Back Focus

[Grierson]

Having recently acquired an OSC and been disappointed with the focus of the resulting images (probably down to my own incompetence) I got to thinking about back focus. I have a SW flattener/reducer fitted to my SW ED80. FLO gives the back focus requirement of the flattener as 55mm. As there is nothing on the flattener to indicate where the 55mm should be measured from, i.e. where the lens is in the flattener I have always measured from the rear end of the of the body. I wonder if someone could confirm, or otherwise, that this is correct as there seems no easy way of measuring from the lens. Many thanks - John     

[wornish]

It's from the flat face of the flattener see image below.  The 55mm is the required distance from that face to the actual sensor in the camera.

 

[Grierson]

Thanks Dave, that is the measurement I am using. In the back of my mind I remember reading that the measurement should be from lens to sensor and the sensor in this flattened sits c. 10mm in from the face. Clearly I’m trying to add an nonexistent problem. Thanks for taking the trouble to provide an image.

John