Magnification

Hello people! Let me show you difference between 150mm, 520mm and 1300mm in focal length. So as you can see first image was taken with 1300mm, that means, if you are using tripod, which you have to, you will lose your subject in matter of seconds because of Earths rotation and stuff. :D Also you will need to increase your shutter speed to get sharper image, you can't use long exposure for this one, Moon is moving pretty fast from your camera view point. For this photo I used 5x teleconverter which means I will lose light that goes to sensor and also sharpness which you can see it's not that great.

Camera lenses are not made for attaching another lens to it, they are made for that focal length that you get, every modification means less sharpness and less light. It's same principal as using macro tubes, you will lose light and also focal distance but you will get much closer.