If I have two neutral density filters (e.g. 5 & 10 stop) and the effect I'm after can be achieved with either (blurring of water/clouds) with differing exposure times using either, is there any reason to choose one over the other?
Aside from the actual time, why would a shorter or longer exposure time be preferable?
When you are using ND filters with long exposure times, you are trying to achieve a particular blur effect. The primary exposure control here is the shutter speed. Shutter speed directly determines the quality and amount of the blur effect. And the required shutter speed to achieve that effect is determined by your scene:
Do you want to completely blur the cloud motion, so you can't even tell general shapes of the clouds? That requires longer exposure.
Are the clouds moving slowly? Slower-moving clouds also require longer expsoure.
Do you just want to blur fast-moving water? Thats one of the faster long-exposure scenarios.
Generally though, there's a HUGE difference between 5-stop and 10-stop ND filters. A 5-stop ND will change a composition that was metered at 1 second, into a 30 second exposure. A 10-stop ND filter will change a 1/30 second shot into a 30 second exposure. The 10-stop ND filter will also change the 1 second shot into a 900 second (15 minute!) exposure, rather than a 30 second exposure like the 5-stop ND filter would.
Some rough examples:
Trying to blur a waterfall or fast stream, with lots of tree canopy cover, and/or on a very overcast day, you'll probably be fine with the 5-stop ND filter.
Blurring that same waterfall or stream, with little or no canopy cover, on a sunny day, you'll probably need the 10-stop ND filter.
Trying to blur scattered soft clouds in a light breeze on an otherwise bright day, you'll probably need to combine both filters to get 15 stops of reduction.
When creating long-exposure shots, in general, compose and meter your scene without the filter, in full manual exposure control. Set your aperture to compose the scene with the depth of field you want. Set your ISO close to the base ISO of your camera (i.e., probably 100, could be 50 or 200). The exact value doesn't matter too much. Let the shutter speed be what it needs to be to correctly expose your scene.
Now, with your handy exposure calculator (or mental math if you're comfortable with base-2 logarithm approximation), increase the exposure time for either 5 or 10 stops. Is that exposure time acceptable to achieve the desired look? Then you're set.
If not, you'll have to compromise, perhaps adjusting your ISO up or down a stop or two, maybe eking a stop either way from your aperture to help. It's all about pushing a tiny bit here and a little bit there, to get the best shot you can with what you have.
If I have two neutral density filters (e.g. 5 & 10 stop) and the effect I'm after can be achieved with either (blurring of water/clouds) with differing exposure times using either, is there any reason to choose one over the other?
A difference of 5 stops is quite a lot -- if you change only the filter and the exposure time, a shot that takes 1 minute with the 5 stop filter would require 30 minutes with the 10 stop filter. So you might prefer the 5 stop filter purely on the grounds that you have other things to do than wait around for the camera to collect enough light. For example, you could take 30 different shots with the 5 stop filter in the time that it takes for one with the 10 stop filter.
Also, very long exposures tend to get noisy, so you might get a somewhat better image with the shorter exposure.
On the other hand, it's hard to imagine that you'd really get exactly the same amount of blur with both filters. The movement of most physical things like water and clouds don't change speed, so it's likely that those things would be recorded somewhat differently given the large time difference. So look closely at the images you get from each and see if you don't like one more than the other.
When I shoot with heavy ND filters I do the calculation backwards - I decide on exposure time (suppose a minute or two) and depending on light conditions calculate aperture and filter required.
Because light conditions vary during the day carrying both +5 and +10 filter gives me greater flexibility.
This is a photo I shot early in the morning with a +5 filter (exposure time almost 3 minutes). If I was after the same look by mid-day a +10 would be required.
