An easy way to sell the lens without a word is to present the pictures or videos made from lens providers, often it convinces customers to end up being scammed, paying for the most premium one. But in fact, there are lots of complaints about the outputs due to the impact of overuse of unnecessary technology.
The latest free-form technology in lens surfacing is nowadays most recommended for all multifocal or progressive lens users. While all lens manufacturers are competing in inventing new complex designs to widen the reading field, is it always an improvement? Certainly consumers will hear only good advantages, but never find out a hidden fact. So WHAT and HOW to CHOOSE then?
The cost of the lens varies widely because of marketing plans. When this is not a buyer’s market, it is not about how much money a buyer will spend but it is about how much truthful fact an optometrist or a sale person can reveal. This following information explains the facts how the lens is designed.
There are 3 surfacing types in the market:
- Front or conventional surfacing; – It has distant power on inner surface and addition reading power moulded on outer surface.
- Double surfacing; – It keeps distant power on the inner surface and split reading power on to both surfaces.
- Full back surfacing; – it combines both distant and reading powers together on inner surface, the outer surface is blank or spherical. This is nowadays the best selling type.
All types of surfacing are crafted by various grinding designs:
Imagine a round bucket that is almost full of water standing on the ground. The amount of water represents power of the lenses that we need, it cannot be more or less. The top water surface represents reading visual field and the shape of the bottom imitates to surfacing design.
- Standard or Moderate design.
The first image simulates moderate full back and front surface designs which offer benefit in some particular purposes. A mild prescription with this design could have nearly the same performance as in the advanced ones.
- Advanced design.
To gain reading field, they design a shallower tray with reduced bottom attached area that can hold the same amount of water and still be easily sitting on the ground, more complicated software is added to digitally grind the lens. The main purpose is to eliminate distortion resulting in better reading, intermediate vision and comfort especially in high spherical and cylindrical powers. Advanced full back and double surface designs are highly recommended in higher powers.
New progressive users usually have low addition power for reading. If the distant power is very low also, they can get very little benefit from the advanced design. This group of buyer often pays for something they do not actually need by relying on convincing medias only.
Both designs above are produced according to factory predetermined set-up parameters, if the optical frames have standard parameters when sitting on the face, e.g., curve, pantoscopic tilt angle and vertex distance, there is no need to find any other lens that is more expensive than the advanced design to gain a better output. There are also individual custom design options added to optimise the performance or suit specific demands and purposes of use by changing the proportion between far, near and intermediate powers. The frames sitting and eye point position should be consistent with the options. Consultation with an optometrist will guide to the proper solution.
- Custom design.
What if it is a bucket that has a heart shape instead of a round one? What kind of surface at the bottom should be to maintain the same reading area as in the former advanced design? When the frames are not ordinary shape, the lens surface needs to be custom crafted from parameters of the frames while sitting at certain height position on the nose. The bottom surface must be uneven causing a small center point of gravity, then it cannot be placed randomly but critically on that certain spot. Misplacing just 1 or 2 mm. by not wearing the glasses at the initial set-up position or in later days the frames are slightly bent out of original shape, the result will be extremely deficient.This design is not invented to widen reading area in the common-shape frames but to solve the problem in extraordinary ones. Most of the lens providers name it as “high-end quality” surely to top up the selling price but do not emphasise that it is not necessary in ordinary frames. In addition, this custom design should be prescribed by a qualified optometrist who has lots of expertise, all measured parameters must be precisely corrected otherwise the result can be upside down.
In conclusion, when shopping for a pair of progressive eyeglasses, always ask an optometrist for the factory initial set-up frames parameters and look for ideal ones by checking the parameters when wearing on the faces. The same frames on different faces turn out to different parameters, that is why ordering on-line is not recommended. If the parameters of the chosen frames are not within that range, consult an optometrist if it’s worth taking a custom designed lens. The more complex design requires more balance control on the nose and ear positions.
There is another tip to choose the frames, it is recommended to have the frames that sit firmly, height not less than 25 mm., higher than 28 mm. is preferable. The eye position is very important, it cannot be too low or high. In any frames size, when dividing them into 3 equal parts horizontally, by looking in the mirror eye position should be very close or exactly at the upper dividing line.
The key to success are: choosing proper frames, specifying individual needs, having a binocular vision test and precised measurement of pupillary distance (PD). In case there is binocular vision problem or the eye height position between both eyes are very different, some minor options added in any design would rather be more reasonable and ethical than being convinced to pay double or triple price for a hi-end fancy design.