Met a lady at the weekend who asked why a picture was quite noisy. After a couple of questions realised that she wasn’t familiar with the ‘1 over rule’ in photography. The particular picture was taken before the sun had risen at a long focal length (possibly 300mm or more), therefore chose a high shutter speed with an f-stop of 3.5 to f4. Obviously this will push the ISO to a high value to get a good exposure, therefore introduced some colour noise into the picture.
Modern day cameras have amazing high ISO values with minimal colour noise, but if we are not careful can still cause some colour noise in the picture. There are great tips on getting the ISO down to something reasonable in awkward shooting situations (especially low light).
The ‘1 over rule’ is one of them, it suggests, that the slowest shutter speed that you can get a sharp image from your camera whilst hand holding it (i.e. no tripod), is to not go lower than 1/the focal length of the lens. I.e. if you have a 35mm lens, then your slowest shutter speed hand held is 1/35 second, if using a 300mm lens, then it will be 1/300 second. But what if you have a telephoto zoom lens? Well it will be the focal length you are using at the time, i.e. on a 70-300mm zoom lens, it will be 1/70 at 70mm and 1/300 at 300mm. Of course it may be that you want the colour noise, or the blurriness in your pictures, so sometimes this rule of thumb won’t apply, depending on your creative vision.
Hope that helps you work out that exposure and give you a good guide for when you are hand holding the camera in low light.
Oh, and as caught by @sPECTre on Twitter, don’t forget to consider the crop factor of your cameras sensor. It may be a 0.6, 0.3 or something different (like a 4/3’s format), so please refer to your owners manual and multiple the 1/the focal length of the lens by the crop factor.