All digtal camera quote an ISO range which measures the sensitivity of the sensor. It’s measured just like traditional film where the higher the ISO, the more sensitive it is. What this means is if light is low, a higher ISO (more sensitive sensor) will mean you can keep on shooting using the same shutter speed and f/stop. The downside is that the higher the ISO, the more noisy the picture becomes as stray light gets picked up by the sensor. It’s usually a small price to pay if the alternative is no picture at all
Most consumer digital cameras have an ISO range of ISO 100 to 400 with the noise levels being noticeable but acceptable at the higher ISO. A didital SLR usually has a much highter ISO range, and much better noise handling too.
My Canon 300D has an official ISO range of ISO 100 to 1600, but add in the popular firmware hack and the ISO goes to 3200.
Here’s a little secret for you though. It’s possible to extend the ISO range way beyond even that, all the way to ISO 25 to 12,800, and it’s a trick that can be donw with pretty much every digicam out there, extending your ISO range too! Yes, it works for Nikon DSLRs as well.
This is a shot of flowers in our garden taken at ISO 100:
Here’s another shot, but this time I’ve over-exposed it by +2 stops, still at ISO 100. Then on the computer, I’ve brought the levels back down to normality. This gives an effective ISO of 25!:
At ISO 25 the shot is very, very noise free. It’s a perfect setting for studio work where the slower shutter speeds aren’t a problem and quality is everything.
On the other end of the scale, here’s a shot at ISO 3200:
And here’s the last one, at ISO 3200 but under-exposed to -2 stops. That’s taking the ISO all the way to 12,800 (!!!) when the image levels are adjusted:
The more I look at that last image the more I think it’s still too dark and would have taken a little more lightening up. Ah well.
At that level the shot is extremely grainy; in reality ISO 12,800 is only useable for very low-light work where the image is going to be used as a moody monochrome shot. In other words, it’s perfect for night photography!
There you have it. By manipulating the f-stop by +2/-2 then correcting one your computer, it’s possible to extend the ISO range of your camera. Even a consumer digicam could go from ISO 25 to 1600 using this technique, though watch out for the noise at the high end.