lens thickness calculation - prism

[essegn]

I know that it does exist the way of lens thickness calculation, where the lens diameter is not measured from optical center , but from geometrical center of frame / foil - the result is the smallest possible diameter.

The difference between geometrical center of foil and optical center gives decentration - prism.

I was told that result of this approach would be the thinner lenses - when lens calculation is done to exact lens shape / frame it is possible modify the shape of the lens from circle to ellipse and the smallest diameter of lens is used.

The smaller the diameter the less lens thickness would be.
In general, if any prism is empolyed to the calculation (not prescribed prism, but because of decentration) the result is that lens gets thicker toward the base.

How does it work ? I would appreciate if somebody provides me with formulas related to this topic.

[Rafael]

If you determine the diameter of the lens by optical center, you will have more material for example on the nasal side of you lens. Your lens looks then thin nasal in the diameter that you have produced.But if you block the lens and edge it it will be thicker nasal if its a plus lens.I think we are talking about plus lenses. If you calculate the lens with centration data you can get better results but not always (but for the custumer till he don´t cut it down to the final form it will appear nasal thicker than you lens because you lens has be manufactured in bigger diameter, and the adventage is also you have not sharp edge lenses in the coating which can creat dipping problems and less flexing of the lens edges. And depending on power and axis you can generate also thinner lenses, and you can use smaller Blancs or generate bigger diameters from smaller Blancs.