Does the Elliptical lenses will reduce thickness? If it does, what is the logic behind it.:nerd:
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Does the Elliptical lenses will reduce thickness? If it does, what is the logic behind it.:nerd:
Please explain more, what do you mean by Elliptical?
elliptical means the lens will not be in round shape.
eg: normal lenses can be 65mm or 60mm diameter but elliptical will be 65/60 or 70/60
you can see it on this below pic.
Attachment 9405
Zeiss always following elliptical shapes as their standard (doesn't matter what power is) for progressive lenses
Elliptical shaping is simply better/easier to edge, without slippage. Many frames now have smaller B measurements, and the uncut lenses are now reflecting this.
The LOH generators have had the ability to crib oval shapes for some time. The others may also have it.
There should be no difference after the lenses are edged, just when uncut.
The reason some manufactures use elliptical blanks is purely to reduce material costs, most of blank is lost in processing (surfacing first, edging second). The end product will be the same, just less is cut away because there is less to begin with.
I dont think cost reducing is only the reason. I have noticed that for the lenses which is near to 90degre axis and plus powers are thinner if we make it in elliptical diameter.
Whether an elliptical shape will result in a thinner lens depends on how the lens thickness has been calculated.
In the U.S., lenses are usually ordered along with either a tracing or a set of frame measurements, and it is left up to the lab to determine cutout, blank size, and thickness. In that case, the shape of the uncut blank is likely to be completely irrelevant, because lens thickness will have been calculated using the frame shape or dimensions.
In Europe, it’s more common for opticians to order lenses by blank diameter. In that case, ordering an oval shape (e.g., 70mm x 60mm) rather than round (e.g., 70mm) may result in a thinner lens, at least for lenses having a plus power in the vertical dimension (including many progressives).
With the advent of freeform and automated processing, many labs now require that a generated lens not have any knife edges, because a sharp edge may tear up the polishing pad, and conformable lap – so there may be a “minimum thickness at crib perimeter” specified, which has to be considered in the thickness calculation. In that case, the uncut shape again may impact thickness.
Shanbaum, how do you achieve an oval shape on a sphrical lens without cribbing, molding or using more asphericity in a given meridian (non-PAL)? With decentration? If it's with the latter, how would it be thinner than say knife edging the correct cut out diameter?
You don't, without cribbing. When you order lenses by blank size/shape (per my example), it's almost certainly going to be cribbed to that dimension, and that's the dimension to which it's calculated. In that context, "cut-out diameter" is unknown - the presumption is that the ordering optician has figured it out (a presumption of which I'm skeptical, but it's what they do).Quote:
Originally Posted by optical24/7