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Thread: Homemade 4 x 5 enlarger back

  1. #1

    Join Date
    Jul 2020
    Posts
    4

    Homemade 4 x 5 enlarger back

    Hello,

    I made a graflok 4x5" enlarger back. It consists of a black plastic box with white papers on it's inside walls, an about 8 x 10cm led grid and a diffuse glass for about 11cm from the grid. I insert the sheet in front of this glass, with about 5mm gap. The illumination is nearly perfect, with less than 1/3 stop light fall-off on the edges. I insert it to the back of the camera (ToyoView, with 300mm industar lens), and there is a CIRCULAR, major light fall off on the final print. There is no light fall-off on the negative, but I made with the same lens. The illumination is rectangular, not circular, like the fall-off on the print. What can be the problem?

    Many Thanks, Adam

  2. #2

    Join Date
    Jan 2019
    Posts
    316

    Re: Homemade 4 x 5 enlarger back

    It's hard to know without more diagrams or pictures of what you did, but:
    - your light source sounds undersized compared to 4x5" (approx 10x12 cm)
    - diffusion by a diffuse glass usually isn't perfect, that is the light emitted by the diffuse source will be stronger straight-on than at an angle
    - tracing the light path back from the lens to corners of the negative to diffuser will show that you are requiring a "bend" in the light rays at the diffuser, especially since the light source is smaller than the diffuser.

    The condenser in a condenser enlarger takes care of this by refracting the light from the bulb, but for a diffusion enlarger you probably need to do more engineering or overdesign it by making the light source and diffusion panel larger than the negative.

    Also, paper is likely to show up a gradient in illumination that your eye doesn't see well.

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