I am doing a distance leaning optical technicians course and i am having some problems fully understanding the whole issue of the direction of base when figuring out prisms and decentration.
1)
A lens of power -4.00DS / +2.00 DC x90 is decentred 4mm IN and 2 mm UP on a right eye. What horizontal and vertical prisms will be produced.
My answer
Power Meridans
along axis. Sphere power only F @ 90 = -4.00
power merdian 90 to axis - sphere plus cyl -4.00 + 2.00 = -2.00
F @ 90 = -4.00
F @ 180 = -2.00
P @ 90 = C x F = 0.2 x 4 = 0.8^
P @ 180 = C x F = 0.4 x 2 = 0.8^
As F @ 90 is negitive the prism base will be in the opposite direction as the decentration ie. Base out.
AS F @180 is negitive. the prisim base will be in the opposite direction as the decentration. ie Base out
0.8^ BASE Down and 0.8^ BASE Out.
2)What prism will there be at a point 4mm IN and 2mm UP on the same Rx as in question 1? Answer in horizontal and vertical meridians.
I tryed to do this one and got the same answer as the first, feeling very mnixed up i attemped the rest and at this stage have no idea what is going on!!
3) You wish to produce a prism of 3.00^ base up on Rx +5.00DS/+2.00DC x 180 by decentration. state the amount of decentration and its direction for a left eye.
4) How much would you have to decentre a lens -2.50 DS / -2.50 DC x 90 to produce a prism of 5^ base IN and 2^ Base up for a right eye?
5) How much prism will there be at a point 6mm UP and 2 mm IN on Rx. +2.00DS/-3.00 DC x180, right eye, answer as a single resultant prism.
Anyone able to explain how to do these more so then actually doing them <defeats the purpose if i cant understand the "how" part. My Thanks
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