Keep poses the same on all comparison shots. Try one comparison shot with the sun hitting the front of your lens. THE NEGS See appropriate instructions and in-class demonstrations. After processing examine the negatives for exposure and contrast range. Note any irregularities and be sure to find out the reason for them.
Contact print the negatives.
It is easier to make compositioinal choices from positive than from negatives. It is better to make exposure choices from the negatives themselves. Bring them to the appointment. We can often think of ways to cut waste and your cost. A certain amount of waste will always happen.