9-1/2" Dagor Focus Shift?

Discussion in 'Large Format' started by john_mcdonald, Jul 8, 2002.

  1. Familiar with 4x5, I am new to 8x10, and finding it challenging to
    get some negatives fully in focus while retaining an f-stop of
    around 22, this in the best of lighting conditions as far as
    focusing is concerned, full sun with a contrasty subject. I am
    using a 9-1/2" Dagor (779160), and have been trying to get an
    understanding of its focus shift, which I have heard about. After
    producing some negatives that could use some improvement in
    terms of depth of field, I went out in full sun and focused on a
    multi-plane subject (shed in a yard with objects in the horizon)
    where movements would not help, and while viewing through a
    4x loupe, made sure 1/3 in was sharp, then stopped down. As I
    stopped down, more and more came into focus on the screen,
    but focus also became harder to ascertain visually as the screen
    gets quite dark with the smaller apertures, and screen grain
    becomes more apparent.

    How does one determine in what manner the plane of focus is
    shifting as more comes into focus with stopping down? Are
    there some helpful focusing guides with this type of lens?

    Any helpful comments appreciated.
     
  2. A little nose oil or vaseline on the GG will brighten up a spot to check focus when stopped way down. I've personally not felt that there is enough focus shift with Dagors to require refocusing, not even when a single element is used.
     
  3. There are two issues here. The first is the ability to check focus and DOF as you stop down and the second is focus shift.

    DOF first. Yes, as you stop down determining focus is more difficult because the GG becomes dim, but also because DOF is increasing i.e., there isn't the usual 'snap into focus' effect you get at wide apertures (which have a narrow DOF). Bill's suggestion of a little vaseline does help to brighten the screen (more expensive solutions include Bosscreens, fresnels etc). Use a loupe to check the focus of different areas of the screen. Don't use an overly powerful loupe because the texture of the GG will also be magnified. Strictly, you should pick a loupe magnification based on our enlargement. So if you're making 16x20's from a 4x5, use a 4X loupe. A good darkcloth does facilitate the ability to view the image as you stop down. Opinions vary about the most useful. The BTZS darkcloth is often suggested as a very good design. I find a thick T shirt to be the most convenient.

    The focus shift issue. Focus shift is due to uncorrected zonal spherical aberration. When used wide open, light rays from the periphery of the lens as well as from the centre contribute to the image, but uncorrected zonal spherical will mean that rays from different parts of the lens are brought to focus at slightly different points. What this means is that there is a sort of range of best focussing points. As you stop down, you are increasingly using light only from the center of the lens. So, in a sense, it is partly an optical illusion. The best focus point does not really change - it is just that our judgment of the best focus point changes. It also explains why some folks like Bill do not experience focus shift. If you judge focus primarily by using local contrast, you are likely to experience focus shift. One way to determine if you are susceptible to it and to get rough rules of thumb about compensation would be to focus on something really bright like a light bulb. Now as you stop down, the bright bulb still remains very bright and serves as a high contrast target to snap focus into. Try this exercise with the light bulb switched off (and focus wide open and refocus stopped down, using a pencil to mark the focus points on the monorail or bed). Then repeat with the bulb switched on. In the field, it might be helpful to carry a powerful flashlight etc which can serve as a high contrast target to place in the scene - remembr to remove it before releasing the shutter. However, I'm with Bill on this - careful time under the darkcloth is worth a good deal more than all of these tools. At the end of the day, it is important to realize that best focus is a human judgment.

    Cheers, DJ
     

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