Robert, you'll have different options.
One could be to move the whole library of Photos/iPhoto to an external HDD. That would ensure, that your pictures would be kept together. But that would mean you have to connect that external HDD always when you'll want to work with any single picture you have.
Another option would be to change the location where Photos stores your files. This is described herein:
https://help.apple.com/photos/mac/1.0/?lang=en#/pht1ed9b966d
This matches your initial question of storing new downloads from your camera in another location as long as you copy the pictures directly from the memory card to the external HDD and import links to them in Photos. You'll have to steps to do: 1st: Copy the pics from the memory card to the external HDD, 2nd: import links to them in Photos. And you have to connect the external HDD to your MacBook, when you want to work with the pictures. The advantage would be, that the pictures are available within the Photos app. The disadvantage would be, that you end up with pictures stored in your Photos library and other stored in links on an external HDD.
I would prefer the first option, 'cause it keeps the files together in one single structure.
Regards, Lars