Farpoint Bahtinov Mask for 77mm filter thread

Bahtinov focusing masks allow ultra-precise focusing

You can see directly whether you are in focus or not. No guessing or interpreting difficult diffraction patterns necessary.

The mask fits to:

77mm Filter Thread

And that´s how it works:

Left picture: the center line does not go through the middle of the cross - out of focus
Middle picture: the center line passes the middle of the cross directly - perfect focus<
Right picture: the center line does not go through the middle of the cross - out of focus

While turning the focus wheel, you can see directly how the center line moves through the cross section.