There is a reason for that madness with Seiko, they use zero, 2, 3 or 4 millimeter drops depending on the amount of prism thinning in that model of lens. Prism thinning isn't as necessary in Free-form. So a Zero drop doesn't necessarily mean it has zero prism thinning, just that its calculated at the lowest needed amount. Its not on a lens by lens basis, but will be consistent with that model, so for example all the Surmounts have less prism thinning.
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