Oblique Cylinder Chart Question

**Strab** · 05-05-2010, 08:15 AM

I am a bit confused, in the Oblique Cylinder Chart where 0,180 is 0% power and 90 is 100% power is this the plus version of the chart or the minus?

**YrahG** · 05-05-2010, 08:29 AM

Originally Posted by Strab

I am a bit confused, in the Oblique Cylinder Chart where 0,180 is 0% power and 90 is 100% power is this the plus version of the chart or the minus?

The percentage is a ratio so it can apply to eitehr plus or minus cyl for instance:

90 is 100% in your chart

-1.00 -1.50 x 005

To find the power at the 095 since the axis is 90 degrees away (95-5=90) then the power would be the sphere power * 100% of the cylinder

-1.00 + (1.00) * (-1.50) = -2.50

The same would hold true if the cylinder were +:

-1.00 +1.50 x 005

To find the power at the 095 since the axis is 90 degrees away (95-5=90) then the power would be the sphere power * 100% of the cylinder

-1.00 + (1.00) * (+1.50) = +0.50

**Strab** · 05-05-2010, 08:47 AM

Thanks YrahG for your reply. What I guess I am not understanding is why do I have 2 different charts one says 0,180 is 100% of the power the other one says 0,180 is 0% of the power?

**YrahG** · 05-05-2010, 09:02 AM

The 0/180 = 100% is wrong or you should just get rid of it now. Your chart should look something like this:

0/180 = 0%
15 = 7%
30 = 25%
45 = 50%
60 = 75%
75 = 93%
90 = 100%

If you were to graph the results it would look like a sin wave, you will see some books use a linear progression and for most purposes it works, but loses accuracy towards the extremes.

**musicvirtuoso** · 05-05-2010, 09:32 AM

I was one of those people that was confused by the graph initially, because it didn't follow a simple logical pattern (i.e.: 180= 0%, 90=100%, 45=50%, so logically say at 22.5 degrees away, you should see 25% of cylinder, which is not the case) The formula used to find the applied cylinder power is as follows: F = Fcyl*(SIN(Î))^2 where Fcyl is the cylinder power and Î is the angle between the cylinder axis and the new meridian (aka, the meridian you are testing). For testing and quick-reference, I would suggest memorizing the major ones, as stated above. But if you want a more accurate solution for any instance outside of those 5 axis, you should know this formula. In my opinion, it's just silly to try to memorize a huge chart.

**YrahG** · 05-05-2010, 09:44 AM

Here is a graphical version of a linear and actual chart.

**Strab** · 05-05-2010, 09:46 AM

I had no problem memorizing the chart there is a trick 012456788988765421 though the formula is definitely more practical. Wish it had been in my study material before I memorized the chart I would have saved the time.

**Strab** · 05-05-2010, 10:02 AM

Does anyone know for the ABO exam if they provide calculators or scratch paper? I know they do not allow testers to bring them in with them.

**musicvirtuoso** · 05-05-2010, 10:59 AM

Originally Posted by Strab

Does anyone know for the ABO exam if they provide calculators or scratch paper? I know they do not allow testers to bring them in with them.

Sorry, no calculators... bummer. But rest assured that any of the math you need to do is easy enough to do on scrap paper (which they allow, if I'm remembering correctly).

Update: Now I'm second guessing myself about the calculator. I know I didn't use one... so even if they do allow it, it's pretty simple math.

**Strab** · 05-06-2010, 08:38 AM

Thanks musicvirtuoso! I have heard so many people say how hard the test was. Then others say it is way to easy. I just hope I am prepared well enough because I do not want to have to retake it. I am hoping to start studying for my next certification right away. Things I should have done years ago but life happens.