When you're photographing subjects at night that you want to be sharp, raising the ISO is your best option. The price you will pay is some image noise, but usually that's a small price to pay for a sharp night photo.
If you want to experiment with wild motion effects, like this 10-second exposure of a Ferris Wheel, then using a low ISO speed and long shutter speed opens up a whole range of creative effects.
Basic Consideration: High ISO vs. Long Expoures?
Almost every time I take a night or low-light photograph I ask myself
the same question: Should I use a longer exposure time and keep
the ISO low to get better image quality, or should I raise the ISO so I
can use shorter exposure times.
One issue here is image noise
(the digital equivalent of that grain-like texture that you used to see
with high-speed films): You get more noise at higher ISO speeds. The
fact is that both higher ISO
settings and long exposure times both lead to increased image noise, so
at least as far as noise is concerned, it's really half a dozen of one
and six of the other. I really don't
mind noise in night photos, so I tend to give that issue minimal
consideration (there are also some very good noise-reduction software
programs available if you find that noise is affecting image quality).
Noise tends to be more obvious with smaller-sensor cameras with high
pixel counts--it's one of the prices you pay for having a lot of pixels
on a small sensor. After all, noise is just that: digital interference
from other nearby pixels: the closer they are and the more there are,
the more noise you're going to get. This is why professional cameras
often use full-frame CMOS sensors: larger pixels, better noise
cancellation.
Your real primary consideration should be subject motion. Since you
will be using a tripod a lot of the time (or at least image stabilization), camera
movement won't be an issue. Subject motion, however, is always a
consideration. If you want to freeze action as much as possible, then
shifting to a higher ISO will buy you a few more stops of shutter
speed. On the other hand, very often exaggerating the motion of
nightlights (traffic streaks, fireworks, carnival rides) is a desirable
thing and so using a slow ISO and a longer shutter speed is a good
working combination. So, for me, the larger questions are always: Is
there motion here and do how do I want to capture it?
There will be times when you're shooting without a tripod and you will
be forced to raise the IS0, so go ahead and raise it. I had to raise
the ISO to 1600 to photograph the famous Rose window in Notre Dame
because I was already using the fastest lens I had with me and
tripods aren't allowed, so I simply had no choice. The image isn't as
sharp as I would like, but in situations like this I always feel it's
better to raise the ISO and suffer some digital noise than to simply
stop shooting. Since I shot that photo, however, I've switched to a
Nikon D90 body and that has a top ISO of 3200, so I could have gained
yet another stop had I owned the D90 then. (And I have to say, that even
at ISO 3200, there is very modest noise with the D90.)
Regardless of whether you're increasing the ISO or using subject
motion (as opposed to you intentionally jiggling the camera--which is
perfectly acceptable if that's what you're after) as a part of the
image, I still highly recommend using a tripod at night. A tripod slows
you down a bit (in a good way) and gives you time to carefully consider
your compositions and, more importantly, it also opens
up the entire range of shutter speed and aperture combinations and no
anti-shake technology does that.
One good test, either with a
still or moving subject, is to put your camera on a tripod and then
manually set the ISO, doubling it for each successive exposure: 100,
200, 400, 800, 1600, etc. That way you can see the effects on both
subject motion (if there is any) because as you go to a higher speed
you'll get a higher shutter speed and also on image noise.
To get the wild motion effect shot of TImes Square in New York shown here, I used a long two-second exposure, handheld, and then intentionally jiggled the camera during the exposure.
I shot this photo of the New London, Connecticut harbor using a very long 28-second exposure (at f/13) because I wanted to use a low ISO of 200 and because I wanted the natural flow of the water in the harbor to create a smooth sheen. If you look closely at the image you can see a red line going across (just at the water line of the schooner) horizontally--that's the red running light of a power boat that went by during the exposure. The overall color shift of the image is due partly to the "cloudy day" setting of the white balance and partly to the vapor lamps lighting most of the scene. Still, by using a careful curves adjustment, I was able to keep whites white (the boat, the tents, etc.).
Entire Site Contents Copyright 2012 Jeff Wignall
An Ambitious Dogs Production In Association with Boo Boo & Mama Cat Films Please also visit my Photo Tip of the Day blog.
