Zoom lenses often have fewer aspherical elements than some high-end prime lenses, primarily due to design complexity, cost considerations, and the inherent challenges of creating zoom lenses that must perform well across a range of focal lengths.
Design Complexity and Variable Focal Lengths
Designing zoom lenses involves accommodating a range of focal lengths and maintaining image quality across all these lengths. Incorporating aspherical elements into such a complex optical formula increases the design and engineering challenges. Each change in focal length can alter how aberrations manifest, requiring that aspherical elements be precisely calibrated for multiple scenarios, which can be technically demanding.
Cost Considerations – Expense of Aspherical Elements
Aspherical lenses are more expensive to design and manufacture than standard spherical elements. They require advanced technology and materials to produce, which can significantly increase the lens cost. In zoom lenses, which already require complex arrangements of multiple lens groups to enable zooming, adding multiple aspherical elements could make the lens prohibitively expensive for many consumers.
Optical Performance – Balancing Aberrations
Zoom lenses must correct optical aberrations across different focal lengths, such as distortion, vignetting, and chromatic aberration. While aspherical elements are excellent for correcting spherical aberration and reducing coma and other aberrations at wide apertures, zoom lenses might rely more on a combination of different types of elements, like extra-low dispersion (ED) glass or low dispersion (LD) elements, to balance overall performance cost-effectively.
Physical Size and Weight – Bulk and Heaviness
Aspherical elements can be thicker or have more complex shapes than spherical lenses. In a zoom lens, where space and weight are significant considerations (especially in lenses intended for handheld use), adding aspherical elements could result in a larger, heavier lens, detracting from its portability and usability.
Market and Application
Manufacturers also consider the target market and typical applications of the lens. For many consumer-grade zoom lenses, the slight optical improvements from adding numerous aspherical elements may not justify the increased cost, especially if the target users are general consumers rather than professionals requiring the highest image quality.
