Am I correct that there are just two optical limitations to resolution / MTF: aberrations and diffraction?

Question

Bit of a semantic (and potentially naive) question here really, but I'm writing an article and want to be sure I'm correct.

Can everything that limits the MTF of a lens except the diffraction limit be classified as an abberation? I've found this nice list of aberrations from this question, is the contents of this list (practically) everything that differentiates the crisp image from a very high quality lens from the blurry image of a low-quality one? All other factors such as aperture, focus etc. remaining the same.

Or are there other factors for poor quality lenses not in that list that contribute to the difference, or to a loss of MTF? I don't know for example if the grind/finish quality of the optical elements contributes, or if it's already accounted for with some of the aberrations in the list.

Thanks!

Answer 1

The lens has two polished surfaces that reflect away some percentage of the right rays. The result is a loss of image brightness plus the reflected rays will likely hit the polished surfaces of other lenses and components of the lens barrel. These are stray light rays that eventually bathe the image area. The result is called flare and ghost images. These cause a substantial loss in image contrast and strange artifacts.

Diffraction is the phenomenon whereby light tends to creep around obstacles like the blades of the iris diaphragm (aperture). We are talking about diffraction that is present at all aperture diameters and becomes horrific at tiny diameter. Well studied by John Strutt Nobel Laureate (Lord Rayleigh) Rayleigh Criteria resolving power = 1392 ÷ f-number. Thus f/1.4 = 994 lines per millimeter, f/2.8 = 497 lines/mm, f/11 = 127 lines/mm, f/16 = 87 lines/mm, f/22 = 63 lines/mm. Above for the center of the visual wavelengths of light.

There are 7 major aberrations. All can be mitigated using a compound lens array. However, residual aberrations are always existing. 1. Spherical Aberration 2. Coma 3. Astigmatism 4. Curvature of field 5. Distortion pincushion & barrel 6. Longitudinal chromatic aberration 7. Transverse chromatic aberration.

We also have figure error. The lens shape (steepness of curve) must be exact. This often strays especially near the edges.

Again, the faithful image has yet to be achieved.

Answer 2

The classical aberrations are not caused by manufacturing defects, they're caused by the way light at different wavelengths are refracted differently by refractive materials with real thickness or by the way the same light is refracted differently based on the angle at which it strikes the front surface of the refractive material. Even a lens that perfectly matched its theoretical design would still demonstrate all of the classic aberrations. They're a property of the physics of light and refraction, not a property of how perfectly lens elements are made and assembled.

Some lens designs have more elements with more aberration correction built into them than others. Even if both lenses were manufactured to perfectly match the blueprint, they'd still demonstrate the effects of the classic aberrations. But the design that provides more correction for more of the classic aberrations should perform better if both lenses are made to the same level of manufacturing accuracy.

Any manufacturing defects add to the blur already introduced by the classic aberrations. Manufacturing defects are not aberrations, they're manufacturing defects. As we all should know, there is no perfectly manufactured anything. That's why manufacturers include specifications for tolerances. A tolerance is how far away from the design ideal a manufactured item can be measured and still be considered "close enough" to be usable for the intended purpose. In general, but there can be notable exceptions, lenses with more aberration correction built into their design also tend to be manufactured to more narrow tolerances. Thus these actual lenses usually tend to be closer to those lenses' design's theoretical performance than other lenses with less correction in their optical design.