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Thread: Is focus a plane to plane affair?

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    Is focus a plane to plane affair?

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    A theoretical and maybe silly question. Suppose I have a camera with a well corrected lens at point A and a subject at the point S far from image centre. Let us say that I don't have a ground glass and have to deal with a foccusing scale in getting S in focus. Which distance should I set in camera focus rail? L1 or L2?

    I "tested" that with an Apo-Symmar 150mm f/5.6. I focused a spot on a wall 6 meters away from lens, right in front, on the lens axis. Then I checked another point at the corner in a 4x5" ground glass. My assessment was that focus could be improved there. I fine tuned it and found indeed that in several attempts I had to bring the lens closer to the film plane in order to improve sharpness at the corner. That tells me, without any math, that at least for this lens, the actual oblique distance is the one that has a better chance of being correct if I had to use the distances scale.

    The reason why I am asking that is that I am planning to print scale focusing and use a wire frame finder plus the aid of a laser measure. By doing so I could, in some cases, dispense the ground glass.

  2. #2

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    Re: Is focus a plane to plane affair?

    L2.

    The difference you've noticed is most probably due to the lens' field curvature. Though I have not seen a noticeable field curvature in Symmars....

  3. #3

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    Re: Is focus a plane to plane affair?

    The premise of your question shows an incomplete understanding of how light rays pass through the lens and are then focused on the film/ground glass/whatever.

    That said, the short answer to your questions is that, assuming the film plane is parallel to the focus plane and the lens axis is perpendicular to those, you can just focus at the L2 distance and any object that lies in that parallel plane will be in focus. Note that we must also assume that your lens is plano-parallel and has no curvature of field.

    So, if your camera has fixed lens and film positions, like a non-view camera, you can focus anywhere on the corresponding subject plane and everything in that plane will be rendered sharp on the film. The trick, really, is determining where that plane is in space in front of the camera. I'd be willing to wager that your unsharp corners are simply due to your misjudging where that plane of sharp focus lies.

    Keep in mind, the plane of sharp focus will then move around in space with the camera. Tip the camera up and the plane of sharp focus tips as well. The same applies for pointing the camera down or side-to-side movements.

    If you are using a view camera with movements, then you need to learn how tilts and swings affect the position of the plane of sharp focus. There are lots of resources here and other places on the web as well as textbooks that have been written about that subject. If you really want to take advantage of movements on a view camera, a ground glass is indispensable.

    Have fun,

    Doremus

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    Re: Is focus a plane to plane affair?

    Thanks for the clarification. So it is plane to plane, subject to image, both perpendicular to lens axis. Maybe there are lens imperfections, specially closer to borders, but anyway that is how it should be.

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    Nodda Duma's Avatar
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    Re: Is focus a plane to plane affair?

    not quite. This is why lens designers speak in terms of “surface of best focus”. This surface is not inherently flat. Field curvature correctly implies that different field angles focus at different distances. It is only by designing the optics to correct for field curvature such that the plane of best focus lies sufficiently flat that the plane of the film or sensor lies within the depth of focus. This is a wordy sentence, but it exactly describes how “flat” image planes are actually just “sufficiently flat”.

    If it were not for field curvature, or conversely if the film or sensor could be arbitrarily curved to lie on the sphere of best focus, then optical designs would be much, much simpler.
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    Re: Is focus a plane to plane affair?

    Do curved projection screens fit into this optical design reality?


    Bernice


    Quote Originally Posted by Nodda Duma View Post

    If it were not for field curvature, or conversely if the film or sensor could be arbitrarily curved to lie on the sphere of best focus, then optical designs would be much, much simpler.

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    Mark Sawyer's Avatar
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    Re: Is focus a plane to plane affair?

    Quote Originally Posted by Nodda Duma View Post
    not quite. This is why lens designers speak in terms of “surface of best focus”. This surface is not inherently flat. Field curvature correctly implies that different field angles focus at different distances. It is only by designing the optics to correct for field curvature such that the plane of best focus lies sufficiently flat that the plane of the film or sensor lies within the depth of focus. This is a wordy sentence, but it exactly describes how “flat” image planes are actually just “sufficiently flat”.
    Quite correct. I'll add that process lenses are designed for a very flat focal plane, as those lenses are made for very demanding copy work of flat artwork, so if someone is looking for minimal field curvature, that's the way to go.
    "I love my Verito lens, but I always have to sharpen everything in Photoshop..."

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    Alan Klein's Avatar
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    Re: Is focus a plane to plane affair?

    My LF camera has asymmetrical tilt on the rear standard. So I focus at the distance first. Since the axis is about 1/4 from the top of the GG (it's on the bottom but everything is upside down of course), I'd be focusing first on L1, not L2. How does that affect our discussion?

    Then I'd tilt the rear standard down to get the close item in focus at the other end of the GG. So then which is the best spot to focus close? Can I use any point close-up or the one that has the line going across the GG?

  9. #9
    Nodda Duma's Avatar
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    Re: Is focus a plane to plane affair?

    Quote Originally Posted by Bernice Loui View Post
    Do curved projection screens fit into this optical design reality?


    Bernice
    Bernice, you’re too intelligent to ask a question like that.

    BTW, I’ll be in your neck of the woods at the end of this month, at Photonics West.

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    Re: Is focus a plane to plane affair?

    Quote Originally Posted by Nodda Duma View Post
    If it were not for field curvature, or conversely if the film or sensor could be arbitrarily curved to lie on the sphere of best focus, then optical designs would be much, much simpler.
    Here you go:

    https://patents.google.com/patent/US4241987A/en

    Popular Photography made a big fuss about it at the time, but I've never read that anything ever came of it.

    EDIT: Also, the proposed embodiment is really oriented to roll film, though with sufficient mechanical insight and imagination one could perhaps adapt the ideas to a different embodiment more suited to sheet film.

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