300mm 6.8 Dagor/convertable???

Discussion in 'Large Format' started by emile_de_leon|9, Mar 4, 2003.

  1. Hi all,
    I was thinking of trying my older uncoated 300mm 6.8 Goerz Dagor
    as a convertable lens by removing the front cell. Does anyone know
    what the f stops will be with the front lens removed? Or the focal
    length? Has anyone tried this? What is the quality of the Dagor with
    the front removed? Is the coverage the same? I cant try this out
    till my new camera comes in as I dont have the bellows draw. Thanks
    for your input!
     
  2. The focal length of a single cell from a Dagor is 1 3/4 X the focal length of the combined lens. The aperture starts at F13 or F12.5 approx. Ansel shot 'Frozen Lake and Cliffs' with the rear cell of a Dagor. The single cells work very nicely, you do have to follow the usual rules about convertible single cells though.

    CP Goerz
     
  3. Don't forget to refocus after stopping down.
     
  4. Yes, Dagors work reasonably well like this when stopped down. I think the 12" comes out to about 18" with just the rear cell. You could measure it objectively and recalculate the f:stops from there. I have a 19" lens, so I tend not to use the Dagor for that focal length, though sometimes I use an 8-1/4" Dagor converted on my 4x5" camera because the 12" Dagor is too big to use on the 4x5" (it's front mounted on an Ilex #5 shutter).
     
  5. To repeat what others have already said, you can use a single cell of a Daogr and the resulting focal length will be about 1.75X of the combined elements, and a loss of about 1.5 stops, so in effect what you read as f/6.8 will be an effective f/12.5 or so.

    In practice if you expect to get acceptable sharpness from the single element for landscape work you will need to stop down the lens 3-4 stops from wide open. Stopping down will eliminate many aberrations but not some forms of chromatic aberration. This results in a different focal point for red, green and blue light, especially severe at the edges. To correct use a sharp cutting filter to reduce or eliminate the light from one or more of the colors so that what remains arrives on the film at the same focus. Green and orange or the best all-around filters for this correction but with a specific subject other colors may work better.

    Sandy King
     
  6. Thanks folks for the Dagor info!
    All the best!
     

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