Why does high F-numbers show groundglass? at DVinfo.net

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Old August 15th, 2008, 05:38 PM   #1
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Why does high F-numbers show groundglass?

If I have understood things correctly you should stay more or less wide open with your SLR-lense on a Letus extreme or else the groundglass becomes visible. Right? This is not so much a problem on the Ultimate as I understand it.
But why is it like this?! I cannot see the logic. How does the Letus "know" what aperture is set on the SLR-lense? All the SLR-lense does it project an image on the ground glass. How can the way this image is rendered affect the visibility of the ground glass?
To me it would be more logical if the F-stop setting on the camera would affect this.

Anybody who can explain?
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Old August 15th, 2008, 06:35 PM   #2
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I'm guessing here (not having a DOF adapter). But the higher the f-stop, the great the depth of field. As an example, if you're using a front-mounted filter on a lens and have a speck of dust/dirt on it, you will notice it only when stopped down (f16/f22), wide open, the depth of field is shallow enough to 'see' around the imperfection.
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Old August 15th, 2008, 06:52 PM   #3
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Wide open is not the only way to go... In fact, best images for a head being fully in focus front to back is somewhere in the f 5.6 to F 8 area. Grain showing up or being frowzen by camera , is, in my mind, more a function of the shutter speed chosen. I would not got faster than 1/48 or 1/60, depending on what your are shooting in (24p or 60i respectively.)
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Old August 15th, 2008, 10:07 PM   #4
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I have never seen or been able to figure out a clear explanation for the phenomena of seeing static grain in a vibrating (or even a spinning adapter) at high f stops on the 35mm lens, but it definitely occurs. Weird.
Take a stab at it.

Lenny Levy
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Old August 15th, 2008, 11:19 PM   #5
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I have racked my feeble brain on this without arriving at a conclusion.

The closest I can get is that each little pit in the groundglass may have one or more facets of surface on the pit sides which may at certain positions of the groundglass direct a pinpoint highlight to the camera or reflect a pinpoint of light away from the camera creating a corresponding dark pinpoint.

With the movement of the groundglass, the pinpoints would be random from frame to frame but of such short duration as to not be motion blurred through a slow shutter. The scintillation of pinpoints of light upon a backlit blue flat ocean under a newly arrived fresh breeze might be the best illustration.

My wild guess is that with tighter apertures, the direction of the fall of light upon the groundglass becomes less angular and that extremes of brightness and darkness are less likely to be diluted at the same pit, which with a wide aperture would otherwise be redirecting to camera, the light coming from other directions furthur out towards the lens diameter.

I am not a lens engineer so my comments should be regarded with extreme scepticism.

Last edited by Bob Hart; August 15th, 2008 at 11:22 PM. Reason: error
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Old August 18th, 2008, 04:53 PM   #6
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Bob - thats an interesting theory! I guess that could cause these effects.
Anyway - I know I have read this somewhere but can't find it - what would be the minimum aperture to use with the Extreme before you can see the ground glass?

Also - am I correct in assuming that I could use any aperture on the actual EX1 without any similar groundglass problems or are there issues here as well?
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Old August 18th, 2008, 05:15 PM   #7
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Ola.


Philip Bloom is probably a better brains trust than I on the Extreme/EX1 as my own expience is a total of two days on the combination.

I found when setting up that f4-f5.6 was about the sweet spot for the camcorder aperture. You will observe that more of the groundglass area is pin sharp in the groundglass texture across the entire area.

As you pull relay focus, at this aperture on the relay with the front lens aperture closed to provoke the artifact and motor stopped, you will observe a ring of sharpest focus grow from centre outwards over the image across an area of generally sharp focus. With other camera iris settings, a zone of sharpest focus is more confined.

With a Sigma f4 - f6.3 zoom on front, as the lens is zoomed in, a groundglass artifact becomes apparent on the sky in shots of aircraft I did, so it seems the general rule of thumb of f5.6 as the limit is about right for the Extreme.

I find that there is a little interaction between the camcorder and front lens iris as the critical point comes near.
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