If a well fitted diagnostic lens has a -3.00 power and the over-refraction at 12mm is -2.00, the correct lens power to order is:

To determine the correct lens power to order based on the information provided, we start with the power of the well-fitted diagnostic lens, which is -3.00. An over-refraction is performed to assess the need for additional correction; in this case, the over-refraction shows -2.00 at a distance of 12mm.

To find the total lens power needed, you add the power of the diagnostic lens to the over-refraction result. Here’s how the calculation works:

1. Take the power of the diagnostic lens: -3.00
2. Add the over-refraction value: -2.00

When you perform this calculation, you get: -3.00 + (-2.00) = -5.00

This means that the correct power of the lens to order, after considering both the initial lens and the additional correction needed, should be -5.00.

Correctly interpreting over-refraction values is key in prescribing the right lens, and understanding how to add these values helps in achieving optimal visual acuity for the patient. Hence, the need for a lens power of -5.00, which directly corresponds with the resultant calculation based on the diagnostic lens and the over-refraction given.