Is there a specific point...

[edKENdance]

Is there a specific point when a progressive corridor is considered to be a short corridor progressive? There are corridors of every length out there but when would you suggest a progressive lens is a considered a short corridor progressive? Are there corridor lengths that you would describe as medium length corridors that provide a balance area in between the 2 extremes? Just looking for some thoughts.

[drk]

I'm no expert.

I'd say that with all the variables involved, there would be no rational "cut off". We could devise a general rule of thumb, though.

I'd say that anything that fits into a 16 fitting height and provides 85% of the prescribed add would be a functional definition of a "short corridor".

Having said that, I think another generally true rule is that you'd probably have a corridor about 11-12 mm for that kind of a lens.

Am I so wrong?

[Robert Martellaro]

If we define the corridor length as the distance from the fitting cross to 100% of the add power, I'd put a long corridor in the 14mm or more category, a medium corridor 12mm to 13mm, and a short corridor 11mm or less. Power rate, distance to the first instance of add power below the fitting cross, distance Rx, etc., flavors the above, plus or minus a tad (the last spoken with a Rumsfeldian accent).

[Slim]

Robert, Is that how Shamir does the Auto III? Been trying to find what the 11 13 15 etc means in that lens.

[Fezz]

Robert, Is that how Shamir does the Auto III? Been trying to find what the 11 13 15 etc means in that lens.

Slim,

This thread should help you understand things much better!

http://www.optiboard.com/forums/show...raph-II-Fitter

[DanLiv]

Robert, Is that how Shamir does the Auto III? Been trying to find what the 11 13 15 etc means in that lens.

Don't conflate corridor and fitting height. Lens designers, lab techs, and technical opticians will talk about corridor lengths for purposes of determining what power is where. Most opticians and manufacturers speak in fitting heights. Shamir's 11/13/15/18 are fitting heights, NOT equivalent to Robert's corridor lengths. Corridor lengths are probably more accurate, since fitting heights are partially manufacturer marketing and can vary from brand to brand even if corridors are equivalent. Some lenses provide only about 50% of the full power reading area at minimum fitting height, meaning the fitting height and corridor length are close to the same value.

Back to the original question, The lowest fitting height I would consider "standard" is 17mm, though I wouldn't choose to fit such a fixed corridor lower than 18mm since some manufacturers consider it perfectly acceptable to cut the reading are in half at that height. 14mm and lower are certainly short corridor, and if you want to carve out a "medium corridor" I would put 15-16mm there. It's a nice thing about variable corridors that they address that awkward 16-17mm fitting height where your options used to be sacrifice reading by fitting a standard corridor to low, or sacrifice width and intermediate by fitting a short corridor high.

[Paul Smith LDO]

Conflate?!, Dan, well aren't you the loquacious one. Well said.