Didn't know these existed. Anyone got any experience with these?
Not a big fan of the Variable ND filter; tried a few and in my experience they're just not worth the money. I would suggest you stick with a couple good quality, standard ND filters and forget about the variable-variety altogether. Learn to apply a gradient in post instead and you'll get better results 99% of the time.
How powerful an ND filter you need depends on what you intend to use it for, obviously, but I keep an ND8 (three-stop reduction) and an ND1000 (ten-stop reduction) in my bag and they're about all I've ever needed. The ND8 is good for taking the edge of bright sunlight when I'm on the beach or in the desert while the big ten-stop is primarily used for long exposure shots. As for brands I like Hoya quite a bit but Tiffen makes a good filter too and neither will break the bank. I do have some filters from Breakthrough Photography and they're definitely my all-time favorites, but they're also reeeally expensive. The ones I have I didn't have to pay for, thankfully. A really good, and surprisingly inexpensive ten-stop ND is the "Ice" branded ND you can get from Am-a-zon; they sell for around
$30 for a 77mm filter. I have one, and I like it a lot.
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