So WTF is prism?

I just recieved my new glasses prescription.
I’ve worn glasses for about 35 years.
Don’t need them for most ordinary everyday things but need safety glasses for work.
Sooo I got one new pair for everyday use and the company pays for safety glasses so I ordered another pair for work.
My problem is the everyday glasses paid for by my insurance company have something called prism.
Of course the safety glasses will have the same prism. Thing is everything I see with my left eye is blurred. I don’t know if it is caused by this prism thing or the optometrist just screwed up.
I picked up the glasses saturday and the optometrist wasn’t there so I agreed to try them over the weekend because “you have to get used to prism and progressive lenses”
Well I ain’t getting used to them.
So WTF is this prism thing?

“Prism” is an error in the angle of sight. It suggests that when you look “straighyt ahead” with both eyes, they’re actuially out of parallel by a slight angle. This angle is called “prism”. It’s measured in “prism Diopters”, which is a confusing name, since they aren’t Diopters at all (a measure of optical power), but a deviation of 1 cm at a distance of 1 meter (which makes it 1 milliradian in more common terminology).

Here’s the thing – prism is pretty easy to apply to a lens, but it shouldn’t affect the focus at all. If your prescription is correct in all other particulars, the effect of getting prism wrong ought to be that the image is in focus, but off at a slight angle. It sounds to me as if someone didn’t get the power correct.
You say these are progressive glasses. Were your old ones progressives? If you’ve never worn progressives before the problem might be progressives vs. non-progressives. (Progressives are a way of gradually blending the near and far field parts of what would otherwise be bifocals, so the range gradually changes between the two regions, instead of having a hard line, for those who don’t know.)

The old ones were progressives with lines.Apparently they don’t make them any more. I think I can get used to the bifocal part. I just didn’t understand the out of focus part.
It does make walking through the grass a new experience though. :smiley:
I’m a little concerned because I work with running machinery, Lots of times I’m pretty tired by the end of the shift. IMHO thats when accidents happen.
Thanks for the info

Dammit, that should’ve read “10 milliradians” (per prism Diopter).

I used to make optical devices where we measured the prism Diopter between the two eyes, so I’m familiar with the term as applied to visors, goggles, and the like. I’;ve never worked with prism as applied to eyes, or as an intentional deviation (meant to correct deficiencies in the viewer), so take your concerns to your eye doctor.

It’s called “prism” because it’s an error in angle that’s the same across the entire eye, just as when you look through a prism. It’s mathematically equivalent to “adding” a very thin wedge prism to your glasses. Ophthalmic lens sets have prismatic wedges that they can put in front of your eye. So does a phoropter (that lens thingie that looks like a mad scientist’s idea of grossly oversized glasses they have you look through when testing your vision (“Is it better this way? Or this way?”)

That would be 10 milliradians. Or a deviation of 1 mm at one meter, or one of 1 cm at 10 meters. But, what’s a factor of ten amongst friends? :slight_smile:

Teach me to preview. Sorry, Cal.

I’ve had a prism for a coupla years now, and it took narly 2 weeks to get used to it. But you eventually do indeed get used to it.

My blurry vision in my left lens was caused by a typo on the part of the optometrist.
thanks all for your input

I’m glad to hear this. Typos happen, and we like it when the patient understands that. I’m sure you’ll be back to happy vision in no time.

When the glasses are “fixed”, take your time with your progressives. I’ll be here if you need the “new progressive education” that I tend to give my first time patients :slight_smile:

FTR, prism is often used in eyeglasses to correct double vision, or sometimes to force an eye to do something that the muscles should be doing but don’t want to do.

Prism? Didn’t they compete with HBO back in the early days of cable TV? :wink: