dark center on 16x20 from 6x7 neg 23c

Discussion in 'Enlarging' started by sharris, Sep 10, 2012.

  1. sharris

    sharris Member

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    greetings....recently sourced some new trays and installed ventillation so I could print larger 16x20 prints. i am using a 75mm lens which i know is not optimal and was prepared for soft, under exposed corners on the maiden voyage. however, i'm noticing a burn close to center which I've diagnosed as as a result of the focus being so recessed on the enlarger bellows that it results in uneven light image/ orb. if i extend the bellows without the negative in place, i noticethelight become very even again. is that what you would expect? will a 105mm do the trick? or does my 23c have another issue? i am assuming longer focal length will mean backing up the column further but will it also mean extending the bellows for focus to eliminate the prob? thx, am interested in your experiences.
     
  2. Bob Carnie

    Bob Carnie Subscriber

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    You are seeing minus density on the corners , which is due to lens falloff.
    longer lens will help.. making sure the negative is completely centered to the bulb/light source and lens is also critical.
     
  3. Jim Jones

    Jim Jones Subscriber

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    Many, if not most, enlarging lenses yield a slight hot spot in the center of the image unless stopped down a little from maximum aperture. Also, on condenser enlargers, the condensers may have to be adjusted for the most even illumination from corner to corner.
     
  4. Ian C

    Ian C Member

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    The shortest focal length standard-design enlarging lens specifically intended for 6 x 7cm negatives is 80mm. The 75mm lenses are intended for 6 x 6cm negatives (and can be used for smaller negatives as well).

    When a 75mm lens is brought close enough to a 6 x 7cm negative to focus an image that covers a 16” x 20” print, the light will fall off a great deal at the outer limits and will begin to vignette the corners of the print. The greater the projection size, the closer you must position the lens to the negative. Due to the fixed projection angle of the lens, the larger the projection, the worse it will be.
     
  5. ic-racer

    ic-racer Member

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    Yes, you have observed what others fail to see; the condenser focuses light to a given distance, not a given lens focal length. So, big and small enlargements require slightly different condenser setup.

    However, your major problem (as pointed out above) is that the lens you have does not cover the 6x7 format for a big enlargement (thought it might cover 6x7 just fine for a small enlargement).
     
  6. sharris

    sharris Member

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    thanks again for input. i've spent a lot of time on this issue and want to offer an observation that may help others. the problem primarily stemmed from positioning the adjustable enlarger setting to the 'correct' position indicated for 2 1/4 x 3 1/4. if i backed off this setting prior to negative placement the uneven light distribution resolved sufficiently if not imperfectly to accommodate my 75mm lens until i purchase a new one. Of course moving the enlargement head farther up did result in the antipated fall off created by the smaller light circumference, but the hot spot and crescent distortion is greatly diminished. i just wanted to offer up the observation that for my particular set up the focal length of the enlarger lens being currently 75mm didnt strictly cause a problem. Your input was awesome so thanks, i've learned a ton about the enlargement process and relationships of the elements I didnt fully appreciate before