When shooting EOS Movie footage with the EOS DSLRs that offer EOS Movie capabilities (the EOS-1D Mark IV, EOS 5D Mark II, EOS 7D, and EOS 550D), the accepted rule of thumb is to use a shutter speed that is twice the selected frame rate. For example, if you were shooting at 25fps, your shutter speed would be 1/50sec. However, you will notice there is an issue if you choose 24fps. Since there is no 1/48sec shutter speed setting, 1/50sec is used instead. But if you switch the camera into half-stop exposure increments using the Custom Functions, you will find access to a 1/45sec shutter speed setting. This isn’t necessarily any closer than 1/50sec but it can prove useful in situations where you need to use a slightly slower shutter speed to capture more light to achieve the perfect exposure.