Longer exposures

I want to start shooting longer exposures anywhere between 5 seconds and a few minutes. My most basic question would be is it as simple as doubling the time between f stops when I throw on say a 5 stop ND. For example I'm shooting f16 at 1/2 second. Would throwing on a 5 stop ND take me to about 16 seconds. If I'm off here let me know. I have a 3 stop ND now but havent been successful using it on landscapes in the last few attempts. Thanks

Counting on my fingers to do the math, it's five stops from 1/2 second to 16 seconds with the same aperture, so you should be pretty close. I'd still bracket your shots, though.

Keep in mind that with most digital cameras you start to run into image quality problems starting with a 2 minute exposure.


Kent in SD

There is a simple formula for determining image exposure time.

Take a test exposure at your highest possible iso value and then do:
"Test ISO divided by Final ISO times
your original shutter speed equals final shutter speed"

So an example: assume aperture f/11 (can be everything really)
test exposure at iso 3200 is 30 seconds and i want my final
shot to be at iso 100 then this makes

iso3200 / iso100 = 32
32 x 30sec = 960sec = 16minutes

Hope that made sense
Wim.

I have a 3 stop ND now but havent been successful using it on landscapes in the last few attempts.

Just to make sure....do you have an actual 3 stop ND filter, or one that's labelled 0.3 or 3x?
There's quite a difference, as noted in this guide.

Well upon checking closer by testing it on the camera I have a 1 stop, 11/2 stops, and a 2 stop NDs. Guess I'll be stacking. That might be why Im getting erroneous exposures on a few landscape shots being off a full stop. Using the NDs more often and correctly will have me hooked on them no doubt.