For moderate corneal cylinder, nomograms start relative to K as:

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Multiple Choice

For moderate corneal cylinder, nomograms start relative to K as:

Explanation:
When correcting moderate corneal cylinder with a gas-permeable toric lens, you anchor the starting point to the keratometry readings. Keratometry provides the two principal meridians—the flat and the steep. Beginning “on” K means you align the lens’s toric correction with those corneal axes from the start, so the cylinder power is placed to neutralize the corneal astigmatism rather than offset it. This baseline on the keratometry references gives a more predictable initial fit and better stabilization, because the astigmatic error you’re trying to correct is defined by those keratometric meridians. If you start off K, or begin with the cylinder positioned on a flatter or steeper reference or offset from K, the lens would be less aligned to the corneal cylinder, leading to residual astigmatism or rotation.

When correcting moderate corneal cylinder with a gas-permeable toric lens, you anchor the starting point to the keratometry readings. Keratometry provides the two principal meridians—the flat and the steep. Beginning “on” K means you align the lens’s toric correction with those corneal axes from the start, so the cylinder power is placed to neutralize the corneal astigmatism rather than offset it. This baseline on the keratometry references gives a more predictable initial fit and better stabilization, because the astigmatic error you’re trying to correct is defined by those keratometric meridians. If you start off K, or begin with the cylinder positioned on a flatter or steeper reference or offset from K, the lens would be less aligned to the corneal cylinder, leading to residual astigmatism or rotation.

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