using ND filters

I want to use an ND filter to take shots of running water
ie: streams the sea etc to blur the water.
my question is, is there a formula or rule of thumb to
work out the exsposure time to achive this, the filters I
have are ND4's.
I know its probably the same for film or digital but just
to let you know I'm using a film slr.

Christopher

No problem...

ND4 is 2Ev (two stops)
ND2 is 1Ev (one stop)

See: http://www.darktable.org/usermanual/ch03s04s12.html.php

If you have TTL metering, then the meter should be able to 'just work' - assuming that when the ND is added the exposure is not out of the exposure range of the meter

Matt

ok I understand what your saying but does that render a sharp picture at the correct exposure without bluring the water or does the water blur naturaly because of the long exposure or do I have to add more exposure time to blur the water and will that cause it be over exposed .
in which case do I need a ND filter with higher density.

just trying to understand how it works.

It is the long exposure time that blurs the water, adding the ND filter reduces the amount of light getting to the film/sensor and allows you to increase the exposure time. You may aslo need to opt for a narrow apperture setting to avoid over exposure.

At the sort of exposure lenghts to achieve this it is likley that you require a triod to get a sharp shot.

In the picture below i was able to get the affect by reducing the apperture without an ND filter (did not have one) as there was not much light.

thats all quite clear thank you
if the exposure time is longer that the 30's that most camera's are able to meter to how do I calculate what exposure time I need withou over exposure or is it unlikely I need longer than 30's

This simple table may be of help

Shutter Speed 1/10 sec to 30 sec




Shutter Speed 1/13 sec to 1/4000 sec




Not sure what the effect will be with camera showing 30 sec shutter speed, add 10 stop filter press shutter and wait 8.5 hours

thank you that's exactly what I needed I have saved the files and will print them of and laminate them and keep them in me camera back

thank you tyronet2000 and eveyone else who replied

Christopher.