How long for a long exposure?

[thegman]

Hi, I have a question about length of long exposure time. I know how to calculate exposure time using an ND filter. Recently I have been looking at a lot of shots of aurora borialis and sometimes in the details it will say 324 sec at .......... etc etc. How do you calculate such a long exposure time?

[Manfred M]

Trial and error with guidance from others on the internet.

The longer the exposure, the more tolerant it is to being off a bit. If the ideal exposure is the 324 sec and you are one full stop under in your estimate, you are going for a 162 sec exposure. One full stop over would be 648 sec. With long exposures, close is often "good enough".

[Polar01]

Gerry as Manfred states it is a matter of trial and error, and also depends on what is being shot. It is was slow flowing waters than it would be longer than for fast flowing water. Say is was clouds I would time movement with my watch to see how far they have travelled to get a idea of the time I wanted. When doing long exposures anything over a minute in length I do not worry about exact time, if I overexpose by a minute so what that is only once stop no problem recovering that in post. I find the longer the time the less I worry about being exact on overexposure now underexposure I do worry about, so I always do extra time. Now on shorter times say 1/8" and I let it run over to 1 sec that is 3 stops over, so more care on short times less care on longer times.
Having a problem getting my thoughts in order today. (bad fingers)

Cheers: Allan

[William W]

Associated Information:

With Film, an additional time was added to account for Reciprocity Failure when making 'long' exposures: typically with most Film Emulsions, exposures longer than 1 second, needed to account for the Law of Reciprocity failure.

With Digital Sensors, there may be (usually is) an heat build up with long exposures, which in turn has an effect on (more) noise being generated. This point is important for astro-photography, when, for example, the aim is to make STAR TRAILS as would be the aim described in the OP.

When making Star Trails, there is generally a compromise between the Shutter Speed and the amount of acceptable noise which will be generated; because, obviously, very long Shutter Speeds, like 324seconds to make nice long star-trail would allow for very low ISO's to be selected.

Obviously one motive for selecting a low ISO is to reduce noise; but that Shutter Speed needs to be balanced with the amount of sensor heat that is generated and the subsequent extra noise.

And the salient point is, for any 'correct exposure' for one stop better ISO, the Shutter Speed has to doubled: and at very long Shutter Speeds, doubling that Shutter Speed can mean a lot more heat which is generated.

WW

[jcuknz]

I was used to counting "One hundred and one, one hundred and two" etc from darkroom and other days ... some count one chimpansee two chimp etc and when I get to twenty I mark it with one finger then two etc then on the other hand and then back to the first hand etc ... ... you need to consider the relative difference between different totals which unless one is twice the other the difference is not very marked.

[thegman]

Thanks guys for all your informative input, as usual. From above I take it that it is more ''trial and error'' which I'm happy with. The question was a general question as I've often wondered how people 'calculated' such excessive exposure times.

Cheers.