Progressive Questions

[Spexx]

I don't know much about progressive lenses, whatever i know is from this site and my part time job. What i know so far:

- 3 different focal powers
- Side of the lenses are distorted, can't see through the sides
- There's a fitting cross for height adjustment and pd

What i don't understand is the corridor, where is it located??? what do you mean how many mm's the corridor is dropped from the fitting cross

Short corridor lenses have more peripheral vision???? or more add vision???

[EyeFitWell]

At the fitting cross, the lens is distance only. As you move below the fitting cross into the corridor, that's where the add power is gradually added to the distance Rx. The corridor is the part of the lens where the power is transitioning from distance to near.
Short corridor lenses change faster, therefore you can put the lenses into smaller frames without cutting out the reading. In general, this is at the expense of the intermediate zone (which is about half the add power and is used for things at an arm's length like seeing the computer). They have neither more peripheral or more add. They simply transition faster. (If anything, I'd say they have MORE distortion in the periphory)

Take for example the Rx:
OD -2.50
OD -2.50
Add: +2.00

At the fitting cross, this pair is -2.50 in both eyes.
As you move down the corridor, eventually they will become -2.00, then -1.50, -1.00, and at the very bottom of the lens, it will read -0.50. This is the full reading prescription for this patient. If you don't understand why the power is weaker at the bottom, that's a different topic. Basically, distance+add=reading. In this case, -2.50+2.00=-0.50.

Now, in a standard progressive (not short) the full add is usually present in the lens about 17-18mm below the fitting cross.
What would happen if you put that lens into a frame with a seg. ht measurement of 15? Your patient would not get their full reading power. They would most likely have at best around -1.00 but that is too strong for him to read with. He needs a -0.50.
That's where a short corridor comes in. If your seg height is less than about 18 (I try to go 19 when possible, more is usually better) you use a short corridor lens. Usually the minimum fitting height is around 14-15. That means that 14-15mm below the optical cross, the full reading power is present. So, you can use a frame which offers only 15 mm of room below the pupil w/o cutting the reading out of the lens in edging.

I think it's important to explain what you're doing and make sure the patient is up for it. Short corridors have their advantages (like less head movement) but they definately offer less room for the intermediate and reading areas. (I use the analogy of squashing a sandwich. The three layers are still there, but each is now smaller.)

A person who works 8 hours on the computer per day is not a good candidate. A person who isn't interested in fashion is not a good candidate. A person who is resistant to change and has a hard time adapting to it isn't a good candidate.
Someone who is in LOVE with a small frame and will do anything to use it is a good candidate. Someone who adapts well to new lenses and doesn't use the computer much would probably be just fine.

Make sense?

[Spexx]

Yeah thanx it makes perfect sense. Lengthy but informative response, helps a lot couldn't figure it out myself, thanx for taking the time to explain all that :) most people here wouldn't because the question was 2 basic for them. Now i know what you guys are talking about.