Close to classic crowns and flints (1.5-1.6 or so). Not so much close to more modern glass types with indices of 1.7 or greater. For photographic objectives it's not as important. Very high performance designs have to account for the difference or they can run into trouble.Originally Posted by Bernard_L
Newly made large format dry plates available! Look:
https://www.pictoriographica.com
Miguel's lens does use Canada Balsam as the cement and should be as good as new in a few days. The edges have been blackened as is the custom these days. The first thing the solvent did was to remove the black , showing that the French tradition for pencil writing on the edges continued up into the 20th century!
I think that these later balsam jobs must have contained an additive or have been subject to refining, as there is no sign of the usual yellowing of the dried out edges. No quite so smelly, either.
And this is what the OP's lens looks like this morning!
It has a wonderful mount and the pencil 15 on the lens edge matches the marked XV on the aluminium mount. It also has a hole drilled through the mount in order to insert oil if the thread was difficult.
Nice work!
“You often feel tired, not because you've done too much, but because you've done too little of what sparks a light in you.”
― Alexander Den Heijer, Nothing You Don't Already Know
Magnifique !!!
Thanks a lottttttt... Steven for bringing this cell back to its original purpose, clean and save from "glucoma".
Will post more pics once the lens group will be mounted on its barrel.
Bravo LFF !
A friend used to re cement lenses. He'd put them together after applying cement, then roll the re cemented cell on a flat table so the edges were lined up. As I understand it, some lenses have elements that need to go back together exactly as they came out. Not only in the right order, but even in the same position (clockwise) relative to the other elements. The story goes that only one or two people could assemble a Dagor and align the elements to cancel out shortcomings. Which is why both front and rear cells carry the serial number. This also leads me to believe you might want to mark the edges of each element in the cell on the side as well as the mount so that the elements end up in the same position they were in before, relative to the other elements in the cell (Like their position on a clock). And so that the whole cell (front or rear) ends up in the same position it was in before, relative to the other cell. Modern lenses are mostly ground by computers, or so they tell me, so such concerns probably don't exist in modern lenses. Maybe someone else has more or better information.
This "rolling" of lenses sounds possible for some designs, but many post- rectilinear component lenses have a lens which is insert in a concave "hole" which finishes quite a few mm from the lens edge. Many Petzvals and fast Aplanats/RRs have very thin edged double convex lens which would not be suitable. The sense of touch is surprising good at noting irregularities.
I can't say much about later designs, but neither Petzvals or RRs are sensitive to the exact distance between cells. I just can't believe the comment about Dagor assembly, There are absolutely no adjustment possibilities in their cells.
The adjustment as explained to me is within each cell, turning each element against the next, not distance between elements.
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