# Optical Forums > Ophthalmic Optics >  Theta Formula

## princess

From the given prescription , how to find out the power at axis 180? using the Theta formula 

Example : -4.50 -3.5 X 40
                 what is power at 90 axis ?
       what is the answer ? 


+2.00 +3.00 X 110
       what is the power at axis 180 ? using the theta formula

-6.00 +2.00 X 85
         what is the power at axis 180? 

Please answer these for me as soon as possible
i have a test on this next week

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## Robert Martellaro

> From the given prescription , how to find out the power at axis 180? using the Theta formula 
> 
> Example : -4.50 -3.5 X 40
>                  what is power at 90 axis ?
>        what is the answer ? 
> 
> 
> +2.00 +3.00 X 110
>        what is the power at axis 180 ? using the theta formula
> ...


D90 = Ds + Dc*cos2a

D180 = Ds + Dc*sin2a

Ds is the sphere power, Dc is the cylinder power, a is the axis.

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## HarryChiling

Theta is aften described as the difference in the meridian in question and the major meridian or sph.

In Roberts equation use the sin version and replace a or axis with the difference in axis as described above.

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