Multi-Focal Lens Guide
Bifocal Type Description
The first type of multi-focal lens we will talk about is the Flat-top or "D" shaped bifocal.  The most common used is the Flat-top 28.  The 28 stands for the widest part of the segment. Other variations of this bifocal are the FT 25, 35, FT 40 and the FT 45.  
NOTE: When you measure for a FT style bifocal you measure from lower eye-lid to the bottom of the frame.
Next we will discuss the trifocal which has Three Focal Lengths.  They consist of the distance portion, and intermediate segment, and the near segment of the lens.  The intermediate segment is usually one half the power of the reading segment and increases the reading distance to twice the distance of the bifocal segment.  The focal length is about 24-36 inches.  The most common trifocal used is the 7x28.  The 7 stands for the depth of the middle segment and the 28 stands for the width of the segment.  
NOTE:  When you measure for a trifocal you should measure from the bottom of the pupil to to bottom of the frame.
Another type of lined bifocal/trifocal is the Executive.  This type of lined multifocal has the seg go the entire width of the lens.  Because of the way this lens is fabricated it is almost twice as heavy and thick as other multifocals.  This lens should be avoided.  This lens is also available in a bifocal or trifocal lens.
NOTE: Fit the same as a  Bi-focal and Tri-focal
A Double D Bifocal has a seg at the top as well as the bottom of the lens.  This would be helpful for people who have to be able to read or see up-close above their head as well as below (i.e. electricians)

NOTE:  this should be fit the same as a bifocal but the upper seg should be at the upper eyelid.

A Progressive lens (Also known as a "No-line bifocal or No-line Tri-focal)  is designed to give the patient the feeling of natural vision in all focal lengths because of the precisely ground corridor in which the power gradually increases to the full reading power of the lens.  Due to the design of the lens, there are soft focus areas on the periphery that are not usable.  With a no-line, the better the lens the less peripheral distortion.  Varilux is the best on the marked as of today.
NOTE: This lens should be measured from the center of the pupil to the bottom of the frame.  It is better to err on the side of too low then too high.

For additional information:

See Also: