On Jul 29, 2017, at 1:30 AM, James Saklad jimdoc@icloud.com [iPad] <iPad@yahoogroups.com> wrote:
>>> I really like one that are about 20-30 seconds long (which need a better camera, a tripod, and a neutral-density filter).
>>
>> Why a neutral-density filter?
>
> Let's say a correct exposure for the image (say, in moderate shade) would be 1/60 second at f/2.8 at your preferred ISO level.
>
> If instead you want to use a 30 second exposure, which is 1800 times longer, you will need to close the lens down.
> 1800 times less light through the lens would correspond to about 11 f-stops, which would be f/128.
>
> My camera doesn't do f/128 (and damned few do — mainly giant view cameras).
> Even if I lower the ISO value to the minimum, I couldn't close the lens down enough to get the proper exposure for a 30-second shutter.
>
> Neutral-density filters can reduce the light passing through the lens by anywhere from 1 to 16 f-stops.
>
> They can also let you use a larger aperture (with shallower depth of field) if that is appropriate for your subject.
Thanks. Hadn't thought of that. How would you do the calculation if the metering isn't done through the lens?
I guess you could also do a burst of exposures and superimpose the waterfall in, say, Photoshop, but that sounds painful.
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