ND4 / ND8 Question.

[WelshTony]

I’m planning to take some shots of a local waterfall using the bulb setting with a tripod mounted 40D.

Having read a number of magazine articles on the use of ND filters for such shots I’m still not clear what the difference would be between using ND4 or ND8.

If I use f22 will the difference just be the length of exposure I attempt to use?
What would be the difference between the two shots if I took one 5sec shot using ND4 and one 5sec shot using ND8?

Any advice appreciated.

 

[Flash In The Pan]

An ND4 will result in a 2 stop reduction whereas an ND8 results in a 3 stop one

 

[will69]

If I use f22 will the difference just be the length of exposure I attempt to use?
What would be the difference between the two shots if I took one 5sec shot using ND4 and one 5sec shot using ND8?

When I do this kind of water shot, it is the effect on shutter speed I am interested in, so decide what aperture you want - then add ND filters to deliver the shutter speed.

Usually a shutter speed slower than around 1/3 sec will give a blending effect on water.

So if f22 (for example) is giving a shutter speed of 1/8sec for proper exposure, adding a ND4 filter for the same aperture will reduce the shutter speed to 0"5 (or half a second).

Using an ND8 will add a further stop, reducing the shutter to 1 sec. You can also stack your ND filters if you nee to so that an ND4+ND8=5 stops. But the more filters you put in front of the lens the poorer (potentially) the image could be.