ND filter for 2-5 seconds?

seth

seth

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So, the shots in this thread about 9 or 10 stop ND filters are really inspiring (as are the welding glass ones) but most seem to be aiming for super-long 30odd second type exposures.

I'd quite like to have a go but would be aiming for say around 2 to 5 second exposures to get some people movement in a busy daylight environment. Can anyone advise on the strength/type of filter I'd need to do that?
 
It obviously depends upon how strong the light is but to get anything like 5 seconds on a nice sunny day I had to stack pretty much every ND filter I had and add a poloriser. A day with less sun would be less of a problem.
 
I'll try my hand at the math!

If you take a bright day (assuming EV 15), the "Sunny 16" rule, you get 1/125s exposure at f/16 as your baseline.

Now each stop of ND filter doubles your exposure time. From 1/125 -> 2s you need a 250x longer exposure. 2^8 = 256 so a 8-stop ND filter should give you 2 seconds. 9 stops for 4 seconds and 10 stops for 8s.

I hope I got it right :gag:
 
Vaizki got the maths right - thats pretty much spot on. So you can work it in reverse as well - if you've got a 10 stop filter and you want a 2 second exposure, but your metering reads 1/125th @ f16 on 100 iso, you can shoot with the 10 stop for 2 seconds (8 stops time) , and change the ISO to 400 for the 2 extra stops.

personally for a 2-5 second exposure on my pinhole camera, if it's nice and sunny, I use a 3 stop filter and 50 iso film - or maybe in B&W a 2 stop and a red filter.
 
Ah, thanks for the maths :) I had an inkling that a cheap ND8 (to try the technique out) wasn't going to cut it. ;) Maybe a low number welding glass would be best for playing with.
 
ND8 is a 3 stopper, fine for my f135 pinhole camera, okay for blurring waterfalls etc, but not going to flatted the sea and blur out skies :shrug:

As for welding glass - a #10 welding glass came out as a 12-13 stopper (I tried 3 and they're not completely consistent :lol:)
 
Hmm, I thought I had a spare welding glass somewhere but can't see it in the shed... I do have one but it is covered in weld splatter :naughty:

I suppose a couple of ND8s stacked might at least enable me to get some blur.
 
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