Archived posting to the Leica Users Group, 2000/05/19

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Subject: Re: [Leica] Long lens and filter question
From: "Takeshi Hashimoto" <hashimoto.takeshi@mailcity.com>
Date: Fri, 19 May 2000 02:30:03 -1100

 
- --

On Thu, 18 May 2000 19:06:17   Dennis Painter wrote:
>Takeshi Hashimoto wrote:
>> 
>> 
>> --
>> 
>> On Wed, 17 May 2000 19:15:36   Dennis Painter wrote:
>> >Looking through the viewfinder Peter Niessen wrote:
>> >>
>> >> Dear all,
>> >>
>> >> today I noticed that, when using the series VIII yellow filter on my 4.8/280
>> >> Telyt, the infinity position had changed from infinity to a little
>> >> above the 100m on the distance scale. The filter itself is approx 4mm
>> >> thick. Is this result compatible with what others have observed?
>> >>
>> >> Cheers, Peter.
>> >
>> >Peter,
>> >


No, just the opposite. It moves the focus back (toward the film) by its own thickness, essentially by "delaying" the light as it passes through the glass.
>> >I have pondered this off and on for a bit and cannot think of any way a
>> >perfectly flat piece of glass would shift the focus. I think some others
>> >mentioned that it will but don't think there was any real explanation of
>> >how.
>> >
>> >Not specifically addressed here:
>> >
>> >http://www.schneideroptics.com/white/long.htm
>> >
>> >But they basically claim no effect.
>> 
>> It does by moving the focus back the equivalent of its thickness.
>> 
>
>How does it do this?
>
>Does "moving the focus back" mean that it is as though the object 'seems
>closer' by the thickness of the glass? Would not be much effect for an
>object at infinity if that is the case.
>
>Again, how does it do this? 
>
>Dennis
>


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Replies: Reply from Dennis Painter <dpainter@bigfoot.com> (Re: [Leica] Long lens and filter question)
Reply from Jim Brick <jim@brick.org> ([Leica] Re: filter shift answer)