using ND filters


Link Posted 15/10/2012 - 09:58
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.



Link Posted 15/10/2012 - 10:15
No problem...

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


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

(For gallery, tips and links)
Last Edited by MattMatic on 15/10/2012 - 10:17


Link Posted 15/10/2012 - 11:12
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.


Link Posted 15/10/2012 - 11:38
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.


Link Posted 15/10/2012 - 12:49
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


Link Posted 15/10/2012 - 13:15
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



Link Posted 15/10/2012 - 14:53
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

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