First off, the title shows this is a 2x teleconverter, while the description states this is a 1.4x teleconverter. This is a 2x converter. Secondly, I'm using the teleconverter with the Sony G Master FE 100-400mm which results in 200-800mm focal length. Camera Body: Sony A7ii
So how much light is lost? My camera shows F9 at 200mm and F11 at 800mm.
I first tried shooting subjects that were very far away to test the limits of the teleconverter. It's quite a challenge to keep the camera steady and get focused, but it's not impossible. Don't try to photograph tiny birds like swallows that are 30 feet or more away. You'd be asking for too much.
It's much better to find a subject that is well within reach of your actual focal length or just out of reach. That's where the teleconverter really shines. There's no huge penalty for shooting wide open at 800mm if your subject is in the right range. You will get amazing closeups and if you're into macros you will get excellent detail without having to get very close to the subject. If you're photographing wildlife, this is great since you'll be able to get closer shots without getting physically closer and potentially disturbing your subject.
I didn't notice any major loss of quality in images. Some images came out soft but this is more of a focus issue. You will find it difficult to lock in focus at times. Overall my images were beyond acceptable. When loading up photographs for editing I don't need to do a major amount of post processing in order to make the shots look good. I'm editing just as I would edit any photo without a converter. You won't be spending a lot of time cleaning up quality loss.
It's a little pricey, but I'm definitely satisfied. If you have the right expectations you will enjoy using it.