If a camera needs 7.9v, can I use a 7.5v adapter or a 9v adapter?

7.5V would be a safer bet than 9V. Given you don't know if this was a regulated or unregulated supply, make sure that you get a regulated 7.5V supply, or it will probably be over 10V off-load, which might damage the camera.

D1 D2 ----- +12 --->|--->|---+---in|LM317|out---+------+-----> + 7.9 | ----- | | + [.1uf] Adj [240R] [10uF] | | | | | +---------+ | | | | | [1276.8R] | | | | Gnd -------------+--------+----------------+-----> Gnd
The above circuit will provide ~7.9 volts at up to 1.5 amps if you put the LM317 on a good heatsink, but I would recommend adding a pass transistor if the camera needa more than 1 amp. You can use a 1200 ohm and a 75 ohm in series for the 1276.8R resistance, and the output voltage will be 7.89, which will be fine.
Ed