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Vision training

Vision Training With Loose Prisms

Loose prisms help to train our vergence system (the ability of our eyes to turn in and out). Prisms are triangular lenses that causes light to bend towards the base of the prism, meaning when you look through the lens, the image will shift from its true location.

When you place a prism lens in front of one eye, then the eyes either have to converge (move together towards the nose) or diverge (move apart) in order to not get double vision. We make these vergence movements every second of the day to focus on near and far objects.

Most patients have a range of prism strength that they can tolerate and look through without getting double vision. Many patients with binocular vision problems have a reduced vergence range which may lead to double vision, blur, fatigue and headache. For example, someone with reduced convergence may get double vision when looking through a 10ʌ prism but a normal patient may be able to still see single with a stronger 25ʌ prism. This can be improved with loose prism training. Having a normal vergence range will help the visual system change and maintain its focus across a range of distances.

What you will need:

  • One loose prism (your optometrist will prescribe a certain power for you) from Innovative Eye Care
  • A pen/pencil


  1. First identify which side of the prism has the thick side - this is called the ‘base’. Imagine an Egyptian pyramid - the thickest side closest to the ground is called the ‘base’ of the pyramid! Your optometrist will have told you whether the base of the prism is to be on the outside (further from the nose) or the inside (closer to the nose).
  2. Hold your pen at 40cm from your nose in front of you. Focus on the pen.
  3. Move the prism with the correct base direction in front of one eye ‒ see image (it does not matter which eye). You will see the pen go double.
  4. Voluntarily bring the two images together to make the pen appear single and clear. Hold this for 2 seconds.
  5. Remove the prism. Once again the pen may go double quickly before your eyes- make the two images single again. Continue these cycles for 1 minute (20 cycles) then relax for 30 seconds.

Do 3 of these 1 minute cycles, three times per day.

If you are doing well at your follow-up appointments, your optometrist may swap your prism for a higher power to make the exercises more difficult. Doing these prism exercises on a far away object will be harder than near. If the pen is easy to cycle through at 40cm, move to a clear object such as a door knob at 1 metre. Your optometrist may recommend that you watch TV whilst dropping your prism in and out every 3 seconds!

Remember: For these exercises to be effective you need to notice that the pen goes double first, then becomes single. If the pen never becomes double and someone watching your eyes whilst you do the exercises cannot see your eye behind the prism moving left and right, you may be suppressing one eye - discuss this with your optometrist. Please ask us if you have any other questions about the exercises.

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