ABOVE: Mirror lockup is useful for astrophotography, where you’re using much longer focal lengths and want your stars to be pin-sharp.
There may also be other instances when using super telephoto lenses on a tripod, because longer focal lengths magnify the effect of any camera vibrations.
With most other types of photography this feature is not necessary as any vibration caused by movement of the mirror will be minimal compared to movement caused by handholding the camera. If you’re unsure, you can always perform your own tests and compare results with and without mirror lockup.
Vulnerable shutter speeds
The shutter speeds at which your images are most likely to suffer from mirror movement vibrations are between 1/100 and 1/4 second.
Shutter speeds faster than 1/100 second are too short to be affected by the vibrations. With exposure times of 1/4 second or longer, the vibrations – which fade away quite quickly – only affect the first part of the exposure. Generally speaking this isn't enough to reduce the overall sharpness.
Your camera model may also have an impact – older cameras are likely to be more prone to vibrations, whereas more advanced cameras have a smoother action. Again, if in doubt, do some tests.
Tripod choice
The sturdiness of your tripod will have an effect on whether mirror lockup is useful in your shooting situation. If your tripod is one of the lighterweight versions, or has a ball socket head which can be more prone to movement, then mirror lockup can be invaluable.