The skill to decentre a lens saves both time and money and can be the defining service that separates the practice from the rest of the pack. ROWAN SMITH details the calculations involved.
Welcome to the world of decentration. If you’re lucky enough to work in a practice with an on-site lab, having the ability and knowledge to decentre a lens is a handy tool in your repertoire.
It can sometimes be the difference in fitting a stock single vision lens on the spot rather than the time and expense of a surfaced lens.
To understand decentration in basic terms, it means how far the optical centre of the lens needs to be moved to be directly in front of the patient’s eye when fitted into the frame.
The formula for decentration on a single vision lenses is as follows:
• Frame pupillary distance (PD) – Patient (Px) PD / 2
• For example, frame size 50, bridge 17 = frame PD 67
• Px PD 62
• Using the formula: 67 – 62/2 = 2.5
This means the optical centre needs to be moved in towards the nasal by 2.5mm on each lens in order to achieve the desired PD.
Another key factor is understanding minimum blank size (MBS). It’s easy enough to decentre a lens but it’s important to ensure you have enough room to do so.
The formula for calculating minimum blank size is:
• MBS = Frame PD – Px PD + frame diameter (DIA).
• For example, frame size 50, bridge 17, Px PD 62 and DIA 53.
• MBS = 67 – 62 + 53 = 58mm blank.
As can be seen, the minimum size blank required for this patient is 58mm which allows plenty of room to decentre the size comfortably. Be aware, however, the narrower the PD and the larger the frame, the less room there is to work with. That’s why a correct fitting frame is so important. Optical dispensers can also use the decentring method to create a wanted prism in a lens. This is where you can avoid the hassle of a grind lens and impress the customer.
In order to do this, there are a few things that need to be known first:
1. Lens power must be greater than the prism power.
2. Prentice’s rule.
3. Minimum blank size.
As a good rule of thumb, the lens powers generally needs to be at least double what the prescribed prism power is. If there isn’t enough power in the lens, you won’t be able to achieve the desired prism, no matter how much it is decentred.
Next, it’s important to consider how much the lens needs to be decentred in order to create the prescribed prism.
Keep in mind this is additional to the already decentred patient PD that was mentioned earlier.
This is done using Prentice’s rule of:
• Prism = lens power x decentration / 10
• For example, with a RE +5.50DS lens that we need to induce 2Base Prism In.
• Using the formula: 2 = 5.50 x D?
D = 2/5.5 x10 = 3.6mm
The +5.00DS lens must be moved an additional 3.6mm in to create 2 dioptres of Base in prism.
Remember, in a minus lens the decentration can be in the opposite direction. But this will of course depend on the power and prism direction. If unsure, remember all lenses are prisms. For plus, it’s two triangles base to base and for minus it’s two triangles apex to apex.
It’s also important to remember the greater the lens power, the more prism you can create with the least amount of movement.
Once the decentration calculations are completed, you need to be sure you have a lens size big enough.
This is where you come back to MBS but you also need to remember to add the decentred prism amount. The formula is: MBS + decentred prism.
Of course, this all comes down to the range of stock lenses and what limitations there are.
The key thing to remember is if the stock blank will not cut out, you have lost absolutely nothing. It’s always worth a try because it can potentially be a win-win for the practice and the patient. It saves everyone time, money and can be that extra defining service that distances you from the rest of the crowd.
ABOUT THE AUTHOR: Rowan Smith has 11 years’ experience in the optical industry and is a certified optical lens mechanic and optician. He’s an experienced practice and area manager in rural and metro practices, and is the online service manager for Dresden Vision, which produces fully recyclable and interchangeable frames.