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
A 10 stop filter would be vast overkill for most waterfalls. I shoot a lot of waterfalls, and rarely do I need to use anything other than the polarizer. Most waterfalls are in the woods and thus relatively dark to begin with; waterfall pictures in bright sunlight don't often turn out well. So you don't need a large amount of neutral density to get into the optimum shutter speed range. Although its a matter of taste, in general the best shutter speed for moving water is in the range of 1/5 to 1/2 second. Longer exposures lose detail in the water, and while there are some water features that benefit from that, preserving detail in the flow usually is more satisfying. A 10-stop filter would put you firmly into the multiple second range in pretty much any lighting, but especially with waterfalls.
In general a combination of 2 and 3-stop filters is most useful for general purposes. They can be applied in combination, so you have the choice of 2, 3 or 5 stops of ND. For regular natural density, square filters with a holder aren't necessary, but I do think it is worth investing in such a system. and if you may want to sue graduated ND filters in the future, a rectangular filter, which can be adjusted relative to the scene, is essential.
I don't know that filter sizes for your lenses, but if they are the same, the filter would work on any of them.If not, buying the largest size, and then using a step-up ring to adapt the others, is economical. The is approach, however, prevents the use of a lens hood when you sue the step-up ring.
As has already been noted, there pretty much wouldn't be any filter costing $10 that would be worth having.