I have a Sony a100 camera body that has served me well. I just bought a vivitar 500mm telephoto lens with a 2x adapter. The lens has an adapter that connects it to my camera. The fit for the camera is perfect. No movement at all. The problem I am having is that the camera won't recognize the lens. It displays a message when I try to shoot that says "no lens attached". I have been searching in the camera to trouble shoot this. Please help, any advice would be appreciated.

Assuming your Vivitar lens is manual focus only and lacks any digital components, you would need to change a setting in your "Custom Menu" so that the shutter will operate.

 

According to page 96 (Shutter lock) of your user guide: "You can set the shutter lock to keep the shutter from releasing when no lens is mounted on the camera. When attaching the camera to an astronomical telescope, for example, select (Off; no lens) so the shutter can be released."

 

Hope this helps.

There used to be m42 to A-mount chipped adapters that would allow the a100 to recognize a lens as being mounted. i used one for a while and it did great.

it reads as 50mm in exif data but exposure is something you would have to adjust anyway with it being a manual lens.

https://www.amazon.com/Camera-Adapter-Confirm-Electronic-Minolta/dp/B007M82DN0