Filters.. or I suggest, any piece of glass you put in front of your lens must be considered carefully.
https://www.lensrentals.com/blog/20...anking-of-the-major-uv-filters-on-the-market/
Roger, on cheap filters. This is included in the above link too.
https://www.lensrentals.com/blog/2017/05/yet-another-post-about-my-issues-with-uv-filters/
Optics:
One important thing to remember about filters (this is the same for any lens, window, etc.).is that they reflect light from both sides, both surfaces. This can have a big effect on contrast and overall image quality.
http://www.koppglass.com/blog/optical-properties-of-glass-how-light-and-glass-interact/
http://www.optics4kids.org/home/content/what-is-optics/reflection/the-reflection-of-light/
Polarized light:
"Hence, a picture taken in a direction at 90 degrees from the sun can take advantage of this polarization. Actually, the effect is visible in a band of 15° to 30° measured from the optimal direction."
I point my thumb at the sun and with my index finger at 90 degrees to my thumb, i rotate it across the sky to visualize the band of sky effected by a polarizer - in order to previsualize the effect of the filter.
https://en.wikipedia.org/wiki/Polarizing_filter_(photography)
I often use a polarizing filter to enhance color saturation
http://www.cambridgeincolour.com/tutorials/polarizing-filters.htm
The "Feynman Lectures on Physics" , 3 Volumes, are a great resource if you're into that kind of thing.
http://www.feynmanlectures.caltech.edu/I_33.html
http://www.feynmanlectures.caltech.edu/II_33.html
If the math is beyond your understanding, just read the text and study the diagrams. Much understanding can be gleaned from this. You will have a better understanding of why some surfaces don't respond to polarizing filters and a better understanding of glass transmission and reflective surfaces.
ND - Neutral Density filters: If you use these, be sure to get high quality examples as stacking them increases the surface count.
https://www.picturecorrect.com/tips/neutral-density-filter-tips/