In article , Terry and Tracey <foxcroft@xxxxxxxxxxxxxxxxxxxxxx> wrote
>- --
>>This one I think I know, but if I'm incorrect, I'm sure the right answer
>>will come forthwith. A "normal" lens is focused by moving the elements
>>closer or farther apart by some means, generally a helicoid thread
>>mechanism. In this case, the physical distance between the rear-most
>>element and the front-most element varies. With an internal focussing
>lens,
>>an element, or group of elements, in between the front and rear move,
>>shifting the place of focus but the physical distance from front to rear to
>>not change.
>>
>
>I thought the normal lens just extended. Put a 300mm on a 25mm tube
>(focussed at infinity), or wind it out 25mm, same result.
>
>Internal focussing move elements and can focus closer (generally), but they
>reduce their focal length. This isn't a problem for most situations, but
>once you focus closer and on small things the focal length reduction becomes
>greater. Or so I'm told.
>
>My two lenses that modify their lens groups (Zuiko 50mm f3.5 and Tamron 90mm
>f2.5) both extend. (25 for the 50, 45 for the 90), but the elements inside
>move about and things change. But it still extension.
>
>Foxy
>
There is no need for an internally focussed lens to change its focal
length, and few change substantially.
There are two main reasons for the use of internal focus mechanisms :
1) the design may enable a reduced optical travel over the focus range.
This is more significant with long focal lengths where the change
required movement of the lens to cover the full range of focus distances
can be substantial. Optical alignment can be better maintained if the
overall travel of the active elements is small, although it does need to
be large enough to avoid temperature drift etc.
2) in sealed lenses (common in defence systems) the use of an internal
focus mechanism avoids the pumping effect of moving the entire lens or
the front elements. Even in unsealed systems this has advantages since
it prevents moisture, dust and other contaminants from being pumped into
the lens or camera due to the focus operation.
--
Kennedy
Yes, Socrates himself is particularly missed;
A lovely little thinker, but a bugger when he's pissed.
Python Philosophers (replace 'nospam' with 'kennedym' when replying)
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