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Welcome to FM, alhajri.
I want to do the long exposure photos of clouds, running water, and mist/fog. However, since it will be my first ND filter, I don't know which grade to get. 0.3-2x which I hear is very poor, 0.6x4x which I hear is ideal for most situations and the x9 and x1000 for extreme metal welding photos.
You don't want a Gradated Neutral Density filter; you want a plain Neutral Density filter. For the photos you describe, you will need to reduce exposure overall, not selectively.
It is easy to use basic photographic knowledge to calculate what strength filter you need based on knowing how long the exposure you need. How to do it is below. To blur waterfalls, you might like an exposure of one second (a range from 1/8 to 8 seconds will have pleasing effects).
However, for your stated needs, a reasonable starting point is 1.8 ND (64x, six-stop). The main use of 0.6 / 4x / 2-stop or 0.9 / 8x / 3-stop NDs is for getting shallower depth of field shooting wide open.
Think about it this way, using the Sunny F/16 Rule, which you know, right?, or will shortly know: exposure in bright sun between the hours of 10 am and 2 pm with the sun behind you is f/16 at a shutterspeed one-over-ISO, so 1/100 sec for ISO 100. In practice, with DSLRs and under many seemingly sunny conditions, a bit more exposure is needed, but that will get you close with a workable image. In the field, use the HistoBlinkyMeter with EttR (Expose to the Right), or if that is a mystery then simply center the meter needle in Manual mode.
To get to one (1) second in bright sunlight, you can start by going to f/22 and sacrifice a touch of sharpness. But let's assume you want to be at f/11 for maximum sharpness (though not maximum depth of field).
That would 1/200 f/11. Let's use 1/250 for ease of illustration. Stepping down: 1/125, 1/60, 1/30, 1/15, 1/8 1/4, 1/2, 1 second. That's 8 stops.
So with a 1.8 / 64x / 6-stop ND, your exposure would be 1/4 sec f/11 in brilliant sunshine or 1 second f/22.
With a 3.0 / 1000x / 10-stop ND, your exposure would be 4 sec f/11.
Here is a crop of an exposure test, no filter, ISO 200, 1/250, f/8:
And here is a photograph I made about ten minutes later at ISO 100, 1/2 sec, f/9 with a 1.8 / 64x / 6-stop ND:
For 20 minute exposures that turn seashores into misty water, try a 3.0 / 1000x / 10-stop ND with dimmer conditions.
(Post edited to correct 1.6 filter to 1.8 filter.)
Edited on Jan 12, 2013 at 04:43 PM · View previous versions