Archived posting to the Leica Users Group, 1997/11/04

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Subject: Re: Focussing 70-180 APO addendum
From: Jim Brick <jim@brick.org>
Date: Tue, 04 Nov 1997 21:38:07 -0800

At 09:09 PM 11/4/97 -0800, you wrote:
>At 10:11 PM 11/4/97 -0500, you wrote:
>>I noticed that infinity is not infinity when used with the 2x APO extender.
>>Atlanta's skyline is over 20 miles from Stone Mountain.  The picture was
>>tack sharp when the distance indicator was on 300 feet.
>>Even when used without the extender infinity is not exactly at the
>>'mechanical end' of the lens.  Perhaps 1 to 2 mm from the end to be tack
>>sharp.
>>
>>Is that something to worry?
>>
>>Regards Jorg
>>
>
>No. This is normal and proper. You see, there is no such thing as infinity.
>It's beyond the farthest possible object. Telephoto lenses frequently focus
>beyond what you might perceive as infinity. That mountain range over there.
>If you stop and think about it, it makes perfect sense.
>
>Jim
>

Adding to this, when focussing on distant objects (such as the moon) with
long and very long lenses, you still need to be able to focus *past* the
object and then come back to it. Just like focussing on a person, or car,
or whatever. The process of focussing usually entails focussing past, then
before, then on to the object. With telephotos, this is *way* out there.
Therefore, focussing past what you perceive as infinity, is proper.

Jim