Spherical aberration of a camera (or any) lens is caused by limitation for a lens to keep the entire image, perfectly sharp in focus. This has nothing to do with bad workmanship, but it is physics of optical science.
a typical spherical lens curvature
Any lens, which has a perfect spherical curvature, (which is a section of a perfect geometrical circle) performs best in its middle section. This area is about 2/3rd of the lens. Beyond that on the edges, it fails to focus the image as sharp as compared to the center area. The result is of an image, which is sharp in the middle part, but not sharply focused from end to end.
A compound lens design can eliminate this fault but the lens becomes bulky and heavy because of additional lens elements. (Cost is another factor, which is always in directly proportion with any optical correction of lens defects)
Another solution is to make a lens, which has an a-spherical surface curvature. This type of lens is difficult to machine, but it can reduce some elements from the lens, and lens can be compact and light in weight.
Aspheric lens design
In whichever way a lens is designed, if it is corrected for spherical defect, it will become expensive.
To check this spherical aberration, shoot a flat subject with texture, on widest lens aperture setting. Make enlargement print (10X) of center area and area taken from corners. You will be able to see the difference.
We can also check this sharpness on a computer screen at 400% zoom. A good quality lens is marked with "Aspherical" word on its lens ring, and we can study the lens instruction manual for further details.
For these corrections and rectifications in a lens design, an entry level lens is so different from the one for professional use.
Read about these lens faults in detail:
Blur or defocus of lens: Blurring of an image due to lens fault
Chromatic aberration: How chromatic aberration or distortion affects the image quality
Diffraction: Scattering of light causing diffraction in an image
Field curvature: Field curvature aberration of a lens
Lens flare: How to take care of a lens flare
Loss of contrast: How ability of capturing contrast of a lens is lost
Optical distortion: Physics of optical distortion and how to check it
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