1. Optometrists and opthalmologists, instead of using the focal length to specify the strength of a lens, use a measurement called the power or diopter of a lens. The power (diopter) = 1 / f For example, a 20 cm focal length lens has a power of 1 / 0.20 = 5.0 Diopters

2. Cameras

3. Camera Settings Shutter speed: how long the shutter is open. Speeds faster than 1/100 s are normally used. Fast action requires a very small shutter speed.

4. Camera Settings F-stop : changes the diameter of the iris diaphragm to control the amount of light reaching the film. The SMALLER the f- stop, the LARGER the opening.

5. The f-stop determines “depth of field”. A larger f-stop (a smaller opening)- will produce an image where everything is in focus. A smaller f-stop (a larger opening)- will produce an image in which only the subject is in focus and everything in the foreground and background is out of focus.

6. A TELEPHOTO lens has a longer focal length to magnify images. A WIDE-ANGLE lens has a shorter focal length.

7. Telescopes Refracting telescopes have two lenses, the objective and the eyepiece. The eyepiece lens has a smaller focal length. The objective lens has a larger focal length.

8. Microscopes Microscopes also have two lenses, the eyepiece and the objective. The eyepiece lens has the longer focal length. The objective has the smaller focal length.

9. Fresnel Lens The weight and bulk of a large diameter lens can be reduced by constructing the lens from small wedged segments that follow the curvature of the original lens and collapse down to a thin layer.

10. Augustin Fresnel Fresnel invented this type of lens in 1822.

11. Fresnel lenses can take a small diverging light source and change it into a powerful straight beam of light.

12. Small plastic Fresnel lenses are sold at office supply stores as “Magnifying Lenses” Fresnel Lenses are also used in overhead projectors

13. Large glass lenses that have Fresnel surfaces surrounding a small light source have provided an invaluable contribution to coastline areas for more than 150 years. These lenses are used in…..

14. Lighthouses

15. Tests showed that while an open flame lost nearly 97% of its light, and a flame with reflectors behind it still lost 83% of its light, the fresnel lens was able to capture all but 17% of its light. Because of its amazing efficiency, a fresnel lens could easily throw its light 20 or more miles to the horizon.