I need help understanding something basic.
When we have the choice of fixed corridors (e.g. 14,16,18, 20mm) does this refer to the fitting height or the actual corridor length?
I need help understanding something basic.
When we have the choice of fixed corridors (e.g. 14,16,18, 20mm) does this refer to the fitting height or the actual corridor length?
fit ht i believe :)
"what i need is a strong drink and a peer group." ... Douglas Adams - Hitchikers Guide to the Galaxy
It's the corridor length. It varies a bit on the lens design, but typically there will be around 5mm of near power at the bottom of a fixed corridor. So, for example, a fixed 16mm design will have 11mm of intermediate progression and 5mm of reading. If you order a fixed 16 with a seg ht of 20 you will still have 11mm of intermediate and 5mm + the remaining 4mm of near power. Obviously if you order the same fixed 16 with a seg ht of 14 you will cut off much of the near power...at least that's how I've been educated about it. Hope that helps.
To be clear, what you are specifying is these cases is, in essence, the length of the corridor. However, you still also need to supply the FITTING height. Remember that with corridor length changes, you will be altering all sorts of aspects of the intermediate and near zones, width etc. depending on brand and design ethos. So in effect, you can order a 13mm corridor from some vendors, but fit it at a 28 seg. if that's what you and your pt thinks will best fit their needs.
Aargh. I'm not getting this.
Make it more simple:
Fixed design of 14mm corridor length.
Will the corridor be:
a. 14 mm
b. less than 14 mm???
It usually refers to the minimum fitting height. The corridor length will be about 3mm to 7mm less depending on the design and minimum height value.Originally Posted by drk
Some designs will optimize the corridor length even within the fixed value, due to axial length of the eye (see CORE from Zeiss), and/or due to the lens power, shorter for minus and longer for plus, (see Ovation and others).
Science is a way of trying not to fool yourself. - Richard P. Feynman
Experience is the hardest teacher. She gives the test before the lesson.
The answer is, from your gracious posts, 14 mm.
But what an unusable nomenclature that is! Why not make it "the design optimized for fitting heights of 14mm"? I mean, I don't have the frame of reference to say, hmm..."Sally would do great with about 14mm of corridor on top of the reading zone size, which, according to my calculations for her 20mm fitting height would be about six. No, on second thought, I'd rather her have a 12mm corridor so her near zone will be higher, and her reading zone larger."
That's too much choice.
And I don't want the "variable" option with all these big geek frames or I'm going to get a 20 mm corridor and a 4 mm reading zone or whatever!
Well, RM, your post is intuitive, but you are all disagreeing.
Dr K with Shamir if I order a
Auto 2 11 the intermediate zone would be 6
Auto 2 13 the intermediate zone would be 8
Auto 2 15 the intermediate zone would be 10
Auto 2 18 the intermediate zone would be 13
So if I have a 14 seg and order a auto 2 18 the patient would only have appox 1 mm of reading.
This is my understanding.
Jeramy
B.
For example, Shamir's Autograph fixed 18 has a 13mm corridor length (defined as 100% of the add).
Science is a way of trying not to fool yourself. - Richard P. Feynman
Experience is the hardest teacher. She gives the test before the lesson.
Cool. That is easy to get. Thank you all.
The fixed terminology was started by shamir with the release of the autograph 2. It refers to the minimum fitting height of the design not the corridor length. consider this if their shortest "corridor" was 14 mm than that would mean that it would likely have a minimum fitting height of 18mm.
Just a tangential question, and it veers off into the netherworld of opinion and lens designers and unicorns...
...but how far down do you think people like to look down to read? 10 mm? 15 mm? Certainly not 20 mm.
I know that we have to talk head depressers and eye depressers and some robotic-looking device standing, menacingly, in the corner of your dispensary allocating between the two...
...and there are faint references to lens power's prismatic effect on object displacement...
...but will someone give me "the right number"? I'm going with a nice, lucky "13".
Maybe you can answer this, Robert:
You advocate, I believe, longer corridors for hyperopic and low myopic presbyopes (BU prism from net positive power in the reading zone) and shorter corridors for net minus power in the reading zone (moderate myopes and up).
To me, this seems odd. I'd think it'd be the reverse. I envision progressively-increasing BD prism from minus lenses "compressing" the design, and progressively increasing BU prism from plus powers "expanding" the design.
Can you explain?
6mm to 10mm according to Kozol.
http://www.ncbi.nlm.nih.gov/pubmed/8890443
PAL wearers learn to rotate more, your 13mm, and my measured 15mm are probably typical reading depths, especially in the higher adds.
Science is a way of trying not to fool yourself. - Richard P. Feynman
Experience is the hardest teacher. She gives the test before the lesson.
BD prism with minus powers moves/displaces the image upwards, requiring a slightly shorter corridor compared to emmetropes. Reverse for hyperopes. I have a client- busy for awhile.
Science is a way of trying not to fool yourself. - Richard P. Feynman
Experience is the hardest teacher. She gives the test before the lesson.
Understand and appreciate.
With BD prism or BU prism, it's variable prism (like a gradient prism). I would think that the effect of gradient BD is compression, and vice-versa.
What's more, BD displaces upwards automatically, so with a myope, a "normal" corridor length would "seem short" and with a hyperope a "normal" corridor length would seem long, since BU moves downwards.
What am I thinking wrongly, here?
Sorry drk,
There isn't going to be a one size fits answer to your question. Ex, some lens companies have a 3-4 mm drop from the fitting cross to the MRP, while others have the MRP at the fitting cross, or visa versa. What you need to do is define the brand and the model and proceed from there.
I believe, if the drinks before dinner, dinner with drinks, and the after dinner drinks haven't clouded my brain too much, the induced prism at the near point is a function of the distance power and the distance from the PRP to the near point, regardless of the add power.
Because the object is higher, the power in the corridor needs to ramp up quicker. In other words, less ocular rotation is required, hence our eyes don't get a chance to reach the full add power. See?What's more, BD displaces upwards automatically, so with a myope, a "normal" corridor length would "seem short" and with a hyperope a "normal" corridor length would seem long, since BU moves downwards.
What am I thinking wrongly, here?
Science is a way of trying not to fool yourself. - Richard P. Feynman
Experience is the hardest teacher. She gives the test before the lesson.
We have handy dandy charts from Zeiss and Shamir on the fitting heights and corridor height/width to reference. That way if we have a patient that doesn't spring for that pair of heavily recommended Computer Lenses as an essential second pair (gasp!), we can find the right parameters to plug in. In essence finding more intermediate or near depending on their particular needs.
I don't know if this will help in any way, or if I'm barking up the wrong tree, but these charts are available from your rep, and they do help.
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