By this app, you can use your phone as incident light meter,and you can take a picture of the correct exposure.
This app can measure 'F number' , 'Shutter speed' or 'ISO sensitivity'.
Set these measurement values at your DSLR camera.
The accuracy of the measurement value depends on illuminance sensor on your smartphone.
This app has a function to correct the value of the illuminance sensor.
This app is for free.