This is going to be a little long, but I figured it might help to share what I've learned about lens filters so far...
Watch our for fake name brand filters shipping from china. I've seen plenty of Hoya filters at unbelievable prices. I took a gamble just to see and the one I bought is definitely NOT a Hoya lens as labeled and advertised. It was supposed to be multicoated but from the looks it has no coating at all and it flares horribly.
I leave UV filters on my lenses all the time for easy cleaning and protection. I can't afford high end filters for all my lenses so I've explored some of the cheaper options. You really do get what you pay for (especially in the low end) but I'd say mid-level filters give about 80% of the performance of high end filters at half the price. True multicoating (usually denoted MC, SMC, HMC, or MRC) makes a big difference, especially when you use a UV filter in low light where you'll get flare from (most) cheap filters. I would avoid cheap Tiffen, Kenko, Vivitar, Fototiox, and Opteka filters and filter kits unless you're on a strict budget where a cheap filter is better than no filter at all. I tried a few when I started out, but I learned why they were cheap and ended up replacing them. Read reviews on Amazon, especially for cheaper lenses where people will often call out poor quality or if the claimed multi coating is any good. Just because a filter claims it's multicoated doesn't mean it's good, and not all multicoating is equal!
My UV filters stay on all the time (even under a C-PL), so I have one for each lens. I use C-PL's enough that I have a couple sizes to fit the lenses I use outside most. For specialty filters I don't use very often I buy 77mm filters so I can adapt them to any of my lenses. You can get step up / step down adapters relatively cheap. I like
these Rangers ones because they're thinner than most so I don't worry about vignetting.
For UV filters the
Sigma DG UV lenses have been my go to favorite at around $15-$30 depending on size, but some are harder to find since they've been discontinued. (Check Amazon). The multicoating does a good job of preventing flare in low light or sunlight. Be careful to make sure it's a "Sigma DG" UV filter and not a generic UV filter for a Sigma DG lens if you buy it on eBay.
Another brand I've just started using is
Rangers, which you can get
on Amazon or
even cheaper on Ebay. (I don't think they're popular enough to have fakes yet, but look for eBay sellers in the US to avoid shipping delays.) I picked up a
Rangers Pro UV filter with Gorilla Glass that I'm using on my Sigma 17-50 f/2.8 and it seems to be almost as good as the Sigma DG, but a much slimmer frame. The color of lights reflected off the surface is a slightly different color than the Sigma DG, but I haven't had any flare issues yet. I also have a
Rangers Extreme Slim MRC (multi-resistant-coating) C-PL that I'm happy with so far.
I recently bought a
cheap set of Rangers ND (neutral density) filters to play around with. They're plastic instead of glass and they add some color cast, but I knew this ahead of time from the Amazon reviews. In quick tests they decrease contrast a little bit and do shift color somewhat, but still do a decent job. They'll do for now, and I'll see if I actually use them much before I spend money on better multicoated glass versions. I also ordered a K&F Concept MRC ND1000
from China on eBay for half the price it goes for
on Amazon, so I'll be curious to see how that works out for me.
As another option check Amazon Warehouse Deals. I just recently bought a higher end
UV / IR cut filter to photograph some high temperature objects at 1000°C / 1800°F, so definitely hot enough to worry about black body radiation. The filter was intended to keep infrared radiation from heating up my sensor so I didn't want to cheap out. The one I bought is $140 new but I got an open box one with 'slight imperfections' for $40, and to be honest I can't even tell it's not new. It worked quite well too. The IR cut filter absorbed non-visible radiation so well that it was almost too hot to touch, but the front and rear lens elements were nice and cool. I didn't see a color change on my K-3 with or without the filter but without this external filter, the internal sensor IR filter would have absorbed heat right at the sensor, and the lens would have heated up too.
I hope this helps, and if I remember I'll update this when I've had more time to use my ND filters in the field. Happy shooting!
---------- Post added 04-21-17 at 02:12 PM ----------
One more thing... Keep in mind that Variable ND filters are basically two crossed polarizers, so at wide angles you'll probably get
uneven darkening patterns at higher settings on the filter. You're probably better off buying single strength ND filters and switching or stacking them.
Also, I forgot to mention but thanks to the guys who recommended Breakthrough Photography filters. It's always nice to hear about good products and good companies, so I'll keep them in mind when I buy a filter next.