Thanks!
CP seems straightforward enough. ND has way to many types and options going on, it's making my head spin, I'll have to get back to this after shabbos...
Nah, it's quite simple actually once you know what everything means.
There are two things to know which will enable you to sort everything out: type and strength.
There are three types:
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Solid ND is completely dark and even all across, and comes in a fixed strength.
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Vari ND (also called Fader) is like a solid one, but you could vary the strength.
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Graduated ND (also called ND Grad) is half ND and half clear. (These come in hard and soft transition versions, as we'll as 'sunset' ones, which only have a dark strip in the middle.)
Strength:
This is how strong the filter is, how much light it blocks. There are three ways to describe this strength, and this fact contributes most to the general confusion.
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Stops: This is the simplest way of measuring an ND filter. Each stop of filter halves the light coming into the lens. Hence a 1-stop filter will halve the light, doubling your shutter speed. A 2-stop will quarter your light, quadrupling your shutter speed. And so on.
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ND Factor: This is not based so much on logic as it is on math. Basically every stop is considered 0.3. Hence a 1-stop will be called a 0.3 ND, a 3-stop will be 0.9, and a 9-stop will be 2.7. Basically if you decide the ND factor by .3 you'd have your strength in stops.
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X rating: Again, not very sensical. In this scheme, 2x means 1-stop, 4x means 2-stop, and so on. But it's nearly impossible to use, since at one point it all goes to pieces. For example, a 9-stop, instead of being a 1024x, is actually called 400x. Thanks fully this meaning convention is very rarely seen without any of the others, so you should never have to figure stuff out just by this.
To further confuse things, sometimes you'd see the 0.3 scheme being referred to as the ND factor, and sometimes it'll be the 2x scheme.
So for example, if you have a 1-stop filter your shutter speed will change from 1/500 to 1/250. With a 3-stop, it'll go down to 1/60. With a 10-stop, you'd be all the way down to 2 seconds.
A solid ND could come in any size from 1 to 12 stops, a grad is usually only 1 to 4 stops, while faders allow you to change the strength, usually 2-8 or 2-10 stops.