Modern post processing software, as good as it is, still can’t replicate the effect of some filters. Polarizing Filters, for example, can eliminate glare and reflections far more effectively than post processing software. Unfortunately, good filters can be fairly pricey. A good quality Circular Polarizer in the 77mm size will cost well over $100.
Since most of us have lenses of varying filter sizes, buying filters to fit each lens can get expensive. The solution for many is to use step rings to adapt a different size filter to a lens. While you can use a filter smaller than the lens, doing so will usually result in vignetting, especially at wider apertures. The practical choice is to purchase your filters in the size that fits the largest lens that you own or plan to own. You can then buy inexpensive step-UP adapter rings to fit each lens to the filter.
The correct filter size for a lens is generally found on the bezel that surrounds the front lens element. The size is also often found on the back of the lens cap. It is not the same as the focal length of the lens. The adapter rings come in two styles, step-UP rings that adapt a smaller lens to a larger filter and step-DOWN rings that adapt a larger lens to a smaller filter. Adapter rings are generally marked on their edge with their size. The first number is the lens size and the second number is the filter size. A 67-77mm ring, for example, adapts a 67mm lens to a 77mm filter.
You will find step rings made from a variety of materials; brass, anodized aluminum, and aluminum. Highest quality rings are generally made from brass, have better tolerances, and are less likely to jam on the filter or lens. Hard-anodized aluminum rings are the most common and provide good service. Aluminum rings are cheapest and are more prone to jamming. A ring with a knurled edge makes attachment and removal easier.
Using step rings saves you money and reduces the amount of filters that you need to lug around, a combination that is hard to beat.