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Filed under: Cameras •Training •

The Not-So-Technical Guide to the Sony F35

CRAMMING 12 STOPS INTO A FIVE STOP BUCKET

Anyone who is familiar with the menu system of a Sony F900R already knows the vast majority of the menus in the F35. Several of the primary gamma curves are roughly the same. We’ll go into those in a moment. But first, let’s talk about why we need aggressive gamma curves.

The original HD spec is based on the old NTSC television standard of five stops of dynamic range, encompassing 0-100% on waveform monitor. That’s it. The Rec 709 HD standard only allows for about 2.5 stops of overexposure latitude, between approximately 45% (or 18% “middle” gray) up to 109%, as seen on a waveform monitor.

Our happy misfortunate is that sensors have become much, much better and can now see well beyond the five stops of latitude provided for in Rec 709, but all of our engineering and monitoring tools and data pipelines (HD-SDI) are designed around the Rec 709 legacy of having a 0%-109% “bit bucket”. At a demonstration of the Sony F23 held by Videofax in 2007, local video engineer Fred Meyers described how he came up with the concept of the bit bucket while doing tests for the film Speed Racer. “The trick,” he said, “is not only using the biggest bucket possible, but making sure you fill it all the way.” We’ll talk about this a bit more when we start comparing curves.

How do we cram a total of 12+ stops of dynamic range into a five stop bit bucket?

Those of you who said “bigger hammers” are banished to the grip truck. The correct is answer is “aggressive gamma curves.”

The following graphs are intended to communicate general ideas and are not exact measurements of anything known to man.

The Rec 709 gamma is a very simple one, and although it has a curve to it it’s easier to envision as a straight line:

At around two to two-and-a-half stops over middle gray, exposure hits a hard ceiling and clips. This is completely unlike film, where at some point one stop of exposure change does not result in a one stop difference on film—and the highlights slowly and gradually lose detail until they disappear into featureless white.

Knee circuits help somewhat but never really do the trick. They try to create an artificial slope that’s shallower than the Rec 709 curve but doesn’t roll off gently the way film does:

Knee circuits are notorious for causing color distortions in highlights, which is why we never use them to reign in highlights on flesh tone: the affected areas tend to turn a metallic green color, which does no one any favors.

A curve is fundamentally different. While a knee circuit is just trying to make blown-out highlights look a little prettier by bringing the exposure in the highlights down in an attempt to make detail visible, curves are actually grabbing extended dynamic range information available on the sensor and remapping it to fit into the confines of the Rec 709 spec:

Instead of starting at a knee point and trying to force detail out of bright, desaturated highlights, a curve gently draws information from beyond Rec 709’s normal cutoff down into the 0-109% bit bucket. Sony describes how much information the camera is recording beyond Rec 709’s usual boundaries by displaying a percentage number on the side of the camera. As best I can tell the percentages break down in terms of doubles and halves—the way most everything in photography works:

100% is what would normally have been the broadcast clip point for Rec 709, about two stops over middle gray.

200% is another stop beyond that point that the sensor can see but couldn’t fit into the Rec 709 gamma space without using a curve.

400% is another stop beyond that point.

650% is a normal “limit” to how much information can be pulled off the chip, and represents about a half stop increase beyond 400%, or an additional 2.5 stops of information that normally wouldn’t fit in Rec 709—for a total of 4.5 stops of exposure latitude above middle gray. (In some modes Sony claims a maximum of 800%, or an additional half stop beyond 650%.)

Depending on the curve used and the gain applied the percentage number will increase or decrease, giving you an idea of how much dynamic range the camera offers in that particular operating mode.

What do these curves actually look like? You’ll see on the next page…

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Thanks, Art!  That was excellent.  Next time I shoot with an F35, you’ve probably saved me a couple of hours worth of phone calls and a ton of headache.
-Graham

Posted by Graham Futerfas  on  01/14  at  09:43 PM

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Thanks for your kind words, Graham. Stay tuned to this channel… there’s more. Believe it or not, there’s more.

Posted by Art Adams  on  01/14  at  09:47 PM

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Great piece, Art. The article is a sort of hypergamma curve of information, helping to fill my (small) bit bucket of comprehension to the brim! I’ve just got to work on lowering the noise floor in my brain now…......!

Chris

Posted by .(JavaScript must be enabled to view this email address)  on  01/15  at  04:20 AM

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I try not to get too technical because (1) it makes my brain hurt, and (2) I don’t need to know how to build a camera, I just want to know what it’s doing so that if something goes wrong I can troubleshoot, or anticipate things going awry.

I hope the article helps in that regard. Believe me, I know where you’re coming from—which is why I’m trying to bring it down to a level where we can understand what’s happening without resorting to serious math. (I’m just awful at math.)

The more I learn, the more I learn that there’s more to learn. My focus is on the practical aspects: how does this affect my next job? Theoretical knowledge is interesting, but unless it has a practical application it just adds to my own noise floor.

More to come… stay tuned!

Posted by Art Adams  on  01/15  at  01:23 PM

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Wow this article is better than a Christmas present. I’ve been looking for this kind of info for ages.

But I have some questions after reading all the article:
You said that the NTSC standard goes from 0-100% on a waveform and the REC 709 form 0-109% so this would mean that we also have to convert any footage from any camera that shoots in the REC colorspace down from 109 to 100 .
The hypergammas that only go from 0 to 100 for broadcast purposes still use the REC standard right ?

I’m a little confused about the 109% percent thing. From my knowledge a waveform tells us the strength of a video signal. What does a signal beyond 100% mean and more importantly how is it cuantified by the sensor of the camera ? Doesn’t itmean that at 100% the photosites on hte sensor are already filled with all the light that they can collect ? Where does that 9% extra go ?

I didn’t undersantd form the article what kind of sensor the F35 is using ? CCD , CMOS , another type ?

Thanks again for the great article!

Posted by .(JavaScript must be enabled to view this email address)  on  01/15  at  05:29 PM

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Okay, I’m going to start with the easy one first. It’s a CCD camera.

As far as the 100% and 109% go, I believe 109% is the max under Rec 709, but broadcasters are still limited to 100%. So you can capture up to 109% in the camera, but whether you can use it depends on whether you are (1) broadcasting the image, and (2) if you are broadcasting the image there needs to be a color correction step that can reduce those 109% levels into the 0-100% range using some sort of post curve correction.

If you shoot using the entire 0-109% range and then broadcast it the transmitter will hard clip the whites at 100% and you’ll lose a bunch of highlight detail. Hypergammas 1 and 2 are designed such that if you know you are going to broadcast the footage, and there’s never going to be any post color correction, then there won’t be any surprises when it goes to broadcast.

If you can record the full 0-109% range, and you’re either not broadcasting it or someone in post can reign in your highlights, then it’s well worth it to use Hypergammas 3 and 4 to grab that extra highlight detail.

I believe 100% is called “White” and 109% is called “Super White”. I believe the original NTSC broadcast spec only called for 0-100%, but the digital spec came up with some additional range as a response to camera sensors getting better at handling highlights. The best way I can explain it is “This one goes to eleven.” http://www.youtube.com/watch?v=d54UU-fPIsY

Posted by Art Adams  on  01/15  at  05:41 PM

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Nice analogy Art :D

Posted by .(JavaScript must be enabled to view this email address)  on  01/15  at  11:07 PM

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