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Thread: Determining coverage/image circle of simple lenses

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    Determining coverage/image circle of simple lenses

    Iím interested in getting back into a bit of home brew lens experiments using simple lenses (pos minuscus, achromats, etc). Most online suppliers provide basic data like focal length and lens diameter but not image circle. Is it possible to work out the image circle thrown by an objective based on the standard info typically provided?
    Francesco Fragomeni
    www.FrancescoFragomeni.com

  2. #2

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    Re: Determining coverage/image circle of simple lenses

    The Leitz Telyt-R 400 and 560mm achromats suffers wide open from field curvature and CA accordingto what I read, so I would say it all depends on whether how much you are going to enlarge, if you are going to use them in sets around an aperture like this ( | ) and how much money you are going to spend. I have found the achromats from Pentax S82, Nikon 5T & 6T and Canon 500D to be of higher quality than Maruni +3 achromats
    Link to my bokeh tests https://www.flickr.com/photos/470525...57634939838558

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    Foamer
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    Re: Determining coverage/image circle of simple lenses

    Quote Originally Posted by Fragomeni View Post
    Iím interested in getting back into a bit of home brew lens experiments using simple lenses (pos minuscus, achromats, etc). Most online suppliers provide basic data like focal length and lens diameter but not image circle. Is it possible to work out the image circle thrown by an objective based on the standard info typically provided?

    If the lens doesn't vignette in the corners, a good deal comes down to what you find as acceptable.


    Kent in SD
    Qui sedes ad dexteram Patris,
    miserere nobis.

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    Re: Determining coverage/image circle of simple lenses

    The mencious cover well, but the two factors that are present are curvature of field (where the outer IC is exceeding the plane of focus), and if the iris is installed further in front of the lens (where optimal), there is the mechanics of the barrel that will limit IC...

    Other than that, they produce a large IC...

    Steve K

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    Mark Sawyer's Avatar
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    Re: Determining coverage/image circle of simple lenses

    Terminology-wise, a single element or doublets have a very large circle of illumination, meaning they light up a large area. But they have small circle of coverage, where the image would be acceptably sharp for most uses. The image quality fall off at the edges is from curved field, as others mentioned, but various other aberrations are greatly increased away from the center too.

    Because of the very short "barrel" these simple lenses require, illumination is very wide, but falls off as the visible area of the lens decreases farther out due to the viewing angle.

    How fast the degradation of the image occurs will depend on the individual lens and the user's standards for image quality, so there's no clear formula, but I've found that for portrait use, a standard portrait focal length for the given format works well with some acceptable degradation at the corners. Better lenses like the Imagon or Kodak Portrait Lens won't lose much, but something very crude like a biconvex magnifying glass will be noticeably out-of-focus outside the center.
    "I love my Verito lens, but I always have to sharpen everything in Photoshop..."

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    multi format
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    Re: Determining coverage/image circle of simple lenses

    Hi Francesco

    I have bought a bunch of Plano convex lenses from Supply Shed over the years and they have been very helpful in telling me
    approximate coverage, I can't remember off hand which ones I used in a 22x14 sun print / retina / obscura camera I made a handful of years ago but but it threw a huge image. The surplus she folks
    were good at pointing me in the right direction. You could also take a peek at Reinhold Schable's website ( http://www.re-inventedphotoequip.com/Home.html ). he has information on meniscus lenses.

    Good Luck!
    John

  7. #7
    (Shrek)
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    Re: Determining coverage/image circle of simple lenses

    You're obviously not looking for coverage or image circle if you're shooting with meniscus lenses, because those are defined by acceptable resolution and lack of aberations within a certain field. You want those aberrations in a soft focus lens, that's what makes it 'soft' and IMHO 'interesting', otherwise you'd be shooting with the latest Schneider or Rodenstock lenses. So all you need is circle of illumination, and you can control how bad (or good) the aberrations are by how much of that circle of illumination you're using, that is the aberrations will get more extreme the further off-axis you go. So a focal length 1,000mm lens on 4x5 will be reasonably 'normal'-looking, even 'sharp' if you use an aperture, but a 100mm lens will have more extreme effects around the edges.

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    Re: Determining coverage/image circle of simple lenses

    I have observed some of my lenses have back cells with positive power, shorter focal and aperture brighter than the combination, my 20 inch f7 Ilex has a 280mm f3.5 back cell. The 240mm Symmar now sold had a 420mm back. Also my 14 inch Ektar has a positive back cell usable for photographic adventures.

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