10-Bit Log from Sony EX1

1st test with the Kodak Print LUT applied.

I started playing with this as a bit of a joke and was a little shocked that it started to work.  To me, this was a little bit like trying to get a Cessna into space flight.  Cessnas are good but they were never meant to be in space.  The EX1 is great at what it does but was never intended to produce 10-Bit Log.  But here it is…

I was testing the Atomos Samurai recorder (more on that soon) and was thinking, how do you test a 10-Bit, HD-SDI recorder with a Sony PMW EX1?

People often fixate on things like 4:2:2 Vs 4:2:0 colourspace or other encoding factors in assessing a camera’s recording quality.  In reality things like these colourspace issues only affect the amount of colour information by about 20% and most of that is interpolated out anyway.

But 10-Bit recording is different, it gives you 400% of picture information for only 125% of the data compared to 8-Bit.  The only trick is that the difference is generally not visible to the naked eye.

Uncorrected Log image from Samurai

10 Bit-Log Background

This extra information is in the subtleties of the tonal range across the colours.  In designing the Cineon digital film format, Kodak decided to use this vast palette of tonal information to capture the raw images off a film negative as accurately as possible.  So they emulated the logarithmic exposure curve of the image on the neg.

The result was that the Cineon Log format also emulated one of the great strengths of film in it’s capacity to record highlight and shadow information beyond what was seen in the print.  A Log image needs to be corrected by applying a Lookup Table or LUT to emulate the printing process and create a normal looking image before colour grading or image manipulation can be done.

The Log exposure curve of Kodak 5218 neg (500T)

The Cineon file format and it’s successor the DPX file became the standards in Digital Intermediate work for the big screen.

The first digital video camera to recreate the 10-Bit Log format was the Thompson Viper and it used it’s internal digital processing to re-map the tonal values from it’s image sensors to match the Kodak Log Curve.  Although the Viper itself was impressive, when I first tested it I discovered that it required a hard disc recorder about the size of a bar fridge to to record 10-Bit Log!  This situation was soon improved when Sony introduced the HDCAM-SR tape format that could record in 10-Bit on a recorder that was a bit bigger than a carry-on bag (HDW-SRW1).

Since then the 10-Bit log image recorded to either tape, hard disc  or solid state memory has enabled a number of high end cameras to gain acceptance as a viable alternative to film.  These include the Panavision Genesis, Sony F23 & F35 and the Arri Alexa.

All of these use variations on the log concept… such as Panavision’s P-Log and Sony’s S-Log.  These make use of an exposure curve similar to Cineon Log, with room to spare at the top and bottom of the range and a very low contrast image to capture as broad an exposure as possible.

 The Experiment

The first idea was to see if I could use the EX1′s settings to create something that looked a bit like a log image and experiment with how that reacted to being stretched back out to a normal image since this would be a dramatic test of the 10-Bit recording in the Samurai.  When I took this image into Combustion, I tried putting a Kodak 2383 Print Stock LUT on it and although it wasn’t right, it wasn’t nearly as wrong as I expected it to be.

So I tried going a lot further with the settings and with a little bit of trial and error, got something that responded pretty well to the LUT and could be colour graded effectively behind the LUT.  That’s the image of the leaves shown above. Click on the image and then again to see it at full 1080p res, but bear in mind that there are some JPEG artefacts from the web image.  This was also just a guess at exposure from the histogram.

To determine a better exposure level, the Sony S-Log white paper was a very useful source of information.  They determined that a Kodak Grey Card exposed at 38% digital exposure provided the optimum latitude.

The image below was shot with the Log setup and exposed at 320 ISO (or ASA) for 38% with an incident reading balanced for full sun.  The Kodak China Girl image overlayed provides a reference.  Although I have modified the LUT for this shot, the China Girl (from a DPX file) shows that the Kodak reference image responds in a very similar way to the same corrections.  This was done by comping the China Girl over the source image, nesting these and applying the LUT and colour correction to the nested images as one.

Original Log Image

Original Log Image

With Look Up Table Applied

 B-Log

Not blog… B-Log.  If Sony and Panavision can do it, why can’t I? :-)  Using the new Log settings as a base, I then started to make a bit more efficient use of the tonal range the way that S-Log and P-Log do.  The following pictures were shot using those settings and again rated at 320 ISO for a grey card at 38%.

 

"B-Log" Test in Log form.

"B-Log" Corrected.

This is just a camera in a carpark on a rainy day, but the image it’s recording is retaining highlights nearly 5.5 stops over key exposure.  The reflections of the sky on the silver car were returning a reflected reading of 4.5 stops overexposed, and weren’t clipping.  Meanwhile, there is still detail in the tyres of the foreground car that are around 5 stops under.  That’s in the ballpark of the F23, Genesis and F35.  To get the highlights on the car to be well and truly clipped, I had to overexpose the main image by 2-Stops.

 Conclusions

These are very much preliminary tests.  I’m not sure yet whether these settings would be reliable enough for a real project and the big question is if the signal to noise ratio of the EX1 will handle these extreme settings in different situations.

What I can say, so far, is that the EX1 can be pushed much further than I previously imagined and that the Samurai is able to capture all of that detail.

The next steps will be to check how it responds to a sequence of shots and to more pretty pictures than what I’ve tested so far.

More info and for video images included in Samurai Review.

 
CLICK HERE  to download the B-Log Setup File for the EX1.

 

UK based DoP Alister Chapman has been following up on this and has done some great tests with both the EX1 and his S-Log equipped F3 and is getting some even more intriguing results on his blog.

 

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