Prism in top half of lens only ? ? ? HELP!

[skirk1975]

When the Dr. only wants prism in the top half of at FT28 we use a fresnel cut in half over the line. Fresnel is really on a temporary solution in most cases, but when nothing is wanted in the bottom half, then he wants them to contunue with the press on prism. This is difficult as they are not designed to last. Does anyone have any other ways to combat this issue ? :cry:

[mrba]

Make him a Benji!!!! I think Clive can do it for you.

[Jacqui]

Sounds kinda like a slab-off.

[rhondaboman]

Skirk1975

There's a great discussion on this subject but I don't know how to link it, so, let's see if I can get you lost giving directions. At the top of this page hit search, then type in "lab? Prism in distance only of a Bifocal?"

You'll be directed to several discussions related to this subject, just scroll down to the one with this heading. Happy Hunting ; )

[keithbenjamin]

Here it is...

http://www.optiboard.com/forums/show...stance+Bifocal

-KB

[chip anderson]

Franklin

I have just done this, took pictures and am almost finished with an article which amoung other things I intend to post on optiboards. It's so simple even I can do it. Cut and edge two pairs of single vision lenses exactly as if this were two single vision Rx's for identical frames. Split lenses on 1mm above (near portion) seg. line. And 1 mm below (distance portion) of seg line. Carefully finish removeing the remaining 1 mm with first a table disc sander. Then finish the last .2 mm or so off with a course, then smooth hand file. Mount both halfs in frame. If desired glue both halves with epoxy (can be removed from polished surfaces with acetone without damageing polished surface if you get sloppy.)

Suprise you are now as technically capable as Ben Franklin's circa 1784 optician.

Chip

Note: This only cost you the price of two pairs of cut and edged lenses and you didn't wait days, weeks, or months to get it back from a high priced speciallty lab.

[Sean]

Chip

What is your prefered method used to "split" the lenses?

[chip anderson]

If you are very, very carefull, split them with a cable drive dremmel tool with a fibre bit. This for cutting oversize only. One lays out lens on axis with a line drawn about 2mm wider than the final cut. Split here then trim very carefully with sanding disc, etc. as described. On glass I would use: Safety goggles, and a diamond wheel for cable drive dremmel tool.

Chip.

[jofelk]

Example:
: If base up left lense, then slab right seg with base up to match center in segs.
: If base down in left lense, then slab left lense base up to match center in segs.
: same for prism in right lense.

[Sean]

If you are very, very carefull, split them with a cable drive dremmel tool with a fibre bit. This for cutting oversize only. One lays out lens on axis with a line drawn about 2mm wider than the final cut. Split here then trim very carefully with sanding disc, etc. as described. On glass I would use: Safety goggles, and a diamond wheel for cable drive dremmel tool.

Chip.

Thanks :)

[Clive Noble]

Chip's got it right, the only point I'd differ on is that I never use glue, I did on the first couple of pairs that was many years ago and it always turned out looking messy. Today, after maybe 30 or 40 jobs I make sure, and this takes a lot of time and patience, that the 2 sections butt up very well against each other without any gaps, and it can only be done by hand, fine grinding on emery paper against a flat surface getting the angles exactly right, checking frequently and measuring with calipers so both eyes are the same!

I actually enjoy this work even though I groan when one comes through.

I've attached a few pics of a recent job where I had to replace the distance lenses after 18 months, the distance prism had increased to 10^ out each eye. The near remained at 3^ out

[Billy Brock]

Awesome photos Clive , so cool to see the specialty work ....... Texas Ranger & I made a few pair in the old days for pilots with overhead panels that required an unusual focal range ............ nice job !

B

[Andrew Weiss]

How do you hold it together?

When you did use glue to hold the two halves of the lens together, what glue did you use? And without the glue, are you relying on the eyewire pressure to hold the lens together and in place, or do you use a different kind of binder? If you're using eyewire pressure, doesn't this cause potential problems in zyl frames and undue lens stress in metal ones?

I checked out the old thread and it was GREAT.

[chip anderson]

Glue

I don't usually use glue. But you can use Epoxy on zyl. Chris Ryser says you can use Cannadian Balsam and I know this works on glass (or at least it did when we had labs that could grind a cementable wafer). I have not tried Canadian Balsam on any plastic.

Now the reasons for glue if you use it, are: I keeps the lenses from popping out of frame on impact ( and no, I don't think this will pass drop ball tests) and keeps dirt from collecting between the lenses. But as I said I don't usually use glue. I will if the patient has problems with either of the above.

Chip

[Clive Noble]

Andrew, I am using pressure from the frames to lock the lenses in place and as I mentioned, if they do butt up perfectly without any gaps, they will hold up very well. These lenses usually are so thick that there is no evidence of any stress there.

Chip is once again correct that we do have one old lady who regularly comes in with a destroyed frame and 4 component lens pieces, she drops them, sits on them, lets the dog play with them but fortunately, it's a cheapy frame that's always in stock and funnily enough the lenses are always undamaged...... maybe because they do come away on impact rather than just cracking up!

That's also an interesting comment on the 'drop ball test'
Fortunately we don't have to drop any balls here in Israel, but how do you guys get around this problem Stateside?

Clive