Bahtinov focus mask for Sigma 150-500 zoom lens

This is a focusing aid used for astrophotography.

This particular one is made specifically for use with a particular zoom lens, the Sigma 150-500 APO DG OS used at full focal length.

The mask fits into the lens hood - it just slots in.

It helps you accurately focus your lens by mounting it in front of the lens and pointing the camera at a bright star. You then see a particular patern in the viewfinder or liveview and adjust the focus manually untill the pattern is symmetrical. On