Provide a valuable resource to your clients or customers by linking to this resource page. Just place the following link on your website.
Sagittal depth, often referred to simply as "sag," is a specific measurement used in optics to describe the curve of a lens. It measures the perpendicular distance from a flat plane connecting the edges of the lens down to the deepest point of the curve in the center. In simple terms, it tells you how "deep" or "steep" the bowl shape of the lens is.
read more about sagittal depth of an eyeglass lens ...The sagittal depth is directly related to how steep the curve of the lens is. Imagine a lens is a slice taken from a sphere. If the sphere is small, the slice will be very curved and deep, resulting in a high sagittal depth. If the sphere is very large, the slice will be flatter and shallower, resulting in a low sagittal depth. This measurement defines the physical shape of the lens surface.
Optical labs use the sagittal depth to calculate exactly how thick a lens needs to be. For a "plus" lens used for farsightedness, the sag helps determine the center thickness. For a "minus" lens used for nearsightedness, it helps calculate the edge thickness. This calculation ensures the lens is made with the minimum necessary thickness to be safe while remaining as light as possible.
The width, or diameter, of the lens also changes the sagittal depth. If you take two lenses with the exact same curve, but one is wider than the other, the wider lens will have a deeper sag. This is why choosing a very large frame often results in thicker, heavier lenses. The larger diameter requires a deeper curve to maintain the same prescription power.
Modern lens designs act to reduce sagittal depth to improve appearance. Aspheric lenses are designed with curves that flatten out toward the edges. This flattening reduces the overall sag of the lens. By lowering the sag, the lens does not bulge out as much. This allows the lens to be thinner, lighter, and sit flatter in the frame for a better cosmetic look.
Sagittal depth is even more critical for contact lenses than for glasses. A contact lens must vault over the cornea without crushing it. If the sag of the contact lens is too deep (steep), it will seal tightly to the eye like a suction cup, cutting off oxygen. If the sag is too shallow (flat), the lens will be loose and slide around excessively, causing blurred vision and discomfort.
No. Center thickness is the total amount of material in the middle of the lens. Sagittal depth is just the measurement of the curve's height from a flat plane. Center thickness includes the sag plus the minimum edge thickness required.
Yes. High-index materials bend light more efficiently, so they do not need curves that are as steep. Steeper curves are not needed, so the sagittal depth is lower, resulting in a flatter and thinner lens.
It is difficult to measure accurately without a special tool called a depth gauge or a spherometer. Opticians use these precise instruments to measure the surface power and depth of a lens.
You do not need to worry about calculating sagittal depth yourself. However, if you have a strong prescription and want to avoid thick, bulging lenses, talk to your optician about "aspheric" or "high-index" lenses. These options are specifically designed to minimize sagittal depth, giving you a flatter, thinner, and more attractive pair of glasses.