Just a note that aperture position is one of the "design freedoms". There's some choice involved because different positions are the "sweet spot" for minimizing different aberrations. The sweet spot for minimizing coma may not be the sweet spot for flattening the field curvature, and neither may be right for minimizing spherical aberration, (although users of meniscus lenses usually want that aberration). And the farther out you move the aperture, the less coverage you have, which may also affect the decision if you're into wider views.
Anyways, your sweet spot may not be someone else's sweet spot.
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