Functional Characteristics of Lenses

Transcription

1 Functional Characteristics of Lenses Kellye Knueppel, OD, FCOVD Great Lakes Congress 2018 Goals of lecture: Understand optical characteristics in order to expand the uses of lenses in prescribing glasses (and in optometric vision therapy.) Understand the optical reasons that very small power lenses can be very powerful. 1 2 Light Transformers Lenses and prisms are light transformers. While you can calculate how they transform light, you can not predict human awareness or behavioral changes because of the transformation. What do lenses and prisms do? It is more appropriate to ask, What do patients do in response to lenses? John Streff, OD, DOS, FCOVD, FAAO 3 4 Lens transformations in two directions Towards the person Away from the person Traditional Thinking re: Optics of the Eye We know that the image formed must be real, because it is projected onto the retina (which is like a screen). Any time an image is projected, it must be real. Karen Wu, The Optics 5 6 1

2 Convention: Optics of the Eye Light rays drawn from left to right Light reflects off of a surface and goes toward the eye where refraction occurs so that the image is projected onto the retina. Traditional Ophthalmic Optics Refractive conditions are viewed as a misfocusing of the optical system that is the eye due to problems with any of the optical parts including the eyeball being too long or short for the system. Light must then be refracted by a corrective lens in such a way as to move the focal point back in myopia and forward in hyperopia. 7 8 Optics of the Eye Myopia 9 10 Hyperopia Consider Inverting the Optics What happens to our thinking about the optics if we consider the person actively looking through the lens? (Rather than just considering the light coming from the object through the lens to the eye.) Classic model becomes confusing: In hyperopia, the remote point will be negative (behind the eye, as it were), which is rather abstract, but that is the way it is. 11 Look at the ray tracings from the opposite direction! 12 2

3 Looking Through the Lens Thin Lens Equation 1/d i + 1/d o = 1/f d i = distance from lens to image d o = distance from lens to object f = focal distance of lens Concave Lens Concave Lens Optics: Real Object anywhere in space always creates a Virtual Image closer to the lens and smaller. (Small In) We can calculate the location of the virtual image if we know the lens power and the distance the object is from the lens Concave Lens Perception: Person looks through the lens at the virtual image and must put it back out in space to the distance they think the object actually physically is from them. This perceptual problem is compounded by optics in the periphery of the lens. Prescribe Symmetrically When Possible Brain must look through the lens in front of each eye and deal with the transformation of light through the lens. Much easier to do with same lens on both eyes Much easier to do with simpler prescriptions Eliminate cylinder or reduce as much as possible

4 Convex Lenses: Two Situations Optics: Real Object between the focal point and the lens creates a Virtual Image farther away and larger. (Large Out) We can calculate the location of the virtual image if we know the lens power and the distance the object is from the lens. Perception: Again, the person looks through the lens at the virtual image. Don t forget optics in the periphery of the lens. (Periphery = anywhere in lens not at the optical center.) We need to consider projection and perception to understand where in space the person perceives the object to be Low power plus lenses are important to think about in this category. With low plus lenses, most near objects will fall closer to the lens than the focal distance, so a virtual image farther away in space and larger is created by the optics of the lens. (Remember, we are looking through the lens.) Useful Working Distance (distance from lens to focal distance) is greater for smaller power Focal distance of is 50 cm Focal distance of is 100 cm (1m) Focal distance of is 2 meters Focal distance of is 2.7 meters Focal distance of is 4 meters Focal distance of is 25 cm Focal distance of is 16 cm

5 Convex Lens (Object between Focal Convex Lens (Object Beyond Focal Distance of Lens) Leverage of a small dioptric change is greater for low powers than for large powers The focal points for and are 4.33 M apart The focal points for and are M apart Convex Lens (Object Beyond Focal Distance of Lens) Optics: Real Object beyond the focal distance of the lens creates a Real Image on the opposite side of the lens. In this case the calculated location in space of the real image ends up in the person s eye/head or often quite a distance behind the person. Of course, now the optics of the eye would also have an effect on the final calculated distance of the image. Perception: hmmm the person looking through this lens has some interesting perceptual transformations to make! Web Sites with Interactive Lens Models This is fun to play with if you like optics and maybe can help you with understanding optics if you don t: Interactives/Refraction-and-Lenses/Optics- Bench/Optics-Bench-Refraction-Interactive Optical Characteristics of Lenses and Prisms See handout in appendix for comprehensive list of optical characteristics of different types of lenses and prisms. These are the optical transformations of the lens and can be determined mathematically. Be careful in prescribing as the individual s perceptual response may be quite different (often opposite.) The individual viewing through the lens interprets the transform within the context of her or his frame world. 29 Thin lens vs. Thick lens optics All of ray tracings so far have been thin lens optics. Ophthalmic lenses are more complicated. Power of the lens away from the optical center (OC) is not the same as at the OC. Prism effects must also be considered. 30 5

6 Important Visual Spatial Perception Concepts SOLI vs. SILO Form Constancy Figure-Ground Monocular Cues to Depth [Note: These are reviewed briefly in another lecture hour.] 31 Small vs Large Powers Small lens powers Greatest effect related to the projected response How the person views the change looking through the lens Large lens powers Greatest effects on the viewed response How the person views light after passing through the lens toward his eyes 32 Quick Review of Thin Lens Optics Concave Lenses (Minus) Image always CLOSER and SMALLER Convex Lenses (Plus) Object between eye and Focal Distance Image always FURTHER and LARGER Object beyond Focal Distance Image BEHIND patient Things to remember on Tuesday: Prescribe symmetrically between eyes when possible When prescribing cylinder match at least one meridian if possible Be cautious when prescribing above Small power lenses can have large effects