Archived posting to the Leica Users Group, 2000/05/20

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Subject: Re: [Leica] Long lens and filter question
From: "Takeshi Hashimoto" <hashimoto.takeshi@mailcity.com>
Date: Sat, 20 May 2000 10:06:36 -1100

 
- --

On Fri, 19 May 2000 23:32:07   Dennis Painter wrote:
>Takeshi Hashimoto wrote:
>> 
>> 
>> --
>> 
>> 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
>> >
>Takeshi 
>
>I am speechless, well nearly so, to learn the you think a filter shifts
>focus at the film plane by an amount equal to the thickness of the
>filter.
>
>No wonder images made with the wide angle lenses on my M are hopefully
>out of focus with a filter attached ;-)
>
>Yes, the speed of light is less through glass than air, but if the glass
>has flat parallel surfaces then when we focus on an object at infinity
>the delay is equal for all rays of light. Thus about all that happens is
>there is a time 'parallax' of perhaps a few picoseconds, between
>exposure made on two identical setups, one with a filter, the other
>without.
>
>Seriously, what you said just doesn't hold water. Maybe what you said is
>not what you meant but I think the listmembers must be tired of deleting
>this by now.
>
>Best of light,
>
>Dennis
>

Many long lenses take into consideration the presence of the filter, which MUST be in place in such lenses (e.g., the 400 f/2.8. The effect is not linear, but depends upon the focal length. Perhaps someone at Leica could enlighten us.


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