Just purchased a older B&L lensometer with a set of prism rings.How do you use prism rings when verifying prism?
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Just purchased a older B&L lensometer with a set of prism rings.How do you use prism rings when verifying prism?
The idea is to keep the mires inside the 5 prism diopter rings inside the lensometer to read the amount and direction of the prescribed prism. Start off with placing a prism ring's base opposite the prescribed prism. Use the axis wheel on the eye piece to align with the direction of prism. ( 180 for in or out prism, 90 degrees for vertical, or compound the prism to get the axis and amount you are looking for.) Example if you have 2D up and 2D out in a right lens, it compounds to about 2.75D at axis 135. You would align the eye piece axis at 135 and look for 2.75 up and out.
As far as using the rings; You can use any ring that will neutralize the prism to 5D or less since 5D is the limit of the lensometer without rings. Example;
9D base out is prescribed. Use a 5D ring aligned at 180 Base in. When you spot the lens the OC should be at 4D out along the 180 line. ( 9D out - 5D In = 4D out).
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