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Thread: Stupid question about lenses, vignetting, groundglass illumination

  1. #1

    Stupid question about lenses, vignetting, groundglass illumination

    Hi guys,

    I'm new to LF and just got a 8x10 LF camera and a 240 Sironar-S lens.

    Today, for the first time, I looked at the groundglass with the lens and saw that the image is well illuminated only in the center. At the edges it gets quite dark.

    Everything was set up correclty, no movements such as rise or fill, I realize that I don't have too much room with the 240 ...

    When shooting a sheet of 8x10 film, how come the image will be well lit overall whereas on the groundglass only the center is really getting much light?

    Is there a difference between what one sees on a groundglass in terms of vignetting than what one will get on film?

    I know this might be an easy answer, but I'm new to this kind of photography, i.e. working on a groundglass ...

    Kind regards

    Paul

  2. #2
    Louie Powell's Avatar
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    Re: Stupid question about lenses, vignetting, groundglass illumination

    Paul -

    Welcome to the crazy world of LF.

    A 240mm lens is a wide angle in 8x10 - the diagonal of the 8x10 image frame is 12.8 inches, so the 'ideal' normal lens would be 325mm.

    There is always light loss at the edges of the image frame - even in 35mm. Remember the inverse-square law - light loses intensity as the distance between image (source) and film increases. With LF, this is more of an issue, and it's not uncommon for there the image in the ground glass to be dimmer at the edges of the frame. That's something that we deal with by using a dark cloth, and simply by sucking it up and living with it.

    A greater concern is when that loss of light translates into a loss of exposure on the film. Obviously, it happens in every case, but its not always noticeable, and good printers tend to use a bit of edge burn both to increase separation at the edges and also to compensate for this effect. (It happens in enlarging also.) But when you are using a wider angle lens in LF, that reduction is exposure is sometimes significant. Architectural photographers often use 'center filters' to compensate for this effect. A center filter is a graduated neutral density filter that is denser in the middle, and gradually fades to clear around the edges. Don't rush out to buy one until you conclude that it is really necessary - they are expensive.

    The other consideration is the coverage of the lens. The image circle thrown by the lens at the typical lens-film plane distance must exceed the diagonal dimension of the frame in order to avoid vignetting. Your lens has a 372mm image circle which is only slightly greater than the diagonal of the film plane. That means that the vignetting that you get with the camera controls zeroed out may be negligible, you you won't have much margin for rise, fall, swing, tilt, etc. The point is that vignetting and loss of light at the edges are really two different things - loss of light is simply the inverse-square law at work, while vignetting is where the image circle is too small, and the front of the lens is actually shading one or more corners of the image frame.

    It's hard to differentiate visually between true vignetting and simply the loss of light at the edges. One trick is to look into the lens with no film-holder in the camera - if you can see all four corners of the ground glass, then you won't have any true vignetting. Of course you still could have noticeable darkening at the edges.

  3. #3
    8x10, 4x5, ..., Tessina
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    Re: Stupid question about lenses, vignetting, groundglass illumination

    There are a couple of points to consider.

    Rodenstock recommends your lens for use on 5x7 format maximum.

    The Rodenstock literature shows an image circle of 372mm, as Louie said, but that's at f/22. When you use the lens wide open that diameter decreases dramatically. This probably explains the difference you're seeing between the ground glass image and the film image.

    I can't find published specs for the Sironar-S wide open, but as an example the comparable Nikkor W 240/5.6 has an image circle of 336mm @ f/22 but only 278mm wide open. That's a big difference, considering the diagonal of your 8x10 film is 325mm, which defines the minimum image circle to fully cover the film.

    The full-open aperture is available primarily as an aid to focusing. The recommended taking aperture for your lens is f/16 to f/32.

    Center filters are not appropriate for this focal length. They're used in situations where the angle of view exceeds roughly 100. Your Sironar-S has an angle of view of about 75 at taking aperture, less when wide open.

    - Leigh

  4. #4
    Stefan
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    Re: Stupid question about lenses, vignetting, groundglass illumination

    Quote Originally Posted by PaulSchneider View Post
    When shooting a sheet of 8x10 film, how come the image will be well lit overall whereas on the groundglass only the center is really getting much light?

    Is there a difference between what one sees on a groundglass in terms of vignetting than what one will get on film?

    The ground glass, unlike the film, works by diffusing the incoming light from the lens. This diffusion is not equal in all directions, most of it ends up going along or near the axis of the incoming light. If you position your head so that you have the corner of the GG between you and the lens, you will notice that the corner is a lot brighter than when you had your head in front of the GG center.

    If you were to put a matte white sheet of paper, which reflects in all directions, in the place of the GG and looked from inside the camera, the image would look just like it is captured by the film. The corners would still be darker than the center due to vignetting, but this depends on the image created by the lens and will be visible in the final image as well.

    What camera do you have? A modern ground glass is brighter than an old one, and you can also add a fresnel lens to minimize the "dark corners" effect on the GG.

    Edit: Apart from this, as the others have pointed out, there is also vignetting due to the lens. When you focus and compose you likely have the lens wide open resulting in heavy vignetting of the image as created by the lens. When you stop down to take the picture, vignetting is much reduced. The corners are not made brighter by stopping down, but they are made brighter relative to the center of the frame, resulting in an image with less vignetting.

  5. #5

    Re: Stupid question about lenses, vignetting, groundglass illumination

    Wow, thank you Louie and Leigh,

    that explains it all, I mean I forgot that there's a big difference between wide open and stopped down. I actually wouldn't shoot at 5.6 anyway ... but to get a family portrait in an interior setting means it won't be easy at f16 ...

    brave new world!

  6. #6

    Re: Stupid question about lenses, vignetting, groundglass illumination

    engl: I have a 8x10 chamonix ... so at what aperture do you think will one not notice the light falloff too much? I wonder whether f8 is already ok ...

  7. #7
    8x10, 4x5, ..., Tessina
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    Lightbulb Re: Stupid question about lenses, vignetting, groundglass illumination

    Quote Originally Posted by PaulSchneider View Post
    ... but to get a family portrait in an interior setting means it won't be easy at f16 ...
    That's why they invented studio strobes.

    LF cameras don't have hotshoes.

    - Leigh

  8. #8

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    Re: Stupid question about lenses, vignetting, groundglass illumination

    As Engl noted, the bright center of the ground glass is just a hot spot. The shorter the focal length, the more you get that hot spot. If you move your head around under the dark cloth, you'll notice the hot spot moves a bit.

    Vignetting is a different phenomenon than fall-off. In fall-off, the corners are darker than the center because they are farther from the lens, but unless you go really wide on 8x10 (150mm or shorter), it's not an issue. Vignetting is when the lens simply does not deliver light to the corners, usually because the mechanics of the lens blocks it.

    The 240mm Sironar-S is a modern plasmat with a lot of coverage. It won't vignette on 8x10 at any f/stop, even when focused at infinity, where the coverage is smallest, and it will even have a bit of coverage left over for movements. I use a 215mm plasmat on 8x10 without any issues. BTW, your lens is also convertible to a longer focal length if you remove the front element, but the f/stop scale will change.
    "I love my Verito lens, but I always have to sharpen everything in Photoshop..."

  9. #9

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    Re: Stupid question about lenses, vignetting, groundglass illumination

    Quote Originally Posted by PaulSchneider View Post
    Wow, thank you Louie and Leigh,

    that explains it all, I mean I forgot that there's a big difference between wide open and stopped down. I actually wouldn't shoot at 5.6 anyway ... but to get a family portrait in an interior setting means it won't be easy at f16 ...

    brave new world!
    An 8x10 family portrait indoors will require lighting/flash unless they are in a very bright space or are working in a non-traditional mode. The traditional expectation is that everyone is pretty much in focus and not blurred. You may need stopping down more than f/16 if the group is large. Remember that this lens is also wide angle and you know what happens to people at the edges of wide angle images. They appear stretched...

  10. #10

    Re: Stupid question about lenses, vignetting, groundglass illumination

    yes but strobes change the look totally ... i prefer the natural light look of the struth portraits for example, take this image, what aperture do you think this was made at? It is a 240mm lens with portra 400 inside an office. I do not have enough experience see from the dof what f stop it is ...

    http://www.flickr.com/photos/thomashobbs/5280705782/

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