Depending on the program you used. In Tv the shutter speed is set by you. In all others IIRC it's going down to get more light onto the sensor. I think you should read your manual before asking such questions, it's obviously that you need lower shutter speeds when shooting in bad light. Don't want to sound like a teacher, but that's one of the most basic things in photography and something everyone should know.
QantasA332 From Australia, joined Dec 2003, 1500 posts, RR: 35 Reply 4, posted (6 years 7 months 2 weeks 1 day 22 hours ago) and read 2555 times:
A few examples, if I may (click for larger versions)...
Those two were shot at ISO 200, 2.5 sec (Tv mode). I chose these settings carefully: it was quite windy, and I didn't want the fireworks to "blur" laterally too much. So, I knocked the ISO up to 200, allowing myself a somewhat faster shutter speed to get the right effect.
1.3 sec, also ISO 200. I didn't want the upper bursts to blur too much - laterally or otherwise - so that I would get a "sparkling" appearance.
Oly720man From United Kingdom, joined May 2004, 6225 posts, RR: 11 Reply 5, posted (6 years 7 months 2 weeks 1 day 22 hours ago) and read 2547 times:
I did some the other night (300D and Sigma 18-50DG) and was using 30s @F22, ISO100 to get fireworks in the garden.
As to shutter speeds, to get an exposure you need a certain amount of light and at night there's less of it, so you need a longer shutter speed. Have you never noticed that on cloudy days the shutter speed is slower than on sunny days? It's slower in the early morning or late afternoon than at midday?