What are Shelf Bevels?

After the lens is quickly cut down to shape (called the "Rough Cycle"), it is edged using what's commonly referred to as a, "V Bevel Finishing Wheel". This wheel has a small groove (illustrated to the left) which will leave a small "v" shaped out-dentation while leaving the rest of the lens flat. This is the most common bevel type that's used on most frames.

This picture (left) shows how the lens looks when edged using a "V" bevel wheel and this is what fits into the groove of most frames, ensuring a good fit and to keep them from falling out the frame during everyday use. There are variations to this kind of bevel, but this is the most basic and common bevel used to fit into spectacles.

The Shelf Bevel

The shelf bevel (sometimes referred to as a step bevel), is usually found in sunglasses that curve around the face. There are several advantages for using a shelf bevel instead of a v-bevel, with the main advantage being that it helps to keep the lens secured in the frame.

Due to the higher than usual curve in many styles of sunglasses (some safety glasses, etc.), regular beveled lenses will be more likely to pop out the frame (especially the top or bottom). 

Some frames will actually require a shelf bevel because the back of the frame window is made higher than the front (as seen in the picture below).

 

A regular "V" style of bevel won't fit in this style of frame correctly, and it will be "pushed" out by the higher ledge. Many novice Opticians will make the mistake of trying to edge lenses for this style of frame using the "V" bevel and will keep re-edging the lenses to a smaller and smaller size; not because they're too big, but because the back edge of the frame just won't allow the hide-a-bevel (another name for the "V" bevel) to seat correctly in the frame. 

 

Conclusion

To summarize, a shelf bevel is a special type of bevel found on many highly curved sunglasses and some safety frames. By trimming extra material from the lens edge leaving a "shelf" around the perimeter, it makes it easier to snap securely into the frame and help prevent it from popping out.

Still not sure if a frame has a shelf bevel or not? Usually it's found in curved sunglasses (like the picture at the top of this article); but to determine for sure, simply remove one of the lenses and inspect the bevel type the lens has. An even more accurate method (Note: some demo lenses are made thin already so they don't have to have a shelf bevel, but lenses with prescriptions will usually be thicker and will thus require a shelf bevel for the best fit), look at the inside of the actual frame window.

If the back of the frame window is significantly higher than the front, it will likely require a shelf bevel to properly fit prescription lenses into the frame.

The picture below shows a side-by-side comparison of the difference between a regular v bevel lens (on the left) and one with a shelf bevel (pictured on the right). Notice how the lens on the right, the bevel pushed is all the way toward the front of the lens, and the bevel is much deeper; this allows it to snap securely in the frame and give a very clean appearance.