On Shabbat it is prohibited to carry from one domain to another and to carry an item for more than four cubits within a public domain—if no eiruv is present. Therefore, one may only walk outside on Shabbat wearing items that are considered either a garment (beged) or an ornament (tachshit), in which case they are considered “nullified” to one’s body. Anything else would be considered a burden (masui), and one would not be permitted to go out with such an item on Shabbat, even if one were to wear it. Moreover, even items that would be considered “nullified” may not be worn on Shabbat if there is a possibility that they may fall off. Nor may one wear an item if there is a concern that one may take it off and carry it. (If the item is attached in a way that it cannot be taken off, we would not be concerned that the individual would remove it.)1

One who requires eyeglasses in order to see properly, and therefore wears them all the time, is allowed to walk outside with them on Shabbat, for they are considered a garment and not an additional item. While earlier poskim were concerned that one's glasses might fall and thereby cause one to carry them, contemporary poskim write that this concern is generally no longer applicable.2 If however one’s glasses are on the verge of breaking or are loose, one is not permitted to wear them on Shabbat. The concern is that one might carry the broken part or fix the glasses, both of which are not permitted.

Reading glasses however are not needed for walking and are therefore considered “extra,” so they are not permitted to be worn outside. In addition, there is a concern that they might be taken off or they may be placed above one’s forehead. Bifocals may be worn if one wears them all the time.3 One who is just starting to wear contact lenses should not go outside with them on Shabbat; there is a concern that the contacts might be removed if they irritate the eyes.4

Regular sunglasses worn solely to protect from the sun may not be worn outside, even in a situation where they help with walking, because they may be taken off when the individual reaches a shady area and they may mistakenly carry them. The same holds true for clip-on shades. However, prescription sunglasses are permitted since we are not worried about them being taken off, even in a shady area, because they are needed for vision at all times.5 Prescription glasses that change color in the sun are also allowed since they are like regular glasses in the shade. (Color change on Shabbat is not an issue since the change is temporary.)

Similarly, one who must wear sunglasses due to a medical concern would be able to wear them outside on Shabbat.