Copied & pasted this, but it is true that off the shelf readers or inaccurately measured specs can cause eyestrain. A pair of prescription reading glasses can be bought in a high street opticians for £25 or less. It's worth having a look around.
Great Vision Requires Accurate PD Measurement
The optical center of your eyeglass lenses is the part that gives you the truest vision, and it should be directly in front of your pupils. To determine how to place the lenses in your frames so the optical center is customized for your eyes, the eyeglasses lab needs to know the distance between your pupils, or PD.
It can be tricky to measure your own PD, somewhat akin to trying to cut your own hair. Dispensers need lots of practice to be able to measure PDs correctly, and even experienced opticians have difficulty taking their own in a mirror.
Most online optical providers present you with several ways to go about this important task. Some suggest the simplest method, which is to have your prescribing eye doctor or an optician take the measurement for you. Alternatively, they offer step-by-step explanations of how to take your own PD in the mirror, or how to have a friend take it for you.
The way that eyeglasses fit your face and head is another important issue. Frames that are too large or that don't fit the bridge of your nose properly can slip. Not only is that uncomfortable, but you also can get a headache if you're not looking through the optical centers of the lenses.
Unskilled at choosing your own glasses? At brick-and-mortar stores, an optician is there to help you.
Alternatively, if your glasses are too small, they will be uncomfortable to wear; they can pinch the sides of your head and leave red marks on your temples. They also can cause discomfort behind your ears or on your nose.
It can be challenging to try to fit yourself with eyeglasses, but some online stores offer tips. One site suggests that you select eyeglasses in a size that is close to your current glasses, or that you measure your head with a measuring tape or use your hat size as a reference.
It's important to note that the prescription strength and weight of your lenses also play into whether your eyeglasses will be comfortable and look good.