Excalibur Glow Lens  

The Excalibur lens has been used to set almost every freestyle record the world over. The optical designer who engineered the Excalibur lens, designed it for one purpose, to be used in a bow scope. Most other scope lenses are designed for eye glasses then adapted for scopes. We found that usually a simple or cheaper solution is never an improvement. A curved plastic eye glass lens is very difficult to make into a perfect optical center. It is impractical to change lenses if they are not optically centered because you will have to sight-in every time they are changed. Couple this with the fact that a curved lens generates reflection.

THE LENS MAKES THE DIFFERENCE

Will a bow scope help you shoot more accurate? The bow scope has become a popular item for the archer that wants to improve their accuracy. A scope can actually work against an archer if they don't do their homework first before purchasing one.

The science behind a quality scope starts and ends with the lens. Understanding the difference in lenses will help the archer make the correct choice. The lens controls the magnification of the target image and where the arrow will impact the target. The optic center of the lens is the determining factor.

The optic center is very seldom in the center of the lens unless it is made that way. The easiest way to make a magnified lens is to buy an optic lens from the eyeglass industry. These lenses are usually made of plastic curved blanks that can be cut into any size or shape. These blanks DO NOT start with the optic center in the center. It takes a special machine and time to find the "true" optic center of these blanks and the machine used by the local eyeglass provider is not capable of doing this. So close enough has to be good enough. Unfortunately for the archer that uses this type of lens it will jeopardize his/her accuracy. The jeopardy of the accuracy is that if the lens is removed after sighting in the optic center is also moved when replaced. An archer must first mark the position of the lens before removing it and reinstalling the lens in the same position will help prevent this problem. Testing the true optic center is easy to do. Start by sighting in with out the lens, then install the lens and see if it shoots in the same place. And if you replace one lens with another the same thing can happen. The longer the distance to the target the more obvious the margin of error will become.

Reflection off the lens is another big concern. The eye glass industry uses two types of lenses "meniscus" and "a spheric". These are both curved type lenses that work very well in eye glasses when they are right in front of your eye. Hold these lenses out 34 inches from your eye and see how much reflection is created. These lenses are inexpensive and are easy to make. Anti-reflection coatings are put on lenses to enhance light transmission through the lens. This is an important factor when you are looking through a lens 34 inches from your eye. As the magnification of the lens increases the more light transmission is needed. Light waves will penetrate lenses in different ways according to their design. A curved lens bends light waves, the greater the curve increases the more reflection is created. The only way to reduce reflection is to coat the lens. Glass lenses, can have coatings baked on them while plastic lenses are to soft to have the coating baked on. Because of this, the coating will not take much abuse and will scratch easily. Continuous cleaning will eventually render a plastic lens useless because of small scratches that will show up on the coating.

The best type of lens is one that has a "true" optic center and is one that will not reflect. This lens is called a "Plano Convex" lens, flat on one side and curved on the other. This style of lens is difficult and more expensive to make, but will not reflect and can be made with a perfect optic center. The "Plano Convex" is usually made of glass since the best type of lens is one that has a "true" optic center and is one that will not reflect. The "Plano Convex"  is usually made of glass since the procedure doesn't work very well with plastic. If the lens is NOT A CERTIFIED OPTIC CENTERED LENS then it should NEVER be removed from the scope for it can affect your accuracy. Removal of the lens will cause the archer to miss proportionately to the amount the optic center is off.

THE IMAGE IN THE LENS FOLLOWS THE OPTIC CENTER
See illustration


Rotation of the optic center will cause impact of the arrow to be in a different place.

The lens design is only one factor when choosing a bow scope. The housing the lens is held in is another very important factor. The lens needs to draw light from its edges. If the lens is held in a dark housing, light will not be transmitted through the edges of the lens. When using higher magnification lenses usually anything over a 4 power or a.50 diopter the image will grow darker if the light can't be drawn in from the edges of the lens. Use of a clear housing will allow the light penetration needed to have a brighter target picture.

Proper peep size is yet another factor as to what the archer can see. The more power on the scope lens, the smaller the peep hole has to be. If the archer isn't using the proper size peep the lens will never focus correctly. The focal length between the eye and the lens determines the power of the lens and is usually 34 inches for the true power.