the crack or change the prescription

The fundamental difference between these models and prescription reading glasses are promising, they each eye / shot the same prescription, generally 1 - 3. Every person and every eye prescription glasses customization, and for each often contain different prescription. If needed, they can also correct astigmatism.
"Reading" is very cheap in bulk, trends. Device fabrication process, the frame is made in one piece, so can not be reused or change the lens, if any difficulties, such as in the lens of the crack or change the prescription. Even more high quality reading, can be made with the metal frame without screws, thus can not be changed, or it is easy to adjust.
Prescription glasses and frame, can be repeatedly used, if you change the prescription. The lens center PD (Tong Ju), in your eyes, while the RR center in your framework. The prescription is by definition is more accurate.
Because the strength of ready readers lens is an approximation (and often self diagnosis), they can cause eye fatigue and the trend is not accurate - although this does not cause any lasting harm your eyes, it can be uncomfortable, if long time use of glasses
In general, if you are lucky enough, the two eyes had the same prescription, it may be worth at least some ready to buy "off the shelf" for backup readers - but it is worth considering presbyopic glasses also provides, to make sure that you take the alwys best take care of one of your most valuable assets. As often as possible to change your lenses really conducive to protein, calcium, fat, because a day has been collected and unsettling eyes, and increase the risk of infection.