Why low ISO for night-time photography?

[zespri]

DP class #5 was on top of a hill overlooking the city. It was bitterly cold, I was wearing long-johns underneath my trousers, gloves, woolly hat, the works. Due to the cold we didn't stay for the entire duration of the class, and I suspect didn't get as much out of it as we otherwise would have.

One thing I did learn was to put my camera on iso 100 for this kind of shot. I accept that it has made the shot better, but I don't understand why. I thought that iso 100 & 200 are for daylight photos, and for a night-time photo I'd be working at the other end with perhaps 800.

Can anyone explain why I was instructed to use iso 100? I did ask him at the time, but due to language difficulties, wind blowing in our ears etc, I don't think I absorbed much of anything he said.

[candleman]

hhmmm.. Mt Eden.. good spot..

glad to see you're having a good time with your class.

Iso=sensor sensitivity.
so the longer exposure at a higher ISO will in effect multiply the noise in the shot. (thats how i understand it anyway)

IE the highly sensitive sensor is operating for longer and therefore more noise.
you can demonstrate this by trial and error.. shoot ISO800 in day and then get the same exposure at night (longer exposure time) and you'll probably notice more noise.
I tried something like this on top of Mt Wellington.. i'm pretty sure i got more noise.

i'm no expert.. but hey .. theres my 2c

[zespri]

Morning!

If it's just about the noise, then why don't we shoot every photo in iso100?

Originally Posted by candleman

so the longer exposure at a higher ISO will in effect multiply the noise in the shot. (thats how i understand it anyway)

[Nicole]

Originally Posted by zespri

Morning!

If it's just about the noise, then why don't we shoot every photo in iso100?

Well, I try to shoot every photo at as low of an ISO as I can

But I agree with what candleman said, it's all about the noise, especially if you start doing longer exposures. I mean, you were only doing 4/5 sec exposure on this one because of the lights, but once you start getting up into 30 sec and longer exposures, you'll likely get some noise even at the low ISOs, so put it up to 800 and it would be exceedingly grainy.

[MrJulius]

If you need a faster shutter, i.e. moving objects at night, higher iso is needed.

[Mr Guy]

Because not every single shot is shot on a tripod!

The answer to your question is here:
Exposure: 0.8 sec (4/5)


For a cityscape that you want crisp, noiseless results in, that's perfect. That'd be a heck of a blurry toddler, or firefly, and forget about hand holding that shot!


*Edit* PS Lovely shot!

[candleman]

i'm going to sound like an evolutionary scientist for a moment..

"I think the reason could possibly, probably, maybe, could be".. because in well lit (daylight conditions) the shutterspeed (exposure time) is so short the sensor doesnt have time to produce noise.

average daytime shutterspeed could be 1/100th second to 1/1000th second
whereas at night you're going for 10 seconds plus..

thats my (possibly wrong) understanding.. but from my own trial and error i always go for the lowest iso i can (on my camera its 64) when shooting night time.
i'll shutup now.. :|

[kirbinster]

Well how would you shoot a picture of a squirrel in the woods without much light? Even if you have a fast lens you need to be able to let enough light in while at the same time shooting fast enough to not get motion blur as the squirrel moves. The only way to do this in many cases is to boost the ISO up. Here is an example of that squirrel shot at ISO 800:



Camera: Nikon D80
Exposure: 0.01 sec (1/100)
Aperture: f/5.6
Focal Length: 300 mm

[jdepould]

Noise isn't necessarily a result of a long exposure. A short, high ISO exposure can produce noisy results because the sensor may not be getting enough information (light) to accurately render the photo. There's some interference inherent in any electronic signal, and when you amp up the sensitivity, you're amplifying the noise that's already there.

[zespri]

All very interesting replies thanks guys, I think I learned a little bit from each.

I have always left ISO to Auto... I'm thinking that I should now be paying more attention to it.

If I'm to understand this correctly: when deciding what combination of ISO/aperture/shutter speed to use, I should try and get the lowest ISO I can at all times, unless there is a reason to sacrifice it in order to get faster shutter speed (such as kirbinster's squirrel).

So from here, would I set my iso at 100 when taking any new shots, and tweak it from there if I have to? I mean to say, is 100 the best place to start in most circumstances?