Originally Posted by
Darryl Meister
This is certainly an interesting discussion topic. It's probably important to clarify a few terms and concepts here.
Essilor is referring to text legibility when discussing vertically oriented astigmatism. They are not suggesting that people necessarily prefer one form of ocular astigmatism over another, but that uncorrected astigmatism at an oblique axis makes it slightly more difficult to read printed text. An astigmatic focus forms a blur ellipse on the retina instead of a sharp image point. When trying to recognize letters, these blur ellipses are least troublesome when oriented vertically (that is, when the major axis of the ellipse is close to 90, so that the horizontal strokes of the letters are blurred instead of the vertical strokes) and most troublesome when oriented obliquely. Horizontally oriented ellipses fall are in the middle in terms of recognition.
The surface astigmatism in the lateral regions of a progressive lens is generally oriented at an oblique axis. While it is not feasible to completely shift these blur ellipses to an entirely vertical orientation, lens designers can at least minimize the "oblique-ness" of their orientation. (Though, as with anything in modern progressive lens design, this comes at a bit of a cost elsewhere.)
Now, back to your question... At this point is important to distinguish between the surface astigmatism of the progressive lens design, the ocular astigmatism of the wearer, and the oblique astigmatism produced when looking through peripheral regions of any lens. We can ignore the wearer's astigmatism in this discussion, since it must be independent of the lens design for a semi-finished lens.
Tangential and sagittal power errors are generally discussed in the context of oblique astigmatism -- or the lens aberration that lenses suffer from when the correct Base curve isn't used for a given prescription. These power errors will interact with the surface astigmatism of the progressive design, and many modern progressive lens designs account for this. However, the orientation of the astigmatism axis in oblique astigmatism varies depending upon your direction of gaze, so the tangential and sagittal power errors aren't strictly "vertical" or "horizontal." For example, the tangential error may result in a vertically oriented blur ellipse when looking up or down through the lens, but when looking side-to-side the blur ellipse becomes horizontally oriented. In other directions, the blur ellipse would be oriented at any angle in between.
Consequently, minimizing the tangential error will won't necessarily accomplish the design of goal of minimizing unwanted astigmatism at an oblique orientation. Instead, Essilor's progressive lens designers most likely use a merit function that includes a mathematical term specifically associated with astigmatism at an oblique axis. This term -- along with any other terms in the merit function -- is then minimized during the lens design process. Further, if ray-traced astigmatism is evaluated instead of only surface astigmatism, the combined effects of oblique astigmatism and surface astigmatism can be controlled simultaneously.
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