Lenses are the main part of the glasses because they compensate refraction impairment that causes your week eye sight. Therefore, first of all you should choose lenses correctly while ordering glasses. So what you need to know when choosing lenses is:
• Optical characteristics (papillary distance, diopter) –the optician must give you this information after your eye examination.
• Refraction coefficient (index) of lenses – is the most important characteristic of glass lenses. The general rule - the higher the INDEX, the thinner the lenses and the smaller the distortion of lenses with the same diopter. The more powerful lens you need the less difference you will see. This means that lenses with the high INDEX will be lighter, they will less distort your eyes and face, and it will be easier for you to select thin and stylish frame for them. The modern lenses have INDEKS – from 1.5 to 1.9 for optical glass and from 1.5 to 1.7 for optical plastic. Lenses that are close to 1.5 index is considered to be low indexed and with index that are higher than 1.6 is high indexed.
• The material of lenses. Nowadays there is a choice between glass and plastic. Plastic, as a material, is much softer and more damageable than the glass. But all modern high-quality plastic lenses are provided with scratch resistant coating, which makes them as solid as glass.
• The advantage of plastic lenses is lightness. The weight of plastic lenses is half less than the weight of glass. That’s why your glasses with plastic lenses will be much lighter. Glass is more fragile than plastic, because plastic is more resistant to cleavage and its fragments are less dangerous. Plastic offers great opportunities for processing, especially for tinting lenses.
• The advantage of glass lenses is that the refraction coefficient (index) of glass lenses is higher than of plastic lenses. This means that in some occasions glass lenses may be thinner than plastic ones.
• Anti Reflective coating of the lenses is special technology that significantly improves the optical properties of lenses. Anti Reflective coating increases the transparency of the lens on 8-10%. Because of the greater transparency and lack of visual interference lenses with anti-reflective coating are more comfortable for the eyes and cause less tiredness. Lenses with anti-reflective coating make possible to see better while driving a car in the darkness.

So the process of choosing glasses is better to start with the selection of lenses, especially if correction is needed.