Bahtinov focus mask for camera lenses with 77mm filter threads. Snaps into a UV or Skylight filter with 77mm threads. FILTER NOT INCLUDED, MUST BE PURCHASED SEPARATELY. The Bahtinov focus technique works best with longer focal lengths. It gets less useful with shorter focal lengths. The practical useful limit is around 100mm focal length or longer.
In 2008, Pavel Bahtinov, a Russian amateur telescope maker and astronomer, published an ingenious new way to easily focus your DSLR camera, webcam or CCD camera for astrophotography. The method is simple, very intuitive, and the device has universally been named after the inventor: The Bahtinov Mask. Farpoint manufactures this "lifesaving" astrophotography tool out of virtually indestructible ABS plastic. Available in many different sizes and types to fit all your imaging needs.