Ari, all lenses are hot, center to edge. Non wide angle lenses have about 1/3rd stop fall off from the center. So, unless someone ground a screen to encompass this fall off you can only reduce or eliminate it with a proper fresnel screen
Ari, all lenses are hot, center to edge. Non wide angle lenses have about 1/3rd stop fall off from the center. So, unless someone ground a screen to encompass this fall off you can only reduce or eliminate it with a proper fresnel screen
1600 grit wet/dry. Any finer and it is clear.
Oh, I realize that, Bob, but there's no provision on a Wehman to fit a Fresnel. I don't like using them much, anyway.
Eric, I went up to 1000, which I feared would clear the glass to its original state. I dared not go further.
There are just soooo many kinds of "wet/dry" paper, as well as options of "plexiglas" or acrylic, and even differing polishing solutions (water, glycerine, paraffin oil, salad oil - seen it all), that one needs to be more specific, unless you're trying to retain a personal trade secret. And then if you go to power polishing, add a whole other layer of significant variables. Even grit ratings differ between not only type of abrasive, but systems of measurement. In the finer grits, the European standard is different from the US; and there are even other standards. Kinda like metric versus Imperial systems, then throw in something wacky like Whitworth. That's just with sheets abrasives. Powder or lapidary grits, a whole other game. But for me, fresnels are much easier to classify - they all go into a single "don't like any of em" column in my book.
For plexiglass, I really love the brightness and detail I get when lightly sandblasted. Takes about 45 seconds to do an 8x10 sheet nicely. The garnet grit in the blaster had been used a bit, and kept the pressure low.
Sent from my iPhone using Tapatalk
I made an 11X14 'Plexi' with DA and 1000 grit
For for a camera I never use, it is very useable
However I prefer my DIY glass
Maybe this year...
Ha! I still have my tube of grit that was a gift to me from Will Whitaker all those years ago...
Problem most of the auto parts store variety of Permatex valve grinding paste (mentioned earlier) is that it is a "multi-grit" paste - based on the theory that as one lapps ones valves (as I have on several occasions) the coarser grits will get pushed out from the contact surfaces as the grind progresses, ending with the finest grit in the paste. Very difficult to get this to work properly (coarse grit gets sucked back in, leading to over-grinding) and for all of my future engine work I'll be using single-grit pastes...thank you very much!
I suspect that for the purposes of grinding plexiglass for a ground glass, a single grit paste should also be used. Valve paste also comes mixed as a water or oil based formula...and I'd suggest water based for the ground glass.
When I grind my own glass screens, I use my grit from Will W. (made for grinding telescope mirrors, which Will had actually done) and I believe it is 500 grit corundum (or carborundum?). At any rate...I could imagine that for grinding plexiglass, the suggestion of using valve grinding paste might actually be better than using the typical dry grit diluted with water scenario. Just a hunch!
Then again, in pinch as was pointed out...if you can lay rows of magic tape onto the plexiglass carefully enough, and do this horizontally - you will have a not-too-distracting series of faint lines with which to check your horizons!
I used DA discs dry, they did clog, but took only 3
Posting Permissions
Reply With Quote
Bookmarks