Troubleshooting Prism - Multi-Focal
When it comes to prescribing and troubleshooting prism, single vision lenses are often straightforward - you’re usually solving a single problem. But when you add bifocals or multifocals into the mix, you’re tackling multiple visual tasks, which means multiple opportunities for prism-related issues.
Below, we break down the key concepts, practical tips, and real-world scenarios to help you understand how to handle prism in bifocals.
Why Prism is Different in Multifocals
With bifocals or progressives, you’re no longer dealing with just distance vision. You’re managing distance and near - and sometimes intermediate. That means your patient may need different prism corrections for each zone.
For example:
- They might need vertical prism to correct an imbalance that only affects reading.
- Or they may have convergence issues that require extra base-in prism at near, but not at distance.
Common Patient Complaints
A typical complaint we hear goes like this:
“My patient just got new progressives but can’t read out of them. Their distance vision is fine. Each eye individually is clear.”
When you hear this, it’s time to dig deeper:
- Is the vision clear in one eye?
- Is it clear in both eyes?
- Is the problem only at near?
- Was the prism trialed for both distance and near?
These answers help you uncover whether the issue is fitting, design or the prism itself.
How Prism Can Differ at Near vs. Distance
A classic example is a vertical imbalance - this is where a slab-off or bi-centric grind comes in to balance the image height when the eyes converge to read.
Another common scenario? Convergence insufficiency - when eyes don’t turn in enough for near work. In these cases, you may need extra base-in prism at near only.
What Are Your Options?
Depending on the situation, here are your tools:
1. Single Pairs or Multiple Pairs
- Some patients do better with two pairs: one for distance, one for near.
- Others may be fine with a bifocal or progressive designed to handle the different prism needs.
2. Decentered Bifocals
With bifocals, you can manipulate the near PD to induce the needed prism at near using Prentice’s Rule:
- The amount of prism = lens power × distance (in cm)
- But there’s a limit - if the required prism exceeds what you can achieve with decentration alone, you’ll need another solution.
3. Franklin (Split) Bifocals
Remember Benjamin Franklin? His split bifocal design still works. If standard decentration can’t get you enough prism:
- Combine two separate lens segments.
- Use a different base direction or prism amount for distance and near zones.
4. Smart Use of Lens Design
Prentice’s Rule is a guide - not a law.
You can adjust perceived prism by:
- Tweaking base curves.
- Mixing different bifocal styles (e.g., round segment on one eye, flat top on the other).
- Adjusting corridor lengths in progressives (e.g., a longer corridor in one eye, shorter in the other).
Advanced Tactics: Bi-Centric Grind
A bi-centric grind (a more accurate name than “slab-off”) means you’re creating a second optical center in the lens. This balances the image height at near, solving vertical imbalance problems.
Fun fact: Older ribbon segments made this easy. Today, you can replicate the same effect by pairing different bifocal shapes or carefully balancing corridor lengths in progressives.
The Bottom Line
- Always ask the right questions - Is the prism issue distance-only, near-only, or both?
- Don’t assume one size fits all - Single vision troubleshooting rules won’t always apply to multifocals.
- Know your options - Decentration, slab-offs, bi-centric grinds, Franklin splits, and design tweaks can all help.
- Keep the math in mind - Prentice’s Rule is your friend, but not the final word.
If you’re stuck with a tricky prism case, give us a call. We love weird questions and we’ll help you find the best solution for your patient.
