, if the image is sharp it stops and locks focus, if it is not it then moves the focus forwards a step and repeats the cycle until it gets that okay, or reaches the end of the focus scale at which point it starts back again to where it began.

Obviously this is computer powered and running at a fair clip, so rather than an hour of step-check-step-check- before locking focus, it charges through the range in a fraction of a second.

All the focus limit switch does is tell the camera not to bother with certain ranges - if you know you are shooting birds at 15+m away for example, if you set the limit switch to 6.5m-infinite, when the lens focusses it will ignore all the range from 1.8m-6.5m, theoretically speeding up the focus. If you know your subjects will be varying, set it to the full range and it wil focus right up to the close limits, with the chance of a fractionally longer focus time.

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