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2024-03-28 05:05:29
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Spherical aberration — aspherical vs spherical lens elements

Following up on Monday’s entry on spherical aberration, some lenses use aspherical lens elements to correct out the most egregious problems. The lens as a whole might still have spherical aberration, but it will be reduced over the lens that does not correct for it.

With a spherical lens element, the greater the distance from optical center, the greater the spherical aberration becomes, so that large aperture fast lenses with their large diameter optical elements exhibit more error than smaller diameter lenses in slower lenses.

For example, a 2-inch diameter lens element means that rays are up to 1-inch away from optical center; whereas a 1-inch diameter lens element has rays that are only 0.5-inch off-center. In other words, an f/1.4 lens is going to have a lot more spherical aberration than an f/2.8 lens. While there are ways of correcting the errors with additional elements, this does make the lens larger and heavier.

Lenses with substantial spherical aberration include most f/1.4 and faster lenses, especially older designs like the Nikon 50mm f/1.2 AIS.

The use of aspheric lens elements eliminates spherical aberration for the element involved, while being smaller and lighter than an equivalent spherical lens. The downside is much greater cost (the best aspherical elements must be laboriously ground and polished). But note that a camera lens consists of many elements, so the use of one or two aspheric elements does not mean that spherical aberration as a whole for the lens is eliminated—only reduced.

Observe how a spherical lens element generates a smeared focus effect; this is the haze and low contrast you’ll see with f/1.4 lenses. Stopping down cuts off peripheral rays, which causes an apparent focus shift.

Spherical lens design — rays from periphery diverge
Image courtesy of Carl Zeiss Inc

 

Aspherical lens design — rays at all offsets focus together
Image courtesy of Carl Zeiss Inc

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