An optical system, comprising: a first optical element having a first side and a second side bounded by a circular perimeter centered on an optical axis, and an annular rim extending from a peripheral portion of the second side in a direction principally parallel to the optical axis, the rim having an inner surface, wherein at least a portion of the inner surface is a first convex surface and a peripheral portion of the first side of the first optical element is a second convex surface, the optical system further comprising: a barrel (430, 440) comprising a first conical surface and a second conical surface, wherein: the first optical element is disposed within the barrel with the first convex surface tangent to the first conical surface and the second convex surface being tangent to the second conical surface, and the first conical surface forms an angle β with a normal to the optical axis, the second conical surface forms an angle γ with the normal to the optical axis, and β and γ are related by the equation: β = arctan L + r 2 tan γ r 1 wherein: r 1 is a radius from the optical axis to a first circle of tangency between the convex inner surface and the first conical surface, r 2 is a radius from the optical axis to a second circle of tangency between the second convex surface and the second conical surface, and l is a distance, parallel to the optical axis between a first plane containing the first circle of tangency and a second plane containing the second circle of tangency. a first optical element having a first side and a second side bounded by a circular perimeter centered on an optical axis, and an annular rim extending from a peripheral portion of the second side in a direction principally parallel to the optical axis, the rim having an inner surface, wherein at least a portion of the inner surface is a first convex surface and a peripheral portion of the first side of the first optical element is a second convex surface, the optical system further comprising: a barrel (430, 440) comprising a first conical surface and a second conical surface, wherein: the first optical element is disposed within the barrel with the first convex surface tangent to the first conical surface and the second convex surface being tangent to the second conical surface, and the first conical surface forms an angle β with a normal to the optical axis, the first optical element is disposed within the barrel with the first convex surface tangent to the first conical surface and the second convex surface being tangent to the second conical surface, and the first conical surface forms an angle β with a normal to the optical axis, the second conical surface forms an angle γ with the normal to the optical axis, and β and γ are related by the equation: β = arctan L + r 2 tan γ r 1 wherein: r 1 is a radius from the optical axis to a first circle of tangency between the convex inner surface and the first conical surface, r 2 is a radius from the optical axis to a second circle of tangency between the second convex surface and the second conical surface, and l is a distance, parallel to the optical axis between a first plane containing the first circle of tangency and a second plane containing the second circle of tangency. r 1 is a radius from the optical axis to a first circle of tangency between the convex inner surface and the first conical surface, r 2 is a radius from the optical axis to a second circle of tangency between the second convex surface and the second conical surface, and l is a distance, parallel to the optical axis between a first plane containing the first circle of tangency and a second plane containing the second circle of tangency.