ND filters

[thanos_tj's]

Can I ask something,what's the difference between an ND 2,4,8,16 filter and an ND 400 ? I found an adjustable filter ND2 to ND400.What does this mean ?

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[locutus]

ND filters have a 'number system' that is a bit unneeded....

ND2 -> 1 stop
....
ND16 -> 4 stop
ND400 -> 6 2/3rds stop

Variable ND's are essentially 2 polarising filters stacked together where the rotation angle between the 2 sets the amount of light coming through. These have the disadvantage that they can have stronger colour casts.

[jk0592]

Sorry, but ND400->8 and 2/3s stop.

[xkaes]

The figures mentioned above are Filter Factors for the ND filters. You take the ISO of the film you are using and divide it by the Filter Factor to produce a new ISO to put into a hand-held meter. A TTL meter will adjust automatically -- within the capability of the TTL meter. But, much like Close-up/Supplementary filters/lenses, not all manufacturers use the same designation for the same filter. For ND filters, some manufacturers use a logarithmic designation, which is 0.3 for each decrease of 1 f-stop. So a 0.3ND is 1 f-stop decrease in light, a 0.6 is two stops, 0.9 is three, etc. If you get these confused, you are in trouble. A ND2X from a manuacturer using a Filter Factor designation means a 1 f-stop decrease in light, while a ND2 from a manuacturer using a Logarithmic designation means a 6.66 f-stop decrease in light -- a BIG difference.

I have two ND filters that use one scale and another that uses the other. I have 2, 3, and 4 neutral density filters. The 2 is a Filter Factor filter -- 1 f-stop change. The 3 is a Logarithmic filter -- 10 f-stop change. The 4 is a Filter Factor filter -- 2 f-stop change. So you can see why I need to be very careful!!!

[Christopher Barrett]

Weird, I've never seen ND filters that weren't on the Logarithmic scale, and that is confusing enough without having to remember that there is a whole other system out there!