Executive bifocal thickness question

[daveyboy]

Hi everyone. Hopefully somebody with a bit more experience in executives can help me here. I edged an exec bifocal yesterday and was pretty happy with the result (line was nice and straight and the power was good). The power was simple OD +2.00 OS +1.75 Add +1.00.

The dispensing optician however is not happy with the job and says that the surfacing lab surfaced the lens wrong because the shelf is the wrong way. After edging the top of the lens is thicker than the bottom (the shelf faces down) and the optician says that it should be the other way around (the top should be thin and the shelf should face up). Its been a long time since I edged an executive so I cannot remember how the lens should look. The lab told me that because of the plus power in distance the shelf will face down.

So my question is should the shelf always face up on an executive bifocal? Both the optician and lab are very experienced so I am puzzled.

Hopefully I have explained the issue properly but I can provide more info if not.

Thanks in advance.

[Wes]

The near portion (bottom) has more plus/less minus power, and is therefore steeper, which puts the "shelf" from the flatter distance portion facing down. Your Lab is correct and your optician is wrong, which he would have known by putting the lens in a focimeter, or by clocking it, or by thinking about it for a couple minutes. Also, the higher the add power, the more noticeable this "shelf" will be.

[Don Gilman]

It helps if base down prism is ground on execs as well as PALS , also called prism thinning to help equalize the thickness top to bottom

[DanLiv]

The near portion (bottom) has more plus/less minus power, and is therefore steeper, which puts the "shelf" from the flatter distance portion facing down. Your Lab is correct and your optician is wrong, which he would have known by putting the lens in a focimeter, or by clocking it, or by thinking about it for a couple minutes. Also, the higher the add power, the more noticeable this "shelf" will be.

Snap... nailed it.

[daveyboy]

Thanks for the info Wes. The attachment is very helpful too. The optician was expecting a similar look to a flat top bifocal but I will be able to explain the difference now. There was no prism thinning ordered so I will keep that in mind for the next time too.