It all depends on the circumstances!
First of all, the problem with long exposure is that there are neither cameras nor light meters made for this task.
In this post, I would like to cover the night-time aspect of long exposures. The content of which is also known as the 6-stop rule, converting second into minutes.
This technique involves at least a digital camera having ISO 6400.
Lets have a look at equivalent exposures shutter vs. ISO:
- 0: 1s - 6400
- 1: 2s - 3200
- 2: 4s - 1600
- 3: 8s - 800
- 4: 15s - 400
- 5: 30s - 200
- 6: 60s - 100
The 6-stop rule converts seconds into minutes when using ISO100 rather than ISO6400. Why is that important? Many Astro-photographer are using ISO100 film. Therefore, a quick estimation can be made with an ISO6400 digital camera, before spending hours of waisting film.