If you use Asynchronous transmission then a UART, either hardware of software is used.
Most people understand a 50foot limit but don't realized there is a capacitance limit of 2500pf. This means that cable selection in terms of capacitance per foot will be a limiting factor which can result in more or less than 50 feet. In other words 50ft is a general limit at 19.2.
(17 pf/ft Cat 5 will easily get you over 50 ft. when you follow the capacitance spec)
In addition to capacitance there is an inverse relationship between cable length and baud rate. You can extend the length to several thousand feet at 2400 baud or less. The 50ft spec is also at 19.2 so a faster speed is going to reduce this lenght significantly.
I assume that since you are using a camera that you will be transferring a lot of data at a fairly high high baud rate. This can complicate your baud selection relative to cable length in that you may not be able to slow the camera baud rate or want to wait for slow baud rate transmissions to complete.
You can extend the distance by utilizing a signal booster or conditioner.
Other options include using RS-485 which would require at least one RS232/RS485 converter at the camera and a RS485 line driver at the uC . Or use fiber optic, again converters at both ends will be required along with additional power requirements.
One final thought:
You will need to run your Atmega on a crystal clock source and not off the internal RC clock.