Questions On Refraction

[Deepak]

-whre is patient's far point located with respect to retinoscopist arriving at neutralization?
Second-what is knapp rule and how it applis to rerfraction?
Last-why is curvature of field the only desirable lens aberration in the eye?

[wmcdonald]

Deepak,
Knapp's Law put very simply relates to the anisometropic conditions associated with either axial or refractive ametropia. Simply, if the anisometropia is related to refractive conditions (particularly corneal), then contact lenses will minimize anisekonia. If related to axial length, then spectacles will better correct image disparities. This information can be found in a number of sources, including Borish's Clinical Refraction (Benjamin), and Primary Care Optometry (Grovesnor). The complete explanation is a bit lengthy, but the above is a good summary.

Best regards,
Warren

[wmcdonald]

The far point during retinoscopy is brought to the plane of the retinoscope. If the average arm length is 67cm, then using the Focal Length/Dioptric Power formula D=1/f in meters, we can find the power needed to place the focal plane at 6m or 20 ft in the folowing manner. We convert the formula to cm and find: D=100/67= 1.50D. Therefore to move the far point to approximate infinity (6m or 20ft) then we add -1.50D to our retinoscopy findings. This can also be accomplished by using the retinoscopy lens in the phoropter, which is a +1.50D lens. If it is dialed out when the process is complete, then we are at infinity.

Best regards,
Warren

[wmcdonald]

Curvature of Field is not always beneficial. but dependent on the AMOUNT of curvature. The minimization of aberrations inherent in spectacle lenses is best accomplished when the back surface of the spectacle lens will subtend an approximate 30 degree angle with the first nodal point of the eye. It is found that an inside curve of approximately 6.00-6.50D will do just that. Flat lenses or lenses that are too steep will increase unwanted aberrations. A good rule of thumb was developed by the late Irwin Vogel some years back. To find the approximate BASE CURVE for a plus lens, take the spherical equivalent of the Rx and add 6. Example: +3.00-1.00x180. The spherical equivalent is +2.50 + 6. The required BC is +8.50, keeping the inside curve at 6D! For minus lenses, take the spherical equivalent/2 and add 6. Example: -3.00-1.00x180. SE= -3.50 added to +6 gives us a BC of +2.50. Curvature is important to minimize aberrations inherent in any spectacle lens. This will only apply to standard lens designs. Aspheric designs will be different and require sophisticated programs to design the correct curves, which is why we stronly suggest that the manufacturers guides be strictly followed. Hope this is helpful.

Warren

[Robert Martellaro]

wmcdonald said:
For minus lenses, take the spherical equivalent/2 and add 6. Example: -3.00-1.00x180. SE= -3.50 added to +6 gives us a BC of +2.50.Warren

Warren,

I think there's a typo here. -3.50/2 +6 gives +4.25.

Does the spherical equivalent/2 +6.00 rule apply to plus powers or is it the spherical equivalent +6.00 per your previous example?

Robert

[wmcdonald]

I appreciate the correction Robert and apologize for the confusion. -3.50 DIVIDED BY 2, (which I forgot to do...duh!) is -1.75 + 6, which actually suggests +4.25 as the approximate BC, as it was aptly pointed out by Robert. Now keep in mind this is approximate. You will find some numbers that are not available, such as in the following example:
-2.00-2.00x090. SE= -3.00/2= -1.50+6 = +4.50 BC. Most manufacturers do not offer a +4.50, so use the next flattest, a +4.25 or 4.00.

Regarding + powers, I addressed it earlier...you do not divide by 2, but use only the SE.

Again, thanks for the correction and I hope this is helpful!

Warren

[Darryl Meister]

Warren has done a great job of summing up its applications, so I'll simply add that Knapp's law specifically states that a lens placed at the anterior focal point of the eye (about 14 mm) will produce a constant retinal image size, no matter what the length of the eyeball.

Best regards,
Darryl

[Optom]

Deepak,

First question:
-whre is patient's far point located with respect to retinoscopist arriving at neutralization?
Answer: At retinoscopist(observer's) pupil.
Second question:
-what is knapp rule and how it applis to rerfraction?
Answer: no matter what the amount of axial ametropia, a proper corrective lens located at the anterior focal point of the eye will produce the retinal images of the same size.
Third question:
Last-why is curvature of field the only desirable lens aberration in the eye?
Answer:the curved image formed by a spherical lens will approximately match the curvature of the retina.

Regards,
Optom

[Deepak]

thanx guys! you pple are optical genius.

You ppl must be knowing abt purkinje images,what are its uses in refraction and why we have 5 images?

[Darryl Meister]

Purkinje images are just specular reflections from the optical system of an eye.

Every time light strikes a lens surface (or, more specifically, the boundary between two refracting media), some portion of it is reflected. A hard resin spectacle lens, for instance, reflects about 4% at each surface, for a total of 8%.

However, I am only aware of four Purkinje images: The reflections at the front and back of the cornea (I and II) and the reflections at the front and back of the lens (III and IV). The reflection of the retina during retinoscopy is not really considered a Purkinje image and neither is the reflection at the tear/cornea interface (if so, it would have been called I).

Purkinje image I is the brightest reflection, and is also called the corneal reflex. This is the image you use to take a PD with when using a corneal reflection pupilometer. This image is also used for determining the radius of the front of the cornea for contact lens fitting. The other images are pretty faint, and I'm not aware of any clinical use for them, but maybe Warren or Optom is.

Best regards,
Darryl

[Optom]

Hello Deepak,

Like Darryl, I am aware of only four Purkinje images. These specular reflected images are formed on the anterior and posterior surfaces of the cornea and crystalline lens. My understanding is the first three Purkinje images are erect, virtual and minified and the fourth Purkinje image formed on posterior surface of the crystalline lens is real, inverted and minified.

Darryl has well described clinical use of Purkinje image 1 for corneal reflection pupillometer, keratometer, and keratoscope(placido disc). May be Chip or Warren would know use of other Purkinje images 2-4 or 5 as you say.

Regards,
Optom

[Stopper]

Image (I) can also be used for measurement of ocular allignment using the Hirshberg or Krimsky test. I don't know of any use of the other three images in relation to refraction.

[Deepak]

i m too pleased with knowledge super guys posses here thanx again.

[grace angel]

Hi Warren. Upon reading your reply regarding base curve theory I had to share my knowledge. You were correct regarding getting the spherical equivalent (SE) which equals the sphere power of the Rx added with 1/2 the cylinder power. For a plus lens the formula would be the SE + 6.00D. For a minus lens the formula would be the SE divided by 2 plus 6.00D. Example Rx -4.00-2.00x180. First the SE = -5.00 Since this is a minus lens, we divide by 2 getting -2.50 Then we add 6.00D to equal 3.50D. These calculations are based on Vogels Formula otherwise known as corrected curve theory. If the lens is of ashperical design we simply use the same formula and subtract 2D at the end. If the above Rx was aspherical design the best base curve would flatten by approx. 2D yielding 1.50D as the answer. Thanks for making me think as my finals will be rolling around here shortly and this theory will definitely be on it!! PS Hope all my math was correct!! LOL :D

[wmcdonald]

I appreciate it, and think that is what I said. I did not divide....in a hurry in the first example and corrected it upon a response 2 posts down, which you must not have seen. Mr. Vogel (now deceased) was a friend, and disguished program director at the Opticianry program at Canada College in California (now no longer in existance). He developed the formula BASED on Corrected Curve theory, but it did not define it. Corrected Curve theory comes from research that indicates that to minimize inherent aberrations present in spherical lenses the ocular surface should subtend a 30 degree angle with the first nodal point of the eye. It provides for the minimum amout of aberration possible with a spherical lens of - 6 to - 6.50; ergo the "6" in the formula. Vogel's formula is a good approximation of that theory. Regarding aspheric designs....the ocular curve(s) is based entirely upon the e-value of the lens itself, not on subtracting 2 from the end. Aspheric designs are based on sophisticated computations and I suggest that it would be better to simply follow the manufacturers recommended base curve as a rule, rather than approximate. In class, your professor may use this calculation as a guide, and it is probably close, but will not always be accurate. I am always pleased to see students taking an active role and that you were taught about Irwin's formula. Where are you a student?

Dr. Mac

[grace angel]

Hi again Dr. Mac. Thanks for the feed back on corrected curve theory and Vogels rule. Like you stated, the numbers will not be exact but close. Doesn't the index of the material have something to do with these #'s not being exact??? I wanted to refer to Ellen Stoner's "Optical Formulas Tutorial" where she states that in any particular Rx the base curve that will minimize or rid marginal astigmatism is not the same base curve that will rid or minimize curvature of field abberations. The lens makers compromise the two issues and come up with their own base curve powers. Like you said, its up to us to use their charts. I am proud to say that i am a student at HCC in Tampa, FL. I truly have some of the BEST instructors in the industry. They are great mentors too. May i call upon you again in the future if I have other questions?? I seem to be full of them! LOL Have a wonderful Thanksgiving holiday.

[wmcdonald]

You are more than welcome to contact me at any time. I am thrilled to see students like you getting into the material! Please send my best to my dear friends Laurie Pierce and Bill Underwood. You have two of the best in the country teaching you. No wonder you are so knowledgeable! I met a number of you class mates at the POF/OAA meeting recently and enjoyed them very much. I will be lecturing at CLES in January....hope to see you and your classmates there. It is simply the best Contact Lens meeting in the world! Have Laurie play you my tape on CRT.

Happy Thanksgiving

[keithbenjamin]

Here's an excellent article by the good doc on the subject...
;)

Dr. Warren McDonald: Basic Refraction Procedures for Opticians
(article is at the bottom of the page)

Keith