joer Loc: Colorado/Illinois

Sensor characteristics (i.e., sensitivity, signal to noise ratio, dynamic range, tonal range, color, etc.) all degrade as the ISO increases. There is a threshold where it becomes noticeable and varies according to your standards. This also varies with camera brands and sensor size; larger sensors are usually better.

Some of it is correctable in post processing.

BlackRipleyDog

Yeah, it's hard to hold it steady at those speeds. Experiment with a tripod.

WessoJPEG Loc: Cincinnati, Ohio

par4fore Loc: Bay Shore N.Y.

Not best but better but not at the cost of needed shutter speed of aperture settings.

Alpix Loc: Cambridgeshire, UK

As has been mentioned, it's the signal to noise ratio.
All things being equal, lower ISO will produce a more accurate image.

Think of each pixel as being a solar cell, converting light into electricity.
At low light levels (when we use a higher ISO), we are actually amplifying the signal (and noise).
So when the light (signal) is low, the noise is a bigger percentage of the final signal produced.

This is why putting more pixels into the same space of the cameras sensor, produces smaller 'solar panels', less signal and more noise.
There is a trade off between the amout of pixels vs noise in cameras, hence many expensive cameras are relatively low resolution (and noise).
Having less pixels in the same area ie bigger pixels (solar panels) helps produce a bigger voltage, and the noise is then very small in comparison.

There is a feature on some cameras to reduce noise on long exposures, by taking the shot, then comparing it to a 'blank' shot from the sensor, and removing the difference. This was an old technique used in radio too. Transmit 2 signals, receive it twice, and remove the difference to help reduce background noise.

mborn Loc: Massachusetts

What I use for ISO depends on what I am shooting. Landscapes Native 100-200 depending on the camera. Wildlife 640-1250 depending on the sky cover. Night time sky 1000-3200.

There are ways to decrease noise in PP and that is what I do. I want the image will worry about noise later

pecohen Loc: Central Maine

The higher the ISO the greater the noise will be, so I try to shoot with the lowest ISO that I can. Still, if the lighting is not adequate to get the shot you want then a bit of noise seems a better alternative than to miss the shot entirely.

bull drink water Loc: pontiac mi.

it's about the level of image quality you are after, and what iso you need to get the shot. back in the day we had asa or iso as low as 12, 32, and 64. most of us used 100,and 200 and 400 when it came on the market. when pushing came on the scene many of us went crazy. now we do it by pressing a button.

CatMarley Loc: North Carolina

This, like many other things you will read, has a certain amount of mythology baked in. The first digital cameras that came out had noisy sensors - electrical chatter induced by the amplification of the signal when the sensitivity was cranked up. A little of this mythology was carried over from film, where the slowest film, ASA100, was finer grained than the faster ASA films. The modern sensors have decreased the noise level significantly, so that noise is not a problem until you hit the higher ISO's and it varies with the sensor. But the purists will still tell you that unless you have the largest, latest, heaviest, most expensive and fastest, and use the lowest ISO, you will not get the "best" photos. Take it with a grain of salt!

rb61 Loc: Maple Grove, MN

I feel that this is a much better way to test, for most of your image making, than using a low light level scene.

pecohen Loc: Central Maine

Agreed. But even the latest cameras will show noise at higher ISO ratings and different cameras will behave differently. With my A6000 I find that ISO 800 works without very noticeable noise and even ISO 1600 is not too bad but with my previous camera ISO 800 was very noisy.

When I can, however, I keep the ISO at 400 or less.

WessoJPEG Loc: Cincinnati, Ohio

Don't worry about it crank up those ISOs.
Heres a photo in my dark basement ISO 16000 Nikon D7200.

(Download)

Greenmachine Loc: Chicago, IL

I agree with you completely: My rule of thumb is to use native ISO 100 for DX cameras (with smaller sensors) and ISO 200 for FX cameras as a starting
point, but increase it if you really need to! ---Greenmachine

camerapapi Loc: Miami, Fl.

It is a fact that cameras perform at their best at their base ISO. That ISO for many cameras is ISO 200. The sensor is simply optimized for that ISO in those cameras.
You can use other ISO settings of course but the best quality for your files will be the base ISO used by the camera in use. Noise is very low at base ISO but modern cameras are getting better and better when it comes to handling noise and there are excellent softwares that can control noise in post. In spite of all that base ISO is where you want to be.
This is the simplest explanation I can give you.

moonhawk Loc: Land of Enchantment

Not too bad, but your highlights are blown, and there's a lot of noise when you magnify the download. I wouldn't hang a print from that on my wall.

But, for showing on the internet, it's usable.