Wow, that's hot.
Now to my question: When a lab selects a base curve for a PAL, does the base curve generally follow what would be selected for a spherical SV lens?
I realize that PALs are aspheric, and may be flatter. But how flat? For example, we have a -2.00 PAL on a +3.00 base, and it looks pretty flat and it's causing a very flat front and temple splay when mounted in a zyl frame.
Thinking I would cry foul on that one. You have to keep the base as close to the frame wrap as possible to help elevate the situation you are now having.
And now everyone is going to start throwing in all the variables
Yes, most premium PAL designs use an optimal amount of asphericity to improve the optical performance when flatter curves are used, similar to their SV counterparts. However, backside PALs may not be as cosmetically flattened depending on the distance and add power. In your example, if the add is not too high, I would expect that it would land on a +3 BC or +4 BC blank. Maybe there's a problem with the bevel placement or a frame design defect.Originally Posted by drk
Wow, that's hot.
Now to my question: When a lab selects a base curve for a PAL, does the base curve generally follow what would be selected for a spherical SV lens?
I realize that PALs are aspheric, and may be flatter. But how flat? For example, we have a -2.00 PAL on a +3.00 base, and it looks pretty flat and it's causing a very flat front and temple splay when mounted in a zyl frame.
Science is a way of trying not to fool yourself. - Richard P. Feynman
Experience is the hardest teacher. She gives the test before the lesson.
Always with the variables, who did the lens work(insurance), lens design, material, there are several very good digital lenses that I use for 6 and 8 base curves. You can also specify your FBC depending on RX,material,FPD and of coarse further variables. It never hurts to have a heart to heart with your contact person at the lab. I like to build a rapport with my labs. Just remember, if you don't ask for special instructions to be followed they won't. Don't forget to eat your variables this Thursday.
I didn't attend the funeral, but I sent a nice letter saying I approved of it. Mark Twain
Agree with Robert M. But...you must KNOW the classic BF CC base FIRST, before your begin. In any event, you can:
1. Deviate at least 1D from the Template base curve with little issue.
2. Deviate from the manufacture recommended BC (in FF) at least 2D with little optical issue.
B
Also make sure you or your lab puts a matching lens bevel curve to match frame bevel curve and the temple splay will reduce.
Wow, that's hot.
Now to my question: When a lab selects a base curve for a PAL, does the base curve generally follow what would be selected for a spherical SV lens?
I realize that PALs are aspheric, and may be flatter. But how flat? For example, we have a -2.00 PAL on a +3.00 base, and it looks pretty flat and it's causing a very flat front and temple splay when mounted in a zyl frame.
B
Hotter picture =P If one of these ever hits us im not sure base curves will matter. But seriously, the lab we use can't seem to figure out how to put an 8 BC on a lens. Every time we send a wrap, irrespective of the special notes we put in they splay the frame out. Oakley, however, seems to have mastered the art of the BC and FF with little distortion and beautiful optics. I hate their frames but I love their RX program.
sounds like you need to find a new lab
Unless image size is one of those issues...
Most progressive base curves are chosen by a "chart" recommended by the manufacturer for Progressives. Some charts account for cyl and some don't depending on brand, some its just spherical power.
Most Free-form lenses do not allow you to choose the base curve, its chosen by the computer and the proper OPC Code is checked. For Wraps you must choose the BC though, so there any lens with Wrap Compensations will allow you to choose the Base Curve, so you can use wrap lenses in non-wrap frames for something custom.
Finished SV the Base Curve is chosen by the manufacturer.
Ideally for SV Semi-finished BC would be chosen by Vogel's rule for best optics, but its not cosmetically (its steeper than most people like) so I use Vogel's -1 when I can get away with it.
I think that has to do with the cutter blades on a dry edger compensating for the beefed up back bezel on the steep based frames.
Maybe a lab rat can chime in...
(hijack time !!!)
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