Round, screw-in, filters are what you need for highly attenuating ND filters.

Tiffen have a very interesting variable (2 to 8 stop) silter that looks very interesting - https://www.adorama.com/tf77vnd.html?kbid=12417. It solves a lot of problems that you can run into with a fixed value ND filter (sometimes not enough attenuation, sometimes too much!).  Hoya do a similar filter (1.5 to 9 stops) - https://www.adorama.com/hy77vnd.html.

I've been happy with Hoya filters myself. I have a 400x ND Hoya (9 stops, but no longer in production) that I use for this type of long exposure. Tiffen makes a relatively inexpensive 10 stop filters (https://www.adorama.com/tf77nd30.html?kbid=12417). They also have one which also blocks IR light (https://www.adorama.com/tfw77irnd30.html?kbid=12417). The IR light that leaks through some ND filters could, I guess, have an effect if the camera didn't have a strong IR blocking filter in fornt of the sensor and the exposure time was long. Most people don't worry about IR Transmission. I've not found it to be an issue for me.

The Breakthrough filters are very good (https://www.adorama.com/btx3nd677.html?kbid=12417), but more expensive then Hoya/Tiffem and fixed in value.

Whether a 3 stop and 6 stop filter pair is better than a 10 stop filter probably depends on what conditions you shoot in. You can change exposure time using aperture and ISO setting, as well as stacking a polarizer (which you probably have). I don't know what prices are in Goa, but the 10 stop Tiffen XLE 77mm filter is only $40 here in the US (https://www.adorama.com/tfw77irnd30.html?kbid=12417). I mighty start out with that one and see how things go.

One thing to watch with the variable filters is that they since they consist of two polarizers (usually one linear, one circular), they can produce unwanted polarization effects (such as banding) if the subject is generating polarized light (e.g the sky on a clear day). The effect gets worse with wideangle lenses and high density settings.

Thanks, Bob for the reply.

Actually, I should have clarified that I am looking for a regular ND filter for seascapes/waterfall photography to get that creamy water look and fleecy clouds. Someone has recommended "Breakthrough" round ND filters. They say that square filters are fine but there may be light leakage with them and they need more careful handling. Also, they say that we can buy Breakthrough 3- and 6-stop filters and either use them individually or stack them to get a 9-stop effect, in which case it may not be necessary to get the 10-stop filter (Save money!). I plan to use the filter on my Canon 16-35 F/4 lens with 77 mm dia.

What is your opinion of round filters ? They seem to be easier to use and do not need filter holders, etc. Some Chinese brands like Nisi and Haida are relatively easier on the pocket but I have no idea about their quality, especially with regard to color cast.

One other thing. I live in India where the summers are really hot. In Goa, where they have lovely beaches, even the winters can have very strong sunlight with temperatures in the region of 35-40 Celsius. Should I, therefore, buy a 10-stop filter straight away or go for the more moderate 3 and 6 stop approach? Personally I prefer to go with the 3 and 6 stop filters but I would like to know your opinion.
Thanks for your assistance. 

Do you mean a graduated ND filter for landscapes or a regular (uniform) ND filter for long exposures?

Stacking filters isn't a good idea, but if you have to then the better the multicoating, the better the result is likely to be.

With graduated filters you have two options, round filters that screw into the lens or square filters that are attached to a lend using holder. The square filters (usually 4"x4") are more useful since they allow you to adjust the position of the light/dark regions. Obviously you need an adapter to mount them on a lens. Cokin square filters are cheap ($40 or so), but they are plastic and not always 100% neutral. Tiffen make decent filters (round and square) at reasonable prices, though you'll be in the $250 region for a 4x4 grad ND filter. Their round filters will be $50-$100, depending on the size.

You might want to give Cokin a try if you don't want to spend much. A lot of people use them and are happy with them, though

See https://www.adorama.com/ckndka.html?kbid=12417 for a cokin started kit with 3 ND filters. You'l also need an A series adapter ring to fit your lens (up to 62mm I think). For larger lenses you'll need the larger (and more expensive) Pro series.

If you like the effect and want to get serious, you're probably looking at a $500 or more investment for a set of good glass filters, adapters and a holder.

I am interested in buying a ND filter but don't want to spend a lot of money. Can someone recommend a good brand with minimum color cast suitable for stacking ? I use a Canon eos 6D camera.

I'd be interested in your results.

It's possible that each lens may be different too. Telephoto lenses require much more focus travel than wideangle lenses. It's hard to guess at how the EOS Utility's focus algorithms would handle this, along with the influence of subject distance. I guess that ideally you'd like a constant increase in the spot size (= degree of defocusing) for each click on the adjustment, no matter what focus distance or focal length. As long as the lens reports back the focal length and focus distance, it should be possible to do that, but whether of not Canon did it that way I don't know.

Thanks Bob, I will try this with a steel ruler as a target.
Ken

Sorry Ken, I don't know. One way you could test this would be to have a stepped (or slanted) focus target and see how many "clicks" it takes to move from one focus point to a second focus point a known distance behind (or in front of) the original focus point.  It will depend on focus distance. I assume it's stepping the focus by a fixed number of focusing motor "steps". Those steps will correspond to larger changes in subject distance for more distant subjects than for close subjects.

Hope this gives you some ideas!

Bob

Hi Bob
The Canon EOS Utility allows one to control focussing remotely using a laptop or other computer. If you select fine focussing, do you have any idea how far forward or back the plane of focus moves for one click? I have started to come to terms with focus stacking and I am advised  that altering focus using the lens is better than moving the camera using a focussing rail.
Regards
Ken Lindup

I think one of the issues is that the rate of progress is slowing as the technology matures. Today's entry level DSLRs probably have the performance that "state of the art" models had 10 or 15 years ago. The difference in image quality between a low end APS-C rebel and a full frame pro level DSLR isn't really that much for the applications that most people have. If your making poster sized prints at iso 25600 then the difference might be significant. If you're shooting for 8x10 prints at iso 100, it isn't. Recent new models tend to feature faster processors, more complex metering and AF systems, Higher resolution video (4K), more connectivity and other features not really directly connected to still image quality.

Whether you need higher image quality is a personal decision and depends on how you will be using your images. Personally I'm fine with the IQ of today's DSLRs. I'd like to see higher image quality in smaller, lighter cameras with smaller sensors. There's still room for a lot of improvement there!

Bob, You are exactly right.
I went from a 20D to a 40D and in 2007 to a 1Ds mk III which was the best camera I had ever used by far.
I love it and still use it,I waited in vein for a high MP update, when it became clear none was likely I considered the 5Ds or sr,  but added the 5D mk iv because it seemed like it offered the best body, if not the highest MP. It is a wonderful camera in every way, but I'd be lying if I said I didn't occasionally think about printing 50MP files.  It was no where near as satisfying as the 1Ds III purchase was.
It was a huge satisfying jump in every area, ( remember in 2007 Nikon's flagship was a crop sensor 12mp camera, and my 40D was 10)
I get what they are doing, and why, but it takes a little of the fun out of it.....;-))