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Silicon Imaging SI-2K
2/3" 1080p IT-integrated 10-bit digital cinema w/direct-to-disk recording.


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Old October 29th, 2014, 01:29 AM   #1
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Alexarising the SI2K??

We can all dream can't we. It has become more or less an urban legend that part of the ARRI's secret sauce for the Alexa has been a contrast filter at or near the sensor.

Maybe this is something along the lines of the "carrot" disinformation propagated by the British during WW2 to help conceal the nightfighter radar technology from the enemy.

Whatever, I decided to give it a shot as best I could.

Formatt in the UK had the kindly good grace to send me some 1mm thick samples of filter material to experiment with including some 1/8 contrast filter pieces.

I intend to machine up some custom filter holders to position multiple layers closer to the sensor. For the time being I have crudely attached filter material to the front of the filter holder.

With the existing filter holder and a Metabones Speedbooster for BMPCC in the mix, there is insufficient space between the rearmost optical element of the Speedbooster and the front of the filter holder for more than one piece of filter material to be added.

Adding planar elements into the path between lens and sensor has its drawbacks, most evident when wide-angle lenses are concerned.

The vision linked to is far from scientifically prepared. I cannot colour-grade to save my life. Normal post-production enhancement like sharpening and green-channel suppression for the IRND colour cast have been built over the low contrast look.

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Old October 29th, 2014, 01:37 PM   #2
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Re: Alexarising the SI2K??

I shot some comparative clips, with and without 1/8th contrast filter on sensor and with and without Speedbooster of a Lemac focus chart and of a Dumonde Chart.

There is a difference to be seen in the RGB parade in Premiere CS6 but it is very subtle. The camera histogram shows slightly wider margins each end with the contrast filter. To the eye view there seems little difference unless one looks really hard.

With the Super16mm lens, a 20mm T1.3 Elite at T2.8 there was a very small degree of sharpness loss with the contrast filter in the path behind the lens, only between 5 and 8 sharpness points reported by the camera. With and without the contrast filter, the "A" block of the Lemac focus chart was resolving - just.

With the Speedbooster and a Nikon 50mm f1.4 lens at between f4 and f5.6 for the same lighting, there was no sharpness loss reported by the numbers with the contrast filter in the path. I'm still trying to figure that one out. The "A" block on the Lemac focus chart was resolving distinctly.

I would like to extract some frame grabs but Premiere CS6 exports the whole clip as a .jpg string. If anyone knows how to make it export a single frame like CS5 I am interested to learn.

It would be interesting to see what one gets by putting 35mm cine lenses in front of the Speedbooster.

There would be a bit of tricky engineering involved and not all lenses would be suitable if the rear elements protrude too far rearwards. The Speedbooster Canon dumb version with its slightly wider diameter throat at the mount ring attachment might be the better candidate for a PL-Mount add-on.

Because the Speedbooster enhances contrast, it would be interesting to see what happens with older flarey Cooke Speed Panchros. I don't think they will be usable as the rear element and barrel are set too far rearwards. The Angenieux 25-250 zoom should be a good candidate.

Last edited by Bob Hart; October 29th, 2014 at 01:37 PM. Reason: error
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Old October 29th, 2014, 02:26 PM   #3
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Re: Alexarising the SI2K??

Second try. Exported with no problem, a bit of a conundrum it is, maybe weevils on the circuit board, malevolent confluences of geomagnetic forces, Halloween ??. It's hardly a professional presentation. I have to keep things as short as I can. Anything more elaborate will crash my computer.


Last edited by Bob Hart; October 30th, 2014 at 01:26 PM. Reason: added text
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