If the consultation is done in person at the clinic (final consultation before your surgery / regular checkup at the dermatologist ) then do as @Gatsby said as doctor will use magnification to check scalp and part your hair and see which areas you are hiding with length.
But if its done by video calls and images you send to clinic (initial consultation) and if you choose to keep lengthy hair then :
1) part your hair.
2) use a good resolution camera.
3)use all types of lightning (indoor, outdoor). Harsh light condition photo is must.
4)Use different angles because with some angles the illusion of density changes.
5)Donor area try to keep short hair as if any donor thining then can be detected and overestimation of grafts is not done.
Or keep short hair and repeat steps 2,3,4. Good luck with your consultation.