You may be looking for true mirror lock up (not just the thing people erroneously call that), in which case you would be looking in vain.
The 500-series cameras only offer mirror pre-release (the button below the wind crank).
Only the 2000-series bodies offer true mirror lock-up (you lock up the mirror - and it stays up, no matter what, - until you decide to let it come down again - meanwhile allowing normal operation of all camera functions). |