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Old October 27th, 2006, 06:25 PM   #16
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the zoom is a pretty fast f2.8 28-70mm

dont get same effect with an f2.8 28mm prime

quyen says it's a problem only with zooms and he doesnt know why...
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Old October 27th, 2006, 10:02 PM   #17
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In my other reply I refer to testing the XL flip without the relay on.

The light transmission of the Letus into the FX1 via the same Century dioptre and camera zoom settings was about the same as my own A05 groundglass.

If you study the groundglass texture up close,you will observe the size of the pits and gouges is not a fixed grade, some small, some larger. In turn you get random areas of light transmission or diffusion, some brighter, some darker.

The spinning disk versions have an advantage in that unless the disk speed is synchronised to the camera frame rate, the same area of the disk never turns up twice so the presentation of texture to the camcorder is truly random. The disk can be covered in dust and not cause a problem until the dust comes off onto fixed optics.

The orbital systems like newer P+S, M2 and Quyen's Letus, present the same piece of groundglass to the camera every time. Therefore, although the individual tiny variations of brightness on the groundglass move, they remain in the same neighbourhood, therefore a faint soft fixed pattern will become evident in adverse circumstances. In other orbital devices, this artifact has been described as "swirling". This has also been associated with higher shutter speeds.

I don't know the size of the movement of the others but the movement of the Letus GG is quite small, probably just under a millimetre.

The options are to carefully select the lighting environments, use a finer groundglass, which brings in problems of hotspots and ghosting or to increase the size of the movement (which I call the excursion) to that of the P+S or M2.

My guess is that the other devices use a movement of up to 3mm diameter.

Quyen's current design is wedded to the small and almost silent vibrator motors. There is not a lot of room to make changes inside the tube enclosure without encroaching into the image area.

The upside is, that little motor seems capable of swinging a little more weight and getting the excursion out to about 1.2mm. The downside is, extra countermass needs to be added outboard of the motor on the GG support panel to keep the motion truly circular and this takes back a little of the extra movement gained.

Remounting the motor to face rearward in a thimble or off-standing mount so that the weight runs inside a hole in the GG support panel at the image plane should give more movement with the existing hardware even without extra weight.

To get a little more movement, I might replace the pillars with tightly pitched coil springs using the existing rubber mounting system.

This stuff is all major guesswork on my part and should not be heeded too much as the problem may be purely optical in origin, visible in other orbital designs and simply not remediable. I just don't know enough about the others to make valid comment.
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