# Thread: Does power affect corridor width?

1. ## Does power affect corridor width?

I know that Add power affects the corridor width (that a +1.50 is going to be more generous than a +3.25), but does base sphere power affect that as well? Or is it simply a function of the difference in power? Everything I've tried to search for on this basically just comes back as "add power affects corridor width", without really addressing the initial inquiry. My gut tells me "no" (beyond the fact that higher powers are going to have more limited visual areas w/o aberration to begin with), and that it's more a function of add power than base Rx

2. I would agree that the design is the design and the base curve might have a very limited effect.

So I'll wait for a Guru to tell me how wrong I am;)

3. No guru, but yeah, plus powers have a narrower field of view (and more magnification) and minus powers have a wider field of view (and more minification).

Shamir does a nice job of explaining this on their professional education documents.

4. So same power and design with a steeper base "power" (or do you mean base curve, juno?) will produce a narrower corridor?

I think that is the question.

5. Yeah, my query boils down to:
Does a +4.00 sph +2.00 add have a narrower corridor than someone in a +2.00 sph +2.00 add PAL, assuming all other things equal. Would this be different for myopes? As mentioned, I know that a 2.50 add is going to be more constrained than a 2.00 add, but I'm curious if that narrowing is partly a function of raw power of correction, or if it's strictly due to the difference in power between distance and reading.

6. Originally Posted by juno
Yeah, my query boils down to:
Does a +4.00 sph +2.00 add have a narrower corridor than someone in a +2.00 sph +2.00 add PAL, assuming all other things equal. Would this be different for myopes? As mentioned, I know that a 2.50 add is going to be more constrained than a 2.00 add, but I'm curious if that narrowing is partly a function of raw power of correction, or if it's strictly due to the difference in power between distance and reading.
I suspect that the higher powers requiring more extreme base curves is a major factor here.

7. Originally Posted by juno
Yeah, my query boils down to:
Does a +4.00 sph +2.00 add have a narrower corridor than someone in a +2.00 sph +2.00 add PAL, assuming all other things equal. Would this be different for myopes?
As drk declared, if magnification is increased the field of view decreases and vice versa.

As mentioned, I know that a 2.50 add is going to be more constrained than a 2.00 add, but I'm curious if that narrowing is partly a function of raw power of correction, or if it's strictly due to the difference in power between distance and reading.
The narrowing of field of vision is due to magnification at all angles of gaze. Increased add power also increases unwanted astigmatism, possibly narrowing the FOV depending on the software. FOV at distance and intermediate can also be manipulated by design/software, but remember TNSTAAFL.

Hope this helps,

Robert Martellaro

8. TNSTAAFL, that's what I always say.

9. Yeah, my query boils down to:
Does a +4.00 sph +2.00 add have a narrower corridor than someone in a +2.00 sph +2.00 add PAL, assuming all other things equal. Would this be different for myopes? As mentioned, I know that a 2.50 add is going to be more constrained than a 2.00 add, but I'm curious if that narrowing is partly a function of raw power of correction, or if it's strictly due to the difference in power between distance and reading.

Re-arranging Minkwitz theorem to solve for x:

$x = \frac{(ast * h)}{(2 * add)}$

We can find how far from the umbilic 1.00 D of astigmatism will be present using a 2.00 add and a 3.00 D add with a 16 mm corridor.

$x = \frac{(1.00 * 16)}{4}\\
= 4 mm
$

$x = \frac{(1.00 * 16)}{6}\\
= 2.67 mm
$

As you can see regardless of the distance power more astigmatism is produced with either a shorter corridor or higher add power.
Using the 2.00 add power we see that we have a total width of 8 mm.
As stated by DRK and Robert power will reduce the field of view.

We will use +4.00 and a -4.00 to see the difference.
Let us assume a 27 mm center of rotation which equals 1/0.027 = +37.00 D
Y is half of the width(4 mm)

+4.00
$tan(a) = \frac{4 \times (37 - 4)}{1000}\\
= 7.5^o
$

-4.00
$tan(a) = \frac{4\times (37 + 4)}{1000}\\
= 9.31^o
$

Keep in mind today's lenses are optimized for many parameters to compensate for many of these issues afflicted with older progressives.

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