Answers for consumer PAL questions

[QDO1]

This thread is NOT for debate - it is a thread for optical professionals to post thier definitive answers to common consumer questions

If you are a consumer, and have a question - please start a new thread

If you are a professional, and have a general comment on a consumer issue - post it here. You can then reference consumers to your post - or someone elses, everytime that question your post covers is asked

[QDO1]

Dispensing presbyopes is always a comprimise. A professional dispenser weighs up the comprimise, and finds a soloution for you

Typically the dispenser will be thinking about the following things about you:

• Your prescription
• Your previous spectacles
• Your occupation
• The quality of your vision
• Your hobbies and pastimes
• Time per day driving
• Time per day using a VDU
• Previous problems with your spectacles

The dispenser will then advice you on the following

• Best frame choice(s) for the chosen comprimise

The dispenser will then think about the folowing atributes of the lenses

• The distance portion - in terms of distortion and unwanted astigmatisim
• The proportions of the near and intermediate corridor, in terms of width, length and power distribution
• The lens material
• Saftey aspects
• Base curves
• Refractive index
• ABBE value
• Coatings availability
• Tintability
• Availability (or not) in transitions
• Issues to do with the frame, physical lens mounting, and its impact on lens materials
• Fitting parameters for the lenses, referenced against the real measurements from the frame and your facial features
• Abberation characteristics of the lens materials
• Lens thickness
• Lens weight
• Durability
• Cross reference your prescription, at the required lens size, against the available ranges from the manufacturer
• How you use your spectacles, and the ease of swaping with the other spectacles you wear

The next bit is the most important:

The dispenser measures you up for the spectacles.

"A great lens poorly measured is going to perform worse than a poor lens well measured"

So when you ask - what lens is better A/B? the answer honestly is - neither, the best thing you can do is have a dispensing performed by a qualified, competent and experienced professional. Thier judgement and skill will ensure you have the optimum dispensing for your needs

[drk]

Deleted.

[QDO1]

If you choose a shallow frame for a PAL lens the following senario's apply

• frame fitted with full length corridor PAL with correct fitting height
• frame fitted with full length corridor PAL with increased fitting height
• frame fitted with short length corridor PAL with correct fitting height

these are all comprimises of a sort:

frame fitted with full length corridor PAL with correct fitting height - depending on how shallow the frame is, distance vision is likley to be OK, intermediate probrably OK, and reading non-existant

frame fitted with full length corridor PAL with increased fitting height - Happens a lot this one (the dispenser pretends your real pupil height meets the minimum* fitting height, and is the cardinal sin of PAL fitting - DV will be comprimised by the intermediate section, and the edge of the lens will be even more distorted, the intermediate will be OK, and the reading area is still likley to be truncated

frame fitted with short length corridor PAL with correct fitting height - this is the best option for a shallow frame - te DV will be OK, but limited, depending on how shallow the frame is, the intermediate (VDU) vision will be virtually non-existant, but will be ok for cursory ussage, and the reading area will be small but clear

Summary
Fitting a PAL lens in a shallow frame can be sucsessful, but the result is really a comprimise. The best way to view this is that the shallow frame will really need to be a second pair, where further dispensings will probrably be needed for some specific tasks. Shallow frames with PAL's make great spex's for dress, but probrably dont suit tasks like prolonged driving, or VDU usage. An experienced optician will be able to guide you through the choices, and find you the best comprimises

*Minimum fitting height
Manufacturers specify a minimum fitting height for a PAL. some manufacturers seem to make this number lower than what it really ought to be. One comonly used lens, was suddenly announced to be OK at a lower fitting height, than the one that was specified before (with no design change). The thing to remember is that minimum does not = optimum