Light contains waves of various frequencies. We see things because they reflect light. Cameras use light to transfer the appearance of a scene through a lens to film. While light is consistant, lenses are not, so changes happen to the image. The changes are measured by the Modulation Transfer Function.
Light travels in a straight line. Lenses that are a constant thickness will be thicker further from the center of the lens. That distorts the image. Lens distortion can be corrected. It is expensive to correct distortion so better lenses cost more.
We measure the value of a lens by the accuracy of the final image. We can measure the quality of a lens using MTF. It’s much easier and much less expensive to simply enlarge the image. Better lenses can provide larger enlargements. Better refers to the sharpness and the color fidelity. A black and white enlargement can vary in sharpness and the shades of grey. Color enlargements can vary in sharpness and color fidelity. All that can vary because of the way the light is changed as it travels through the lens.
The shape of early lenses was spherical and it was difficult to maintain that spherocity. Modern lenses vary is spherocity which can better correct the light that is distorted travelling through the lens.
Oh. Then there’s this: Light consists of waves that have both frequency and amplitude. Different colors have different frequencies and each frequency changes in differing amounts going through a lens. A lens may be excellent for certain colors and not as good for others. It is different for brightly lit colors compared to dimly lit colors. the MTF can measure this. So can your amazing eyes by simply looking at an enlarged photograph.
In the final analysis the amazing, wonderful and wonderous eyes have it.
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