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DoF - straight or curved?

Bend The LightBend The Light Registered Users Posts: 1,887 Major grins
edited February 13, 2013 in Technique
I was just looking through some old images, and this one taken with a MF Vivitar 90mm macro, at 1:1.
I noticed the thin "focal plane" is curved.
Is this right, and the focal plane is actually curved, or is it distortion which is not so evident in other lenses or something?

6066094063_4a0a93cac5_o.jpg
Unidentified dragonfly at Ynis-Hir 2 by http://bendthelight.me.uk, on Flickr

Thanks.

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    ziggy53ziggy53 Super Moderators Posts: 23,795 moderator
    edited February 10, 2013
    It's pretty likely that your Vivitar 90mm macro exhibits field curvature.

    To verify, shoot a flat target (with the camera squared to the target) using a macro rail, or at least using a tripod with a crank stem, and capture a number of distances from the subject. You should see different "sections" of the flat subject matter exposed and sharp at different distances from the subject to confirm the field curvature.
    ziggy53
    Moderator of the Cameras and Accessories forums
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    Bend The LightBend The Light Registered Users Posts: 1,887 Major grins
    edited February 10, 2013
    ziggy53 wrote: »
    It's pretty likely that your Vivitar 90mm macro exhibits field curvature.

    To verify, shoot a flat target (with the camera squared to the target) using a macro rail, or at least using a tripod with a crank stem, and capture a number of distances from the subject. You should see different "sections" of the flat subject matter exposed and sharp at different distances from the subject to confirm the field curvature.


    Yes, that's what I thought. So the focal plane is not a plane at all...more a Focus Locus! :)
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    ziggy53ziggy53 Super Moderators Posts: 23,795 moderator
    edited February 10, 2013
    Bend The Light wrote: »
    Yes, that's what I thought. So the focal plane is not a plane at all...more a Focus Locus! :)

    There are flat field lenses, commonly found in film duplication applications, and many more common lenses are "relatively" flat.

    Field curvature occurs when the lens is not corrected for field curvature. Many/most lenses are at least partially corrected for field curvature. When you test your Vivitar 90mm macro against a flat subject and vary the shooting distance, you'll have a better idea of the extent of any field curvature in that lens.
    ziggy53
    Moderator of the Cameras and Accessories forums
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    QarikQarik Registered Users Posts: 4,959 Major grins
    edited February 12, 2013
    hmm..well focal planes are theoretically flat. Lens distortions may result in some curvature..but in the case of this shot, you are shooting a curved surface apparently. This may introduce what appears to be a curvature in your focal plane even though it is not.
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    Bend The LightBend The Light Registered Users Posts: 1,887 Major grins
    edited February 13, 2013
    Qarik wrote: »
    hmm..well focal planes are theoretically flat. Lens distortions may result in some curvature..but in the case of this shot, you are shooting a curved surface apparently. This may introduce what appears to be a curvature in your focal plane even though it is not.

    As far as I remember, and judging by other photographs taken at the same time, the wood is not curved...it is 3in x 4in timber railings and so all the same in all the railings in that wetlands area.

    If you notice the forward facing wings of the insect follow the curve of focus, too...not sure that would happen if the focal plane was flat as it theoretically should be.

    :)
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