If you make a radial light and go into the light lab.
1 Put a blob in flare.
2 Use image and select the this golden fellow.
3 You will find that it takes your image and cuts a circle out of it and adheres to transparency mask. Also you will find that the distance the light source is away from the camera does not affect the size of the image generated. In this example the light source on the right is significantly further away from the one on the left.