Archived posting to the Leica Users Group, 2000/05/20
[Author Prev] [Author Next] [Thread Prev] [Thread Next] [Author Index] [Topic Index] [Home] [Search]- -- 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. Get your FREE Email at http://mailcity.lycos.com Get your PERSONALIZED START PAGE at http://my.lycos.com