Beginner Questions from a Lab Tech

**Roland** · 01-04-2011, 06:01 PM

Hello everybody, I'm new to this forum and the whole optic field. I've got some questions that I would really appreciate if somebody could take the time to answer. Please be patient as I'm just a Lab Tech and have never studied this before.

I'm trying to get a solid technical understanding of what it is I'm actually doing in my job and unfortunately nobody at work has been able to give me a comprehensive answer. Recently this question came up in a test that my boss insists I've got wrong but has been unable to convince me why. It goes something like this:

Given the following prescription:
-2.00 +1.25 x 35
What is the axis meridian with the highest power?
a)35
b)125
c)... (I forget the other choices but I'm pretty sure it's one of the above)

So my reasoning (please correct me if I'm wrong) is that the sphere power is -2.00, the cylinder value +1.25, and therefore the cylinder power is -0.75 (there is a difference between cylinder value and cylinder power, right?). So I figure that since the spherical power is higher than the cylindrical power, then the axis with the highest power would be 90 degrees off the cylindrical axis (which is 35 right?) and therefore the answer should be 125.

My boss claims that the correct answer is 35, but when I asked him to explain why we got into this huge debate and ended up all sorts of confused.

Please help,
Thanks!

**Jacqui** · 01-04-2011, 06:16 PM

Originally Posted by Roland

So my reasoning (please correct me if I'm wrong) is that the sphere power is -2.00, the cylinder value +1.25, and therefore the cylinder power is -0.75 (there is a difference between cylinder value and cylinder power, right?). So I figure that since the spherical power is higher than the cylindrical power, then the axis with the highest power would be 90 degrees off the cylindrical axis (which is 35 right?) and therefore the answer should be 125.

My boss claims that the correct answer is 35, but when I asked him to explain why we got into this huge debate and ended up all sorts of confused.

Please help,
Thanks!

I agree with your boss :D

**Roland** · 01-04-2011, 06:24 PM

Could you please explain to me where my reasoning is wrong?

**NeGlassesGirl27** · 01-04-2011, 06:41 PM

Originally Posted by Roland

Hello everybody, I'm new to this forum and the whole optic field. I've got some questions that I would really appreciate if somebody could take the time to answer. Please be patient as I'm just a Lab Tech and have never studied this before.

I'm trying to get a solid technical understanding of what it is I'm actually doing in my job and unfortunately nobody at work has been able to give me a comprehensive answer. Recently this question came up in a test that my boss insists I've got wrong but has been unable to convince me why.

I'd really hate to buy a pair of glasses from a place where no one knows what their doing :hammer::bbg:

**Roland** · 01-04-2011, 06:52 PM

Originally Posted by Keri_Leigh27

I'd really hate to buy a pair of glasses from a place where no one knows what their doing :hammer::bbg:

I agree, which is why I'm trying to actually understand the subject. I thought I had a working understanding after going through their training, but now I'm being told that I'm wrong but nobody has been able to explain it to me.
Can you please explain to me what I'm doing wrong?

**LENNY** · 01-04-2011, 06:58 PM

Boss is always right!

**Roland** · 01-04-2011, 07:04 PM

Maybe I didn't express myself clearly - I'm not curious about people's opinions, I need someone to explain to me what I'm doing wrong.
My understanding was that given a prescription of -2.00 +1.25 x 35 meant that there was -0.75D power at axis 35 and -2.00D power at axis 125 (90 degrees off 35).
Where am I going wrong here and what's the right way of looking at this?
Thanks

**Jacqui** · 01-04-2011, 07:28 PM

You're forgetting that it's plus cylinder.

Don't worry, I'm teaching you the same way I teach my apprentices.

**finefocus** · 01-04-2011, 07:37 PM

You're over-thinking. -0.75 is the power at 125 degrees, -2.00 at 35 degrees. The sphere power is the power at the prescribed axis. 90 away from the prescribed axis (125) it's the sphere plus the cylinder. Learn to do a lens cross. It will help you avoid confusion at the lensometer, explain transposition, and avoid the dreaded "90-off" error.

**Roland** · 01-04-2011, 07:37 PM

Originally Posted by Jacqui

You're forgetting that it's plus cylinder.

Don't worry, I'm teaching you the same way I teach my apprentices.

Ok, so a lens with a negative sphere power and plus cylinder value always has the highest power in the cylinder axis?
I was under the impression that cylinder power = sphere power + cylinder value.

**Roland** · 01-04-2011, 07:44 PM

Originally Posted by finefocus

You're over-thinking. -0.75 is the power at 125 degrees, -2.00 at 35 degrees. The sphere power is the power at the prescribed axis. 90 away from the prescribed axis (125) it's the sphere plus the cylinder. Learn to do a lens cross. It will help you avoid confusion at the lensometer, explain transposition, and avoid the dreaded "90-off" error.

Ah. Is this always the case (that the sphere power is the power at the prescribed axis) or is it only the case with + cyl values? Is the prescribed axis not the same thing as the "cylinder axis"?
I'll have to look up what you mean by doing a lens cross.

**optilady1** · 01-04-2011, 07:52 PM

the sphere is the power at the stated axis

so -2.00 -2.00 x 180 means that at the 180 axis there is -2.00 D 90 degrees away, you algebraically combine -2 and -2 = -4.00 D

so -4.00 +2.00 x 90 means that at the 90 axis there is -4.00 D

-3.25 + 1.75 x 35 means that at 35 there is -3.25 D and 90 degrees away, at 125, is -1.50 D.

Don't think of the 'power of the cylinder' or what ever. At the axis, is the sphere power. Use the cylinder, whether it be positive or negative, as the direction to go on a number line. You are either going to go in the direction of the positive or the negative, in the amount of the cylinder.

Learn how to do a lens cross. it's a great tool to 'see' what a lens looks like. I've found it to be extreemly helpful, since I need to learn by seeing, not by reading.

**Roland** · 01-04-2011, 08:08 PM

Originally Posted by optilady1

the sphere is the power at the stated axis

so -2.00 -2.00 x 180 means that at the 180 axis there is -2.00 D 90 degrees away, you algebraically combine -2 and -2 = -4.00 D

so -4.00 +2.00 x 90 means that at the 90 axis there is -4.00 D

-3.25 + 1.75 x 35 means that at 35 there is -3.25 D and 90 degrees away, at 125, is -1.50 D.

Don't think of the 'power of the cylinder' or what ever. At the axis, is the sphere power. Use the cylinder, whether it be positive or negative, as the direction to go on a number line. You are either going to go in the direction of the positive or the negative, in the amount of the cylinder.

Learn how to do a lens cross. it's a great tool to 'see' what a lens looks like. I've found it to be extreemly helpful, since I need to learn by seeing, not by reading.

Thank you, I think I understand now.

**pezfaerie** · 01-04-2011, 08:08 PM

Originally Posted by Roland

Maybe I didn't express myself clearly - I'm not curious about people's opinions,

By the way Roland Welcome to OptiBoard :) ;)

**Roland** · 01-04-2011, 08:12 PM

Originally Posted by pezfaerie

By the way Roland Welcome to OptiBoard :) ;)

LOL Thanks. Ah, online forums.
Like the name by the way.

**pezfaerie** · 01-04-2011, 08:24 PM

Thanks. Everyone here really does mean well and are very passionate about what they do. Look at it as "tough love" ...lol ;)

**abberated** · 01-04-2011, 08:45 PM

Given the following prescription:
-2.00 +1.25 x 35
What is the axis meridian with the highest power?
a)35
b)125

The answer is A)
as long as highest = the most power.
Absolute value from zero.

Usually when describing power at certain axis the word Effective is used.
The Effective Power at the 035th meridian is -2.00
The Effective Power at the 125th meridian is -0.75
Try this...

Transposition

1.Algebraically add the sphere and cylinder:
_____ + _____ = _____
2. Change the sign of the cylinder:
_____ -/+ _____
3.Change the axis 90 degrees:
_____ _____ X ± 090
Final axis must be between
0 and 180
If it is NOT, then
If you added 90 subtract instead
If you subtracted 90 Add instead

Example:
+2.00 +1.00 X 180

1.Algebraically add the sphere and cylinder:
+2.00 + +1.00 = + 3.00
2. Change the sign of the cylinder:
+3.00 -1.00
3.Change the axis 90 degrees:
+3.00 -1.00 X 090

Contact me if you wish, I'd be glad to send you some worksheets. Welcome to Optiboard!

**conantoptics** · 01-05-2011, 01:08 AM

The formular to change the cylinder sign:
A+B*a= (A+B)-B*(a+/-90)

am I right?

**AngeHamm** · 01-05-2011, 09:15 AM

Jeez, people. Can we not just answer the guy's question without hazing?

**Diane** · 01-05-2011, 09:15 AM

Use the lens cross. It will help you every time. It helps you SEE the prescription on a lens and see what it looks like. It helps with transposition. It shows a lot.

-2.00 +1.25 X 035.

The sphere power ALWAYS relates to the axis. The sphere power is in every part of the lens. The axis meridian is where NONE of the cylinder is incorporated into the lens. The FULL amount of the cylinder is added to the sphere power 90 degrees away from the axis. Obviously percentages of the cylinder are incorporated into the lens at other meridians, but that's another lesson.

Hopefully, my slide will upload that I made for you.

Diane

**WFruit** · 01-05-2011, 09:48 AM

Buy this book: http://www.amazon.com/System-Ophthal.../dp/0750674806.

It will be your best resource (other than Optiboard).

Also check out: www.opticampus.com