Originally posted by Shadus
The above photo 'blurness' is due to handshake caused by low shutter speed. You're right, it is due to shutter speed.
However, ISO & shutter speed have no correlation.
Shutter speed is how long the camera shutter opens to let in light to fall on the film.
ISO is a measure of how sensitive the film is to light. The higher the number, the more sensitive it is. Hence, for the same brightness in a particular scene, for eg, if a ISO 100 film needs 1/125sec shutter speed to properly form the image, a ISO 400 film will only need 1/500sec shutter speed to properly form the same image.
What is handshake?
Its when the shutter speed falls below 1/focal length of your lens. For eg, if you're using 50mm. Once the shutter speed is slower than 1/50, it is more likely to see handshake in the end photo. For practical reason, just try to keep shutter speed at 1/60 and above for most of your shots. If you're using long teles like 200mm and above, then use the 1/focal length rule.