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Thread: Optics: varying focus at extremes of image circle ?

  1. #1

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    Optics: varying focus at extremes of image circle ?

    Today I put a 150mm and 200mm lens on my 5x7 camera and focused on a flat subject around 10 feet away. These are not "wide" lenses but standard designs.

    I noticed that with the lens wide open especially, the edges of the subject were out of focus when the center was in focus, and vice versa. Once I focused the lens for the corners, the corners were sharp: I hadn't reached the limits of coverage.

    I tried the same experiment at infinity distance and still noticed a focus shift towards the extremes, although it was less and disappeared with a bit of stopping down.

    I rarely shoot wide angle lenses. Is this normal ?

    We expect a bit of darkening towards the corners due to the increased distance, but focus shift too ?

  2. #2
    8x10, 5x7, 4x5, et al Leigh's Avatar
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    Re: Optics: varying focus at extremes of image circle ?

    The depth of focus of a lens is defined as the distance from the rear node to the film plane along the lens axis.

    If you rotate that line about the rear node, the point of focus defines an arc, not a plane.
    I think this is one reason the Image Circle diameter is usually less than the circle of illumination diameter.

    I believe there are some aspects of lens design that can counteract this effect to a limited extent.

    - Leigh
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    Mark Sawyer's Avatar
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    Re: Optics: varying focus at extremes of image circle ?

    "Flat field" is a relative term. Most modern lenses are close to it, but the wider you go, the more you notice they aren't quite. Process lenses are optimized for flatness of the focal plane, but at the expense of speed and coverage. One reason manufacturers' coverage figures are conservative is that they won't count illumination after the field curves.
    "I love my Verito lens, but I always have to sharpen everything in Photoshop..."

  4. #4
    retrogrouchy
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    Re: Optics: varying focus at extremes of image circle ?

    You have field curvature. While the focus field should in theory be a flat plane, it is often not perfect and can sometimes be significant.

    Petzvals have significant field curvature for example, and for those who also use 35mm cameras, some of the modern ultrazooms have huge field curvature, e.g. the Tamron 17-50/2.8.

  5. #5
    retrogrouchy
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    Re: Optics: varying focus at extremes of image circle ?

    Quote Originally Posted by Leigh View Post
    The depth of focus of a lens is defined as the distance from the rear node to the film plane along the lens axis.

    If you rotate that line about the rear node, the point of focus defines an arc, not a plane.
    I think this is one reason the Image Circle diameter is usually less than the circle of illumination diameter.

    I believe there are some aspects of lens design that can counteract this effect to a limited extent.

    - Leigh
    Sorry but this is just plain wrong. A "simple" lens, ignoring the various shortcomings they all have, has matching planes in focus, not spheres. If you have a planar sensor, you will have a plane in the scene in focus, the angle and placement of which obviously depends on your movements.

  6. #6
    Tin Can's Avatar
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    Re: Optics: varying focus at extremes of image circle ?

    I don't know the science, but it seems an empirical given that edges will not be the same focus. I see it all the time.

    I do not worry about it as I keep my important content within 2/3 of the image area.

    I also shoot a lot of Macro where this is evident, most likely because I am shooting beyond the suggested IC.

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    Re: Optics: varying focus at extremes of image circle ?

    Ken, visit http://toothwalker.org/optics.html and scroll down to the table that shows the dependence of third order aberrations on relative aperture and distance off-axis.

    Curvature of field is affected by both. Stopping down reduces it and it gets worse as the distance off-axis increases.

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    Tin Can's Avatar
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    Re: Optics: varying focus at extremes of image circle ?

    Nice link. Great concise info.

    Quote Originally Posted by Dan Fromm View Post
    Ken, visit http://toothwalker.org/optics.html and scroll down to the table that shows the dependence of third order aberrations on relative aperture and distance off-axis.

    Curvature of field is affected by both. Stopping down reduces it and it gets worse as the distance off-axis increases.

  9. #9
    8x10, 5x7, 4x5, et al Leigh's Avatar
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    Re: Optics: varying focus at extremes of image circle ?

    Quote Originally Posted by polyglot View Post
    Sorry but this is just plain wrong. A "simple" lens, ignoring the various shortcomings they all have, has matching planes in focus, not spheres.
    Sorry, but my comment is just plain right. In fact it's even worse than the simple description I posted.

    As a focused point on the subject recedes (moving away from the camera), the focal distance from rear node to film becomes shorter. So if you expect a planar subject to be in focus, you have a tough row to hoe, since the distance from rear node to image off-axis increases rather than decreasing as would be required to keep the image in focus.

    Quoting from the reference that Dan provided, as regards image focus:
    "With a real lens, the sagittal and tangential focal surfaces are in fact curved."
    Full article here: http://toothwalker.org/optics/astigmatism.html

    The focal distance from rear node to image is a fixed dimension.
    As you move off-axis, that distance defines an arc, not a plane.

    - Leigh
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  10. #10

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    Re: Optics: varying focus at extremes of image circle ?

    Yes, that's an excellent site. I like http://toothwalker.org/optics/astigmatism.html: illustrations 4, 5 and 6 match what I saw on the ground glass.

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