Differentials (jargon), (close up, computer use or diffs) are reduced strength glasses or lenses for close up activity. They have a lower correction than your normalized lenses, the difference between them being usually between 1 and 2 diopters.
People with myopia of -2 D to -1.5 D and lower do not typically need to wear differentials (because with -2 D your distance till blur should be around 50 cm, and with -1.5 D 67 cm, far enough to see your screen with sufficient clarity). The exception is that if you have more than 1 diopter of cylinder correction you will likely need differentials to help manage directional blur. In this event you likely can reduce the cylinder correction by between .25 and .75 D and pair it with a reduction of up to 1.25 D in spherical strength for either little or no spherical (plano) correction.
Below -1 D, unless your close-up distance is around 100cm, you might consider using plus lenses as a differential. EndMyopia used to specify that they be used for low myopia, but they now consider it is a last resort.
The first differentials are typically 1.25 - 1.5 D weaker than your measurement for full distance vision. If you have -0.25 D of cylinder, you have the option of dropping that. If you have -0.5 to -0.75 D of cylinder, you have the option of dropping that and having the sphere only 1 - 1.25 D weaker.
See How to use Contacts with EM if you wear contacts.
Ideally your differentials give you a small amount of blur at your daily near work. The nature of your work, the distance to your work, and your refractive state and cylinder dependence are all factors in choosing your differentials. The ideal distance for near work is not closer than 50 cm. You may get your first differentials and discover you have a headache and you really shouldn't have dropped cylinder. You may find that your work is pretty close or really far and you need to adjust the strength up or down. You may find that you really need detailed vision to do your work and you'll settle for slowing progression with your differentials and do your blur challenge for improvement in your off hours. This is the first place in the program where you really need to start understanding what you're tinkering with and make personalized diopter specific choices.
Why are differentials worn?
- is engaged in close-up activity
- wears more correction than needed when seeing the object that is close up.
Differentials allow the wearer to wear just what they need to, to see up close. This removes the stimulus of hyperopic defocus and stops eyes from getting more myopic. In a sense, it's a quick fix.
Another reason to wear differentials is to avoid convergence issues. When viewing in close proximity the position of your eyes needs to compensate by turning in, particularly when you are looking nearer than 50 cm. Doing this for long periods of time on a regular bases is likely to cause issues. Such issues are easily avoided by wearing differential correction to accommodate a larger distance.
It is useful to apply the 3 hour rule when engaging in close up (near) work.
Always be mindful to be in a good ergonomic position. Leaning in is a common issue and is something you need to be mindful to avoid.
Good ambient lighting is important because your eyes can not see as far or as well in low lighting. Natural light is best but supplement as needed.
When are differentials worn
Read a book with differentials. Use your computer with differentials.
Wear your differential glasses any time you are doing close work. If you have to get up to go to the bathroom, switch back to your normalized glasses. You get used to it pretty fast. "What if I just need to look out my window for like a minute?" CHANGE YOUR GLASSES. If you are asking yourself "should I switch to normalized glasses to look outside the blur horizon of my differential glasses?" the answer is always YES.
But for convenience, you might choose a threshold time and not change your glasses if it would be worn for less than that threshold. As an example, see the 20-20-20 rule page.
What is "close" work? Norms minus diffs = the number of diopters you need to convert to find this answer. For example, if your normalized glasses are -5 and your differential glasses are -3.5, then you need to convert -1.5. You should be able to see 66.67cm with -1.50 myopia! That's about 26.24 inches, or a little more than 2 feet.
- The Eyesight Improvement Equation: Good close-up habits
- When do I wear normalized
- Guide:Reducing differentials
- Guide:Measuring with differentials
- differentials: a video
- The EndMyopia Blog, https://endmyopia.org/faq-items/what-is-the-differential-prescription/
- The EndMyopia Blog, https://endmyopia.org/choose-reduced-glasses-prescription-differential-close-friendly-glasses/