Common strengths of neutral density (ND) filters are 3-stop, 6-stop, and 10-stop. I recommend that you start with a 6-stop, but any of them will be fine.
I find that a 6-stop filter is powerful enough to achieve virtually any goals I might have in terms of lengthening my shutter speed. At the same time, it isn’t overpowering. Let me explain that in the context of the other two choices I mentioned.
The 3-Stop ND Filter
If you get a 3-stop filter, it might not be strong enough for you to lengthen your shutter speed as much as you want in bright environments. Let’s say, for example, that you are shooting outside on a fairly bright day. Your settings are 1/100, f/8, ISO 100 and that shows a proper exposure. You are photographing something like a waterfall or other moving water, and you want a moderate blur to it so decide to slow down your shutter speed. Let’s assume you want to use something in the 5 – 10 second range, which I think is pretty typical for this sort of shot.
You put the 3-stop filter on your lens. That allows you to shorten your shutter speed to 1/12 of a second. That’s not nearly slow enough. You stop down your aperture all the way to f/22, which buys you another 3 stops. That gets you to .6 seconds. Your ISO is already as low as it will go. You cannot get there: your filter isn’t strong enough.
That’s not to say the 3-stop filter won’t be strong enough in a lot of other contexts. The scenario above is about as bright as you will ever face. In many other situations, the 3-stop filter will be fine. I have one as part of my kit, but it just isn’t the one I would start with.
The 10-Stop Filter
If I would have written this post a year or so ago, I probably would have recommended that you start with the 10-stop filter (this change in my opinion shows you how subjective this whole question is). The 10-stop ND filter is a wonderful tool. When it comes to restricting light, it is an absolute beast. I love it.
It leads to some problems though. First of all, your camera cannot see through the lens at all when you have this filter attached. That means you cannot focus with it attached. You cannot meter light either. Your exposure simulation (if you have it) will not work. That means you need to set your camera up on the tripod, focus and meter without the filter on, then attach the filter and change the shutter speed to account for the 10-stops of light you just took away. And you have to do all that without jostling the camera too much. It can be kind of a pain.
In addition, consider what happens when you try to use this filter in low-light situations. Let’s say, for example, that you are in a moderate lighting situation and a correct exposure reads 1/30, f/11, ISO 200. You want to increase your shutter speed to the 5 – 10 second range so you attach your 10-stop filter. If you simply attach the filter and move the shutter speed by 10 stops to offset for the lack of light that will take your shutter speed to 30 seconds. That’s too long for what you had in mind. To shorten the shutter speed to the range you had in mind, you would need to either raise the ISO or open up the aperture. Neither of these is likely to be a great option since it might lead to increased levels of digital noise (ISO) or a shallower depth of field (aperture).
The 10-stop filter is great for bright situations or where you want to use a very long shutter speed. You’ll be able to knock the shutter speed to 30 seconds or longer in almost any situation. I do recommend getting one, but probably not as your first filter.
The 6-Stop Filter
I find that the 6-stop filter can fill virtually all my needs. In fact, my simple walking-around kit when I am not carrying a backpack is simply my camera, tripod, remote shutter release, and 6-stop ND filter.
Let’s consider how the 6-stop filter would function in the two scenarios set forth above. In our first scenario we were shooting at 1/100, f/8, ISO 100 but we wanted to use a longer 5 – 10 second shutter speed, but our 3-stop filter was too weak. Simply attaching the 6-stop filter to our lens and making the offsetting change to our shutter speed gets us to .6 seconds. That isn’t long enough, so we can stop down our aperture by another 3 stops to f/22. When we lengthen our shutter speed to offset this move, that allows us to go to 5 seconds. So we can get there with a 6-stop filter.
Now let’s consider the second scenario. Remember we started at 1/30, f/11, ISO 200 and wanted to use a shutter speed of 5 – 10 seconds, but our 10-stop filter was too strong. Simply attaching the 6-stop filter and making an offsetting change to our shutter speed gets us to 2 seconds. From there I can lower the ISO to 100 and reduce my aperture size to f/16. That buys me another 2 stops, which allows me to use a shutter speed of 6 seconds. We’re in business.
That’s not to say the 6-stop filter will work in every instance. Sometimes you want the nuance of the 3-stop filter and sometimes you want the brute strength of the 10-stop filter. I personally own all three and recommend you build toward that. It is just that the 6-stop filter is very versatile and, when you are picking up your first neutral density filter, it is a good choice. In addition, it is frequently a little easier to use than the 10-stop filter because your camera can usually focus and meter when you have the filter attached.