Differentials are reduced strength glasses for close up activity. They have a lower correction than your normalized lenses that allows you to see clearly only up to the screen. The difference between normalized and differential lenses is usually between 1 and 2 diopters.
People with myopia of -2 D and lower do not typically need to wear differentials (because with -2 D your distance till blur should be around 50cm - far enough to see your screen with sufficient clarity).
The first differentials are typically 1.5D weaker than your Measurement for full distance vision. If you have -0.25 to -0.75D of cylinder, you have the option of dropping that and having the sphere only 1.25D 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 dependance are all factors in choosing your differentials. 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 they need to 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.
- 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/