Question on long/short exposure time

Started Aug 7, 2012 | Discussions thread
mnodonnell Contributing Member • Posts: 639
Re: In theory, yes. (nt)

I mean that in theory, assuming everything is optimal (including a sensor which does not produce noise from the heat of a long exposure) everything will be identical in the long and short exposures.

But that is only in theory. In practice that doesn't happen. Some reasons:

1. Longer exposures = more noise, even at base ISO. At 5 seconds you'd barely notice. At 5 minutes there would be little doubt. This is why most who shoot long exposures turn long exposure noise reduction on. It makes a big difference.

2. No filter is perfect. The most common issue is color casting. Longer exposures and denser filters lead to more color casting. For example, if you took a shot at 1/30 you wouldn't have any color issues (assuming your WB is correct). Take the same shot, same white balance, with a 10-stop ND filter (30 seconds) and your final shot will be tinted either blue or magenta (depends on the filter).

ND filters also have variability in terms of stopping power. A 3-stop ND filter may actually be 2.9 stops. Lee claims that their 10-stop Big Stopper can vary anywhere between 9 1/3 and 10 2/3 stops.

3. The intensity of light can very easily change over the course of your exposure, even when it seems perfectly even.

In terms of resolution I always assume that a long exposure will be softer. To answer your question theoretically any loss of resolution would not be attributable to the extended exposure time. It should be the same for dynamic range.
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