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  1. #31

    Join Date
    May 2008
    Shooter
    Medium Format
    Posts
    60
    You can check infinity focus by aiming at an infinity target, setting the lens at infinity and checking focus at the film plane. A bit of Scotch frosted sticky tape stretched across the rollers makes a good temporary 'ground glass'.
    I do not agree that this way is working (in case of medium or small format cameras) ; when I have to check the focusing, I do it systematically by test shoots with real film (an old wife practice, I guess).

    For two reasons :

    1/ usually, accurate focusing means at least 0.03mm detail resolution on the neg (maybe even smaller on small format). So, are you sure that you can see this - naked eyed - on a makeshift groundglass ?

    2/ when you test for focus accuracy, you should do it lens full open ; let's say, for a MF camera it means f/stop 3.5 . To reach focusing details of less than 0.03mm, you are at this f/stop not allowed a focus plane misplacement error of more than 0.1mm : are you sure that your "bit of Scotch frosted sticky tape stretched across the rollers" is positionned at the same place as a film with an accuracy better than 0.1mm ?

    Bye Paul

  2. #32

    Join Date
    Sep 2004
    Location
    New Jersey (again)
    Shooter
    35mm RF
    Posts
    1,932
    I would first check to see if infinity focus is correct. Then check the calibration of the rangefinder. The images in the rangefinder should align when the lens is at infinity and when the camera is pointed to a distant object. Some people use the moon, but I could never determine if this was correct. I usually took it to work and focused on a building across town.

    I think that the sweet spot for the Tessar design generally has been from f/8 to f/11, possibly f/16 for a medium format camera.

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