Why are round pieces of glass so expensive/cheap protection filter?

Before I begin let it be known that I know very little about filters.

I'm looking at getting a cheap protection/uv filter for use with a rainsleeve. The biggest problem I had with my home-made version(plastic bag) was that the front element kept getting little droplets on it that I had to keep wiping off, and I was nervous about scratching the lens.
With the cheap protection filter I won't worry so much about scratching it.
This is something I plan on using ONLY when it is raining, which rarely happens, so I'm not too concerned with the quality. (Honestly it's just a clear piece of glass, so there shouldn't be excessive negative effects. Right?)
I looked at the Hoya and B+W ones, but the cheapest is about 50$.
Is this something that I could just get a no-name brand at the local bestbuy?

Well, if the filter isn't perfectly flat with plane parallel surfaces, it'll act as another lens element, and distort your picture to a greater or lesser degree. Also, the expensive filters are multicoated like your lenses are, which reduces light loss (a single-coated or uncoated filter can block around 7% of the incident light, good multicoated filters will transmit well over 99% of the incident light) and cuts down on flare from hot spots in the picture or even just off axis. So, no, you don't want a cheap, no-name brand one. Get a quality one like a B+W MRC, Hoya S-HMC, or Heliopan SH-PMC.

All filters have some kind of coating on them. I can't explain the science of it, but a cheap coating, even an expensive coating made for the old film style lenses, can cause diagonal lines in the backgrounds of images made on digital sensors. You will know it when you see it, it jumps off the picture at you. There was/is an old thread about this, but I have lost it, that showed exactly what I am talking about. I see it in the share forums now and then and it comes from using cheap glass filters with cheap coatings on them.

Water isn't going to hurt your lens. Just clean it with a microfiber cloth when you get inside. There is a new chamois type thing now for cleaning filters that will work very well for that.

Polarizing filters might cost around US $50, but UV filters typically cost way less. A typical Hoya filter should cost around US $20 and a Tiffen filter should be only a little more expensive.

Both those are cheap, poor quality ones. The good quality Hoyas do run much more, especially the 72 and 77 mm OP will need for the 17-70 or 70-200 2.8. You'll find any number of posts in here, with examples, of people who thought they had problem lenses when in reality they had Tiffen filters on them.

I'm with Jon on this one. A cheap filter will cause problems. I have seen it when I had a 70-200 f/4 IS, colors washed out and flare. Removed filter problems gone. I picked up a B+W MRC and no more problems. I have MRC's on all my L's and really cant tell the diff.

Cheap = problems!

Mike

Just to pile on, if you think that rain on the front objective lens element is a problem, just try a cheap quality UV filter ... unlike water drops, you can't wipe it to make the problem go away.

Found that thread and some pics of what a cheap filter does.

https://photography-on-the.net …hp?p=2602038&po​stcount=29

Here's that old thread.

So this would be okay?
Or should I step it up to this one, which doesn't have the uv part(though my understanding is the camera already does some uv filtering).

Get the second; the first has a lower-grade multi-coating (each coat of a multi-coated glass surface is "tuned" to a specific wavelength, so more is better; the more layers, the more of the light will be "trapped" and sent through the glass to your lens).

Good optical glass is expensive.

^^^^ Agreed. Quality costs money and you get what you pay for!

GM_of_OLC wrote in post #6052462
......Or should I step it up to this one, which doesn't have the uv part(though my understanding is the camera already does some uv filtering).

Jon wrote in post #6053139
Get the second; the first has a lower-grade multi-coating (each coat of a multi-coated glass surface is "tuned" to a specific wavelength, so more is better; the more layers, the more of the light will be "trapped" and sent through the glass to your lens).

Of those two choices, I agree with Jon and his rationale. But, I think that I should comment on your understanding of the effect of UV. Any remaining UV that makes it through all of the elements in the lens would be extremely weak since ordinary glass is a fairly effective UV filter. While the filter in front of the sensor would take care of the remaining UV, the purpose of the UV filter on the front of the lens is not really there for the benefit of the sensor as much as it is there to reduce atmospheric scatter which can cause a bluish cast to distant parts of the image and reduce flare in the lens ... but a cheap UV filter can actually increase flare.