Archived posting to the Leica Users Group, 2000/05/19
[Author Prev] [Author Next] [Thread Prev] [Thread Next] [Author Index] [Topic Index] [Home] [Search]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