nd filters

[shed301]

I have recently picked up 3 cokin P series graduated ND filters, ( 4,6,9 ), Now i was driving home from work today and a thought occurred to me.... they say you use a nd filter 2 stops back, 3 or even 1 stop back.... now my question is.... back from what?..... i mean what F stop are we stopping back from?. the lowest number on your lens or 2 stops back from the highest number on your lens?... starting to get slightly confused with the nd filters

cheers
Grant

[i speak in math]

From whatever exposure you want the scene to look like.

ND filter block out light. You can get one stop filters, two stop filters and 4 stop filters or even 10 stop. They make all kinds of them. They block light.

If your camera wants to expose at 1/500th, f/8 and ISO 100, then you slap a 10 stop filter on it, you are going to have to change around some settings to get a good exposure. So, to get back to that same exposure you had without the filter, you could go with 2 sec, f/8 ISO 100. Or 8 sec, f/16 ISO 100. Or whatever you want.

[OsmosisStudios]

ND filters affect exposure in that they block light. Your filter may claim to set you back 3 stops, and what that means is that if your meter normally reads something like 1/60, 100ISO, f/4 then you'll end up with something that is 3 stops lower than that, so say 1/8th second instead of 1/60th, or f/1.4 instead of f/4 to get the same exposure levels.

Its generally used to lower shutter speeds for waterscapes.

For instance, If I go to a scene to take photos, set the camera to f/11 and 100iso and it calculates a shutter speed of 1/15th second, but I wanted a longer shutter speed to get a waterfall to appear misty, or a river to appear raging, I could place a 3 stop ND filter to get my shutter speed down to 1/2s.

I have a 3-stop and a 10-stop ND filter: the 10-stop is extreme, for getting extremely long shutterspeeds or long shutterspeeds with shallow depth of field. If used in conjunction with the 3-stop, I would end up with 13-stops stopping power, meaning that the exposure I would get would be 13 stops DARKER with the same settings