Hi, i recently bought a d5300 and i have a 18-140mm f 3.5-5.6 35mm f.18 and 50mm f1.8. I have a hoya cpl filter for the 35mm. I'm considering a ND filters for long exposures of clouds, waterfalls, buildings etc. However they are a little pricey and i blew my budget pretty big. I read 10 stops is the best for waterfalls but will be to dark for other uses. Here are my questions
1) Which lens should i buy the filter for, since they will most likely be landscapes im assuming the 35mm prime will be the better investment?
2) What stop filter should i buy?
3) Are Haida filters good? there is a 10 stop on ebay for $25
4) Should i consider square filters and buying a holder since a pack of 3 nd filters are only around $10 but the 10 stop is $60
Unfortunately, in photography, you get what you pay for. Put a $10 filter on any lens and you turn the lens into an equivalent $10 lens. Doesn't matter if it's a $100 Tamron or a $6,000 Leica Summicron, it will only perform as good as your $10 filter.
B+W and Marumi, make good filters. Tiffen makes good ND's; these are chromatically neutral.
To avoid the expense and clutter that comes from buying multiple filters of the same types, most people buy the filter that fits their largest lens, and then buy a step up ring to match their other lenses to the diameter of the filter.
While you're at it, consider what the diameter of filter thread is on any lens you believe you may buy in the future. Buy your expensive filters to match that size. I use 77mm filters, even though my largest filter thread now is 62mm. It doesn't hurt to use oversized filters; they even help avoid vignetting.
A 3-stop ND filter is good to have to tame harsh mid-day sunlight; more useful than a 10-stop. You could also get a 6-stop filter, and stack the two to get 9-stops. That would be more than strong enough to turn your waterfalls into milky white stuff if you wanted to do that.
When you get past 6-stops you can wind up with color shifts due to the camera's sensor getting saturated with infrared and near infrared wavelengths. Adding an IR blocking filter, or using an ND with an IR component incorporated should take care of that problem.
Each filter you put on a lens increases the potential for creating lens flare and ghosting. Multi-coated filters, like the multi-coated optics used in modern lenses help prevent this, if the coatings are good.