problems with aspheric lenses

[mirage2k2]

:angry: I'm having lots of problems with my lenses. My prescription is quite strong, +6 in both eyes with 3 base out prism in both eyes. My old lenses were high index 1.8 glass non aspheric and the vision was pretty good. My new lenses are high index 1.74 plastic aspheric. The vision is bad, firstly the centres seem to be out by about 5mm (in the direction of the temple) in both eyes, secondly, the off centre vision is awful. If I move my eyes just slightly off centre one eye is always looking through a blury part of the lense (the part nearest my nose)! Its seems to me that the field of vision through an aspheric lense is very small! Is this the case?

I understand that the prism moves the optical centre of the lense - is it possible that the centres "seem" to be out because the image right in front of my eye is coming through a poor part of the lense due to the refraction of the prism?

Please help!!!

[chip anderson]

Go back to the 1.8 glass, it worked. Adding prism to aspheric lenses isn't always the smartest thing one can do.

[eromitlab]

Go back to the 1.8 glass, it worked. Adding prism to aspheric lenses isn't always the smartest thing one can do.

forgive me for asking, Chip, but why is best to avoid prism in aspherics?

:idea: I assume it would have something to do with the non-uniform nature of the front surface curvature, right?

[CME4SPECS]

forgive me for asking, Chip, but why is best to avoid prism in aspherics?

:idea: I assume it would have something to do with the non-uniform nature of the front surface curvature, right?

Miragey2k explains why in his post. The "sweet spot" of the aspheric design is not in direct alignment with his eyes. The amount of prism in this lens has moved the o.c 5 mm away from the center of the asphericity of the lens. A +6 lens with 3 diopters of prism has 5 mm of displacement.

[eromitlab]

D'OH!! yeah, I guess he does... 'just didn't visualize it correctly, I guess. I understand the "sweet spot" idea, it just didn't occur to me at the time. Must come from sniffing too much neutralizer and sharpie fumes. :hammer:

[mirage2k2]

I have been led to believe that the entire lens is a sweet spot when its aspheric!!! But does this idea go straight out of the window when it comes to high index 1.74?

Question then, does a 1.8 glass lense have better optical characteristics than a 1.74 plastic lense because it is glass OR because it is non-aspheric?

[QDO1]

I have been led to believe that the entire lens is a sweet spot when its aspheric!!! But does this idea go straight out of the window when it comes to high index 1.74?

Question then, does a 1.8 glass lense have better optical characteristics than a 1.74 plastic lense because it is glass OR because it is non-aspheric?

One can have an aspheric, and prism. You have an aspheric front surface, which is lined up vertically and horizontally, to the pupil centres (monocularlary) and the prism is SURFACED on the back surface. One ought to adjust the heights of the OC's to compensated for the frontal angle.

The problem might (in this case) be a multiple one - poorly lined up aspheric front surface and poor abbe value lens combined decentered lens

1.8 glass abbe = 39.3
1.74 plastic abbe = 33

[chip anderson]

As I understand or missunderstand aspherics. They are designed to provide best acuity when the eye is straight ahead looking though the center. They are also designed to compenstate for the further distance (from the cornea) as the the moves from the central visual axis. If you add prism (de-center) you will now be looking through a protion of the lens designed for a greater distance from the eye (compensated power) when the eye is looking straight ahead. When you turn the eye slightly away from the center (with prism) you will find the part of the lens designed for central vision (sweet spot). This will not be the same for each lens.

Why look for trouble? You are doing a highly complicaated job that obviously few others will attempt. The patient will be gratefull. Not every job must be pretty. I have some very ugly tools that do thier job very well and I am pleased with them. I have also owned some pretty cars and an ugly truck. Form follows function, not the other way around.

Chip

[QDO1]

As I understand or missunderstand aspherics. They are designed to provide best acuity when the eye is straight ahead looking though the center. They are also designed to compenstate for the further distance (from the cornea) as the the moves from the central visual axis. If you add prism (de-center) you will now be looking through a protion of the lens designed for a greater distance from the eye (compensated power) when the eye is looking straight ahead. When you turn the eye slightly away from the center (with prism) you will find the part of the lens designed for central vision (sweet spot). This will not be the same for each lens.

Thats why you must get prism surfaced on the rear of the lens. you line up the front aspheric portion to the optical axis, and the prism on the rear surface moves the image around as required. If you just decenter a lens then you are going to be looking through the wrong aspheric zone of the lens

[Lewy]

I quite agree! Prism can be dispensed on an aspheric but it MUST be surfaced.

Lewy