When using a manual lensometer to read a lens in minus cylinder power, what is the first adjustment to make on the power wheel?

When using a manual lensometer to read a lens with minus cylinder power, the first adjustment to make on the power wheel is to set it to high plus. This adjustment is crucial because it allows the lensometer to accurately measure the power of the cylindrical component of the lens.

In a lens with minus cylinder power, the spherical power is negative, and the cylinder power indicates the orientation or axis of that correction. By initially setting the power wheel to high plus, the lensometer is adjusted to account for the minus value of the lens, helping to focus the light correctly through the lens being tested. This method ensures that the readings are taken with the appropriate reference point, facilitating a correct assessment of the lens's prescription.

After setting to high plus, further adjustments can refine the measurement as necessary. This approach streamlines the process of obtaining accurate readings and is an essential technique in lensometry, especially when dealing with complex prescriptions.