Archived posting to the Leica Users Group, 2001/11/01
[Author Prev] [Author Next] [Thread Prev] [Thread Next] [Author Index] [Topic Index] [Home] [Search]David Rodgers wrote: > While reading some lug message from the last week or so I read some > comments about scratches on lenses. While scratches on rear elements are > probably worse than on front elements, placement of the scratch is very > important. I had 150/4 Sonnar with a horrible scratch on the rear element > (possibly left by a repair person gone mad with a screwdriver). It never > showed up in the least (even at f4). The only time it was ever a factor was > when I bought and sold it. It greatly reduced the value of the lens. OTOH, > I sold it for what I originally paid for it and I used it for 10-years. > (try that with anything digital!) > > The horrific scratch on the 150 was on the outer third of the rear element, > which led me to wonder; in the mystical world of bent light and > vectors, just how much of a rear element is actually used. Maybe just a > portion of the center? I suppose only a lens designer could answer that. > > Dave While you are correct in that only a lens designer could give some sort of measure I will say that the closer the rear element is to the film the more critical it becomes. Stick a piece of crud on the rear element of the 21mm and take a picture of a large expanse of sky which will be rendered with uniform tone or color. If the crud is in the light path of the rays forming that expanse then you will see the effect as a change in tone. Real world, test the lens and see if the effect shows on film. Dennis - -- To unsubscribe, see http://mejac.palo-alto.ca.us/leica-users/unsub.html