Understanding Cylinder & Spherical Equivalent Refraction

[1UP]

I've been reading up on the various aspects of human visual acuity/correction and I want to see if I'm understanding the concepts of cylinder and SER correctly.

SER is equivalent to the average of the powers in both major meridians.

Example, for a given eye the script is +3.50 -4.50 x 076. In the one major meridian it is +3.50 @ 076, & in the other it is -1.00 @ 166. Averaging those two together gives a SER of +1.25. This would place the circle of least confusion 0.8m from the lens.

However, here's where I get stuck. Let's say the script is +2.25 -4.50 x 076, giving +2.25 @ 076 in one major meridian, and -2.25 @ 166 in the other major meridian. Averaged together that gives a SER of 0.00, placing the circle of least confusion directly in the plane of the lens. In this case, the astigmatism is corrected, but there is no additional spherical correction?

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[KHE]

However, here's where I get stuck. Let's say the script is +2.25 -4.50 x 076, giving +2.25 @ 076 in one major meridian, and -2.25 @ 166 in the other major meridian. Averaged together that gives a SER of 0.00, placing the circle of least confusion directly in the plane of the lens. In this case, the astigmatism is corrected, but there is no additional spherical correction?

In that case, the circle of least confusion is at the lens but it doesn't mean there is no spherical correction. It means that both focal points are equal distance from the lens, one in front of it and one behind it.

 

[1UP]

In that case, the circle of least confusion is at the lens but it doesn't mean there is no spherical correction. It means that both focal points are equal distance from the lens, one in front of it and one behind it.

Ah, okay. So, in that case, if the second script was used for an eye with the first script, in the first major meridian the script would be too weak by +1.25 and too strong by -1.25 in the second major meridian?

 

[1UP]

Ah, okay. So, in that case, if the second script was used for an eye with the first script, in the first major meridian the script would be too weak by +1.25 and too strong by -1.25 in the second major meridian?

Anyone? Is my understanding correct?

 

[1UP]

Seriously, no one here can literally spare two seconds to tell me if my understanding is right or wrong?

 

[hyponasty]

Seriously, no one here can literally spare two seconds to tell me if my understanding is right or wrong?

If +2.25 -4.50 x 076 is used instead of +3.50 -4.50 x 076, the patient is overall undercorrected by +1.25.

That means, both meridiens are undercorrected by 1.25 each

 

[Shnurek]

However, here's where I get stuck. Let's say the script is +2.25 -4.50 x 076, giving +2.25 @ 076 in one major meridian, and -2.25 @ 166 in the other major meridian. Averaged together that gives a SER of 0.00, placing the circle of least confusion directly in the plane of the lens. In this case, the astigmatism is corrected, but there is no additional spherical correction?

The Circle of Least Confusion caused by the spherical equivalent refraction is placed on the retina. Therefore the person will still have uncorrected astigmatism in two meridians as you stated -2.25 in one and +2.25 in the other. Basically instead of focusing the light to a point on the retina the light is being focused to a blurry circle on the retina.

 

[1UP]

Thank you.