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

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Subject: Re: Focussing 70-180 APO addendum
From: Jim Brick <>
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
>>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.

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.