Another Shen-Hao question for setting front standard and lens.

Discussion in 'Large Format Cameras and Accessories' started by kjsphoto, Jul 10, 2004.

  1. kjsphoto

    kjsphoto Subscriber

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    OK,

    Now that I have been using the camera (man I love LF!) I want to make sure I am setting up right. Let say I am using a 90 lens which is a 3" lens I set the front standard to 3 inches.

    When I set the front the camera front rail has not been extended and is flush with the front end. IF I use a 240 lens I set it to 9 inches.

    Then I start to focus. Is this correct when first setting up the lens? I want to do it right so I know where infinity is. That way if I work close up and if I pass the infinity mark I am now 1:1 arena. I also was reading that to get the bellow factor subtract the lens (240 = 9 inches) from the mark say 12 inches.

    The difference between 9 and 12 would be roughly 1 stop. Is this correct?

    Thanks,

    Kev
     
  2. Mongo

    Mongo Member

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    1" = 25.4mm, so a 90mm lens is really a 3.5" lens. This will be important when you calculate the bellows factor (see below).

    Where you start is really a matter of personal preference, but this is the method I use with my Shen-Hao. Just set the front standard near the infinity focus for the lens and go from there.

    To figure out the how much more exposure you'll need, use the formula:

    (extension/focal length)**2 (Divide the amount you've extended your bellows by the focal lenght of the lens, then square the result.)

    In the case of a 9" lens set to 12", this gives you 1.7777. Thus you have a little more than 2/3 of a stop less light, and should compensate by opening up the aperture or extending the exposure appropriately.
     
  3. Mongo

    Mongo Member

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    A small correction to this last paragraph. It should read:

    Thus you have a little more than 2/3 bellows compensation factor, and should compensate by opening up the aperture or extending the exposure appropriately. The bellows compensation factors of 1, 2, 4, 8, etc. correspond to 0, 1, 2, 3, etc. stops. In your case you'd need to add about 1/3 stop to your exposure. (If you want to be exacting, you can take your bellows factor and plug it into the formula (log(Factor)/log(2)) to find the exact change in stops.)

    I apologize for having pulled the trigger too soon on the previous message. I'm a bit sleep-deprived at the moment.
     
  4. kjsphoto

    kjsphoto Subscriber

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    So is the infinity mark for the 90 lens 3.5 inches? IS that where I place the front standard before I start adjusting?

    Thanks,

    Kev
     
  5. L Gebhardt

    L Gebhardt Subscriber

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    On my Shen-Hao I need to place the 135mm lens at about 10cm on the focus scale (based on the mark on the front standard). If you look the rear of the camera hangs behind the 0 mark, so for a 90mm lens I would place it at about 55mm to start with. Just play with it and you can see the exact position (for your lens). I usually go behind the infinity position so I can focus past infinity, to be sure I actually can reach infinity. For closeup work I use a ruler and measure the actual distance. Or better yet put a small ruler in the image and measure it's size on the glass.
     
  6. mobtown_4x5

    mobtown_4x5 Member

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    I'm totally confused by this, I rack my front standard untill the image on the gg is sharp. What am I missing here, is knowing these measurements helpful to achieve better focus?
     
  7. Mongo

    Mongo Member

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    Mobtown-

    Assuming no problems with your camera, you're focused properly when the image on the ground glass is sharp. None of this stuff will get you sharper focus.

    The stuff above about setting the front standard is meant to address the question of, "Where do I set the front standard before I go under the dark cloth?". Some photographers pop a lens on the camera and run the front standard back and forth until the image starts to clear up, and others try to get the camera near infinity focus before they look at the gound glass so they have to move the front standard less when they focus. In either case, you have to look at the glass to focus the image. Neither method is "right"; do what works for you.

    In theory, you could measure the distance to your subject, mathematically figure out your plane of sharpest focus, set all of your movements properly, and take an exposure, all without ever looking at your ground glass. To do this, you'd need to know things you probably don't know (like the exact focal length of your lens, the exact height of your camera, the distance and angle to everything you want in sharp focus, the location of the nodal point on your lens, the exact location of the film plane on your camera, and a bunch of math), your camera scales would have to be perfectly aligned, and you'd have to have scales on all of the movements on your camera. In reality, it's much easier to set up the camera and then focus on the ground glass. Whether your start with the lens near infinity focus or somewhere else is a matter of personal preference.
     
  8. noseoil

    noseoil Member

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    Kevin, start by setting all movements to "0" as far as back, front, plumb, level, etc. all movements. I know where each lens will focus by looking at the bed markings. To find out, just point the camera at a distant object and focus until it is sharp. This is "0" for your infinity focus. Any movement of the bed for a closer object is just the difference between infinity and the new object in cm.

    Example: 125mm lens is at infinity at 7.7 cm on my Shen Hao. I focus, and there is a difference of 4 cm because I have a closer subject. 4 / 12.5 is about .32 (1/3), so I double this .33 and get .66. I add 2/3 of a stop and take the shot. This is a crude way of doing the math, but my mental calculator is a very crude model, so I'm in the ballpark and it works for me.