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Reading levels are usually taken if: a. the eyewear is for reading only, or b. if the amount of vertical imbalance (i.e. unequal powers in the vertical meridians) is sufficient to interfere with near vision. As a “rule-of-thumb” patients tend to read at a level about 10 millimeters below their distance MRP level, so a vertical imbalance of one diopter would give them about one prism diopter of unwanted vertical prism. Most patients can adapt to some vertical imbalance, usually up to about one prism diopter. For greater amounts of vertical imbalance, patients usually benefit by some compensation such as slab-off to minimize the imbalance. A marking pen can be used to mark the reading level. This is done with the patient holding the material in a comfortable, typical position. The patient should look about half way from the top to the bottom of the page and then a mark is placed on the lens so that it intersects the patient’s line of sight. When ordering slab-off it is best to give your lab both distance and near MRP levels and then let them calculate the amount of slab-off compensation. However, do not expect your lab to determine whether slab-off is needed. You must specify slaboff if your patient needs it. When the lab does the slab-off they will do it so that you can measure about an equal amount of base-up or base-down prism with your lensometer when the lensometer rays pass through the lenses at the reading level.