You can choose between 3 bifocal types.
D-shaped segmentIt has a reading segment shaped like a "D" that has been rotated 90 degrees, putting the flat edge of the D facing up. This lens type is great for minus powers as they induce less prismatic jump (movement of an image). These are suitable for lower plus power, generally up to +3.00D. The optimum visual point of the segment is close to the dividing line, so you find it quicker. Available in hi-index 1.61 and Transitions® as well as 1.50, 1.56, 1.59 indices.
D28 - This is the most used bifocal at present. The diameter of the segment is 28mm. Great for shortsighted prescriptions.
D35 - Almost identical to the D28 bifocal. It has a larger diameter reading segment (35mm), giving a larger reading area if required. Great for short sighted prescriptions. The larger size is advised if you do a great deal of close work.
Round segmentsThis bifocal has a circular reading segment, however the line separating the distance and near zone will be smoother and less noticeable than with the D segment bifocals. This type of bifocal is great for plus pows over +3.00D. Available segment as a 28mm diameter in both 1.50 and 1.56 index as well as Photochromic.
R28 - This type is far less noticeable compared with the D-shaped segment, however it is suited better for longsighted prescriptions.
If you currently wear bifocals, it is advisable to stay in the existing shape and size reading segment unless you are having problems with these.