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Thread: 4 x 5 focus problem

  1. #21
    Jac@stafford.net's Avatar
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    Re: 4 x 5 focus problem

    Quote Originally Posted by LotusEsp View Post
    any suggestions on a good loupe?
    I second the suggestion to try a few, if possible.

    I'll leave this thread with just a couple things - greater magnification is not always better, the coarseness of the GG is important and relates to magnification. I have a couple of the late Satin Snow screens and it was just too fine for my eyes (but so very well made). Finally if you use a GG with a hood as some field cameras have, be sure it reaches into the area properly. The later point is why I had to swing away the hood on mine, a real PITA. I lucked out in finding a used Wista loupe, the long, collapsible one; no longer made, I think.

    Good luck!

  2. #22

    Re: 4 x 5 focus problem

    Sounds to me like a ground glass misalignment...that would be consistent as you describe. Loupes should not even be necessary except in very low light or with a really slow lens.

  3. #23

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    Re: 4 x 5 focus problem

    Wow! WOW!!! That Peak loupe is priced to perfection. Just a little less than another lens.....
    Peter Collins

    On the intent of the First Amendment: The press was to serve the governed, not the governors --Opinion, Hugo Black, Judge, Supreme Court, 1971 re the "Pentagon Papers."

  4. #24

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    Re: 4 x 5 focus problem

    Quote Originally Posted by LotusEsp View Post

    I think is sharpness on the GG glass not transferring to the negative.
    Suposing that there is no camera shake...

    You have to spend a sheet or two to diagnose the thing.

    Place a metering tape on the floor, the camera can be some 3 metres far and 1 meter higher, well, it depends on the bellows you have to focus on near objects...

    Then write in a paper the DOF: first and last numbers in focus of the metering tape, so you have the center focus reference.

    Shot wide open and develop... You'll know if the focus center has shifted, by looking in the neg if the same numbers of the metering tape are in focus.

    Is a shift there ? It can be:

    1) Film holder plane inside Graflock clamp has to match the same plane than the ground glass.

    2) Tripod !!!! Be careful !!! if it has some jog... if you make a tilt and after inserting the film holder it moves just a bit... then the focus plane will move a lot !!! impossible to retain the focus in the eyes of the subject !!!

    a tripod can be "elastic", this is it can move away when you make force to insert the film holder, but it is imperative (absolutely) that the camera returns exactly to the same position after you stop making force !!!!!

    3) Some weird glasses have "focus shift", the focus plane moves when you close diafragm, an example is old Symmars in converted configuration, this can be stated by closing diafragm and cheking focus again, this may be done in a dark place and with a subject strongly illuminated with a close light source.


    To know if it is the tripod or the glass plane, repeat the shot but with some tilt movement, and perhaps making a little of force down at the lens (hang some 200grs weight). If problem appears with the shot made with the shift movement, then it's the tripod !!!


    This is what I know... I don't remember other factors...

  5. #25

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    Re: 4 x 5 focus problem

    Shooting a 90mm lens stopped down to f/32 should be a lock on a sharp negative - diffraction should be minimal. Assuming that you are able to focus the image on the ground glass visually (again, should be easy enough for a landscape without using a loupe), to summarize the three most likely sources of this problem:

    1. The ground glass is not positioned correctly, or could even be installed backwards. The "ground" side should be facing the lens, not the back of the camera. Also, check to see if someone inserted a Fresnel lens next to the ground glass (if so, where is it placed?).

    2. The film holder is not seating properly at the back of the camera. What type of film holder are you using?

    3. The film is not inserted into the film holder correctly. Is it loaded emulsion side out, slid all the way under the film rails?

  6. #26

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    Re: 4 x 5 focus problem

    Thanks for your comments I will try and answer them

    I am not using a fresnel at all - only the GG
    The ground side is facing the lens
    I am using a mixture of film holders, Lisco Regal, RITEWAY etc
    I am not sure what (if any) adjustments can be made to the GG position- it's pressed flush into the recess on the back of the camera and held in by 2 clamps to a sprung mounted plate. This sprung plate is then flush against the main camera body. So the only possible movement available would be to bring the GG it further away from the lens by putting a spacer of some sort inbetween it and the mounting board. I assume that would happen if I placed a fresnel in front of the GG anyway?

  7. #27

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    Re: 4 x 5 focus problem

    Quote Originally Posted by LotusEsp View Post
    I will try
    One thing else... in case you can measure the focus shift by focussing a reference point in the "metering tape" and looking at the negative where the focus moved on the metering tape, then you can calculate the shift error (correction_to_be_done) from the ground glass to the film plane by the formula:

    1/F = 1/D + 1/B

    where F = focal length, D = distance to object, B = Bellows Draw


    Then you have a system with two equations, all in mm

    1/F = 1/D1 + 1/B

    1/F = 1/D2 + 1/(B+correction_to_be_done)

    D1 the distance from lens board to the point focused in the ground glass.

    D2 is D1 plus the focus shift in mm found in the negative.

    Note: It is considered that a common lens has its principal point at lens board plane aprox, (in case of a Tele it is a bit forward)

  8. #28
    Jac@stafford.net's Avatar
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    Re: 4 x 5 focus problem

    Quote Originally Posted by Pere Casals View Post
    [snip needless maths]
    Note: It is considered that a common lens has its principal point at lens board plane aprox, (in case of a Tele it is a bit forward)
    The nodal point is usually close to the diaphragm regardless of the lens board position, and telephoto lenses can have it not a bit, but far in front of the first (front) element.

    So much for desktop impressionistic optics. Thanks. NOT.
    .

  9. #29
    Jim Jones's Avatar
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    Re: 4 x 5 focus problem

    If the Shen Hao was originally provided with a correctly installed Fresnel between the ground glass and frame, and the Fresnel has been removed, the ground glass will be displaced forward by about 2/3 the thickness of the Fresnel due to the index of refraction.

  10. #30

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    Re: 4 x 5 focus problem

    Quote Originally Posted by Jim Jones View Post
    If the Shen Hao was originally provided with a correctly installed Fresnel between the ground glass and frame, and the Fresnel has been removed, the ground glass will be displaced forward by about 2/3 the thickness of the Fresnel due to the index of refraction.
    This is not possible with Shen Hao cameras.

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