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Alexa Shutter Question


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#1 Charles Haine

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Posted 15 March 2011 - 06:54 PM

Grading a project shot on the Alexa, I noticed the footage looked a bit like the attached photo.

This seemed kind of strange to me; motion blur makes sense if the rectangle is moving fast enough and the shutter is 1/48, but why does it appear pulsed? As if it's recording two frames and mixing them together? Shouldn't it be a continuous blur? It also seems like one of the two images (or pulses, or bursts, whatever you want to call it) is more exposed than the other. Is the alexa taking two images at different exposures and mixing them together for latitude, like a scanner? Or is there a much simpler explanation I'm missing.

Anybody have any thoughts?

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#2 Patrick Kaplin

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Posted 15 March 2011 - 10:51 PM

From what I understood from an Arri rep during a presentation of the Alexa, they are in fact pulling two exposures from the photosites and using them to increase latitude. Don't know how exactly, but I do remember him stating that the method they used (pulling directly from the photosites instead of affecting the shutter speed of one exposure to add to the standard 180 exposure) maintained the 180 motion blur characteristics.

If someone with some more technical knowledge about this could explain further I'd be very interested!
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#3 John Sprung

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Posted 16 March 2011 - 12:38 AM

The Alexa uses two analog amplifiers and two A/D converters per pixel. It doesn't mix different exposure durations like the Red MX.

What's the rest of the chain that produced that picture? It looks more like an error in some post equipment or software than a camera thing.





-- J.S.
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#4 Mitch Gross

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Posted 16 March 2011 - 12:57 PM

I agree with John Sprung. How was the image captured from the ALEXA, via Sx cards to ProRes or off the HD outputs? What system is being used to post the material, FCP or AVID or other? If not FCP, what generation of the other and how did the file get converted to get into the system?

Looks a lot like a frame merge similar to a 2:3 pulldown cadence. Just not possible coming out of the progressive-only camera.
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#5 M Joel W

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Posted 16 March 2011 - 03:08 PM

This is a huge long shot, but was the footage by any chance shot with HMIs on magnetic ballasts? Shooting with a 180ยบ shutter on such lights will produce almost exactly the phenomenon you've mentioned.
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#6 manigandan srinivasan

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Posted 03 January 2012 - 01:36 PM

I am just a student, i have not shooted with alexa yet. But looking at the picture i just have a guess if am wrong pls correct me
I feel that picture is giving that kind of blur. Because of "CMOS CENSOR" which records image similar to interlys scanning technology. i feel this problem wouldn have come with a camera with "C.C.D CENSOR".Because, i remember i saw a comparison footage of same image shot using cmos & ccd. In which cmos has this kind of image due to its image recording method.

Its just a guess am sorry if am wrong :)
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