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Varying exposure during scanning via a 2-pass method


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#1 Guy Burns

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Posted 30 May 2016 - 06:17 AM

8mm home movies, if the amateur cameraman wasn't careful with his settings, can have the problem of exposure varying widely from scene to scene. And even though varying the frame-to-frame exposure during scanning is undesirable, I can see that there might be benefits in altering the exposure scene-to-scene.

 

Ques 1

Are any high-end scanners capable of doing a 2-pass scan? i.e. the first 'preview' pass works out where the scene changes are and calculates exposure; then during the second pass, the exposure is altered to boost dark scenes, or reduce bright scenes.

 

Some providers offer a manual version of the above – a "best light" service whereby they go through the footage scene-by-scene and balance out any inconsistencies in the lighting, whether under or over-exposed. 

 

Ques 2

Could such a 2-pass scheme have unintended consequences?

 

 


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#2 Phil Rhodes

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Posted 30 May 2016 - 06:19 AM

Quite a lot of homebuilt s8 scanners will do an HDR pass, where each frame is flashed two or even three times (two or three times per colour channel, if it's a monochrome sensor and RGB backlights). This seems to create data which is capable of representing the entire density range of the optical medium, so variations can be worked out in software.

 

But yes, you could do two passes.

 

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#3 David Mullen ASC

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Posted 30 May 2016 - 11:53 AM

There are two-pass scanners designed to maximize the dynamic range available in the original piece of film, but a film scan is not really designed to fix exposures, they are just supposed to get you everything possible off of the original film for later color-correction.


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#4 Guy Burns

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Posted 30 May 2016 - 01:48 PM

Thanks for the responses. Not the most sensible idea I've thought of. Came to me when I was thinking of how Premiere improves the quality of its exports by using a 2-pass render. But a 2-pass scan: a bit sus.

 

However, the mention of each frame being flashed more than once reminded me of something I'd read, and I eventually found it again: the triple-flash of Lasergraphic's Director.

 

Ques

Does triple-flash mean that you end up with three files from each scan? Much like when you set a digital camera to take three exposures: normal, +1, -1?


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#5 Phil Rhodes

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Posted 30 May 2016 - 06:56 PM

Usually the software will combine the three shots into a single, often higher-bit-depth file, much as Photoshop will with three exposures from a DSLR.
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#6 John Rizzo

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Posted 31 May 2016 - 05:41 AM

The lasergraphics director uses a monochrome sensor with RGB light source so in the HDR mode each frame is flashed either 6x or 9x  and it combines the data in the raw output.The Director however is not capable scanning 8mm film, 8mm is only available on the Scan Station which does not have hdr capabilities.   


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#7 Bruce Greene

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Posted 31 May 2016 - 11:03 AM

My Nikon film scanner (for stills only:), has a multiscan feature to maximize dynamic range from the scan.  From my experience, all color negative film's dynamic range fits within a single pass, and multiscan has no benefit.  Multiscan does help sometimes with reversal film (slide film), but it's also limited by the limited dynamic range of the original slide, but it helps pull a little detail out of the most dense areas at the expense of long scanning times.


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#8 Guy Burns

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Posted 31 May 2016 - 02:40 PM

My Nikon film scanner (for stills only:), has a multiscan feature to maximize dynamic range from the scan.

 

I assume by "Nikon Film Scanner" that you mean one of the Coolscans. I was not aware they had a multiscan feature. The Coolscan 5000 has a Multipass feature, but that's intended for reducing noise. See the tests done by Martin Ranger (website now gone) which I've archived here: http://www.mediafire...b/Documents.zip

 

He concludes:

 

While Multipass Scanning does not seem to degrade the sharpness of the scans, it offers little benefit in the form of reduced analog noise. Given the significant increase in scanning time, one might think twice about using it.


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