Squeezing the most out of a 5D Mk II or ALEXA signal

“So if you are a really keen young Camera operator and you see EXTENDED range – you’re going to pick it – I mean who wouldn’t? Normal and Extended – you’re 26 – you’re going to pick Extended,” joked Charles Poynton as part of his lecture at fxphd last week.

What Charles was talking about was the Extended range on the ARRI Alexa, but the same discussion could be had about the Marvels Cine Profile for the 5D vs the Technicolor Cine profile.

The theory is that in many cameras there is a set of levels. Let’s say for the purposes of this article ONLY – that they are between 0 to 100. Now imagine that you discovered that your camera had values mapped for black to white that only went from 5 to 95. In other words, you film with the lens cap on and it would internally store that number at 5. And if you way way overexposed, the best you could do is an internal number of 95. Of course, on your screen or in the cinema it would be black and white respectively – but in effect you have mapped all the range of light levels from black to white just between 5 and 95 – and in theory there is no way to get below 5 and or above 95. It would seem like a waste perhaps? You might even look for the button that says EXTENDED – that did not do that. Why not map between 0 and 100 if that is black and white?

Marvels Cine Picture Style is an option for the 5D. I am sure it works for some people, but I was alarmed when I read their web page. Here is a quote from the Marvel Cine Profile web page, which highlights the problem and reads as if the Technicolor Cinestyle would be inferior. This is frankly at best odd, and at worst, simply inaccurate:

Warning to all users of the Technicolor Cinema-style flat-profile! Using the Technicolor style will severely limit your dynamic range! Blacks are lifted and whites are crushed! With only 255 values between black and white for each color, snooping 10% off at both sides is simple “DR theft”! You remove another 10% of DR when using the Technicolor LUT (via LutBuddy); that’s probably two stops total!

There are many things wrong with this statement.

First it reads like a warning, which is inflammatory. I mean, we are all on the side of getting the best out of our next shoot aren’t we? No need for the drama. But, perhaps, that is just me.

Second, it says the whites are crushed and the blacks lifted. Yes, the blacks are lifted, but the whites are not crushed. Crushed implies that data is clipped off. It is not the whites that are clipped – or even crushed – as long as they are adjusted along with everything to fit into a more restricted range.

Then there is that “theft” remark. Well now, you really need to start discounting the marketing hype ! Why is it not theft? Why is EXTENDED range or using the full range not such a great idea?

For a start, ARRI — one of the most respected camera companies in the world — does not make this ‘theft’ mode normal. It is not the default. Technicolor represents some of the best technical minds in the industry, they have no incentive other than image quality to get this right. Note that Technicolor gave away Cinestyle for free. I will leave that for you to interrupt motives.

But again – why isn’t it theft? The answer is simply noise.

Imagine we only focus on noise in the blacks. That is a sensible thing to do…after all if you have 10 units of noise over a very small dark signal of say 20 units, the noise is half the signal. Man, will I notice that. And yet if I have the same 10 units of noise over a signal up at 70 units (say 70% up the light scale) then 10 over 70 is not half, it is 14% (10/70). That is why we record signal to noise ratios (normally in dB). The same amount of noise in the blacks as in the whites looks worse as there is a higher signal to noise ratio.

One more point before I prove my noise ‘anti-theft’ theory. Not withstanding my point above, you might argue there is actually more noise in the blacks due to heat. Good point. In a sensor, photons come in and they get collected in little conceptual ‘wells’ and the photons cause electrons. Every frame, we read out the number of electrons and bingo we have the light level for that pixel for that frame. But heat also causes electrons, this is the dark body noise; the noise the actual silicon makes given it is hot (and yes it does rise as the camera is on filming longer).

But the clever guys at your camera company know how much we hate noise so they make some sensor ‘pixel’ on the CMOS chip – some normal ‘wells’ or ‘pixels” and then cover them with aluminum, they block them off completely to light. This gives a great idea as to the noise the other active ‘wells’ are getting that have nothing to do with light. Imagine your camera only had 2 pixels – the real one and this dummy one (the one covered over). The main ‘pixel’ well reads 92 photons and the covered over one reads 7 photons. But, of course, the covered one is locked shut so it is really reading 7 photons of noise. By subtracting the dummy reading from the active reading (92 minus 7), we effectively take out the noise in our one and only pixel image. This makes it reasonable to assume that the other active pixel is really only reading 85.

However, noise is not constant. So that one active pixel might have actually had 6 or 8 fake photons of noise. We don’t know. But we all agree if we take out most of the noise it is a lot better off. Even if a small amount of noise remains, most of it is gone.

So how does this relate to our extended range or ‘theft’ remark?

Well, of course our cameras have a lot more than one pixel, but the principle is the same. We sample a set of blocked off dummy pixels, work out the average (this is important), and subtract the average from the active pixels. This gets most but not all of the noise out. Sometimes the average we subtract was not enough; some times too much. And don’t forget there are SO many pixels or sensor points on a CMOS chip that the math is really valid.

To illustrate, I am going to imagine that there are only 10 active pixels and, as above, keep the math simple and use some dummy numbers to illustrate the point. Here are our 10 pixels and the numbers on those pixels when the sensor thinks it is seeing black:

6, 7, 6 ,3, 9, 15, 6, 5, 10, 3

The covered over dummy sensor ‘pixels/wells’ read an average of 7. So lets subtract out the 7 to get:

-1, 0, -1, -4, 2, 8, -1, -2, 3, -4

Note the average of these numbers is zero. Add them up yourself. The average is zero. We took out the noise…and sure, some pixels are up some are down, but on average there is NO average noise.

Now imagine you stored these numbers without any offset. In other words, we store these numbers as per Marvels Cine profile. We set zero to zero and only store positive numbers… we get


The average noise is now … 1.3 – it went up, the image is, on average, now noisier !

If we had some room at the bottom and did not cut off zero at zero and saved the file with zero lifted up, yes, the BLACK LEVEL lifted… we would have less noise. If we just saved some of those negative numbers then as we moved into post, we’d have less noise. No signal processing – no tricks – just simple maths.

but this goes to 11
There is a trend to have an extended range of signals from digital cameras, which is fine. It is an option. ARRI allows this option but does not make this option the default. Technicolor made the decision that we all hate noise and thus they compressed the white to black to this slightly reduced range – but they did not cut off any values – they did not crush the values – they simply remapped them – it is exactly like the scene from Spinal Tap… Marvels Cine claims to go to 11.. “well why not make the loudest 10 and call that the loudest? – yeah but this goes to 11”
But in the case of mapping to the extended range has the side product that you raise the noise floor.
Is this the end of the world? – no. It is just a decision you should make well informed, you dont mind the noise – fine, but lets talk calmly about this and with real facts.

Personally I will stay with Technicolor, for two reasons:

  1. I like it and I hate noise.
  2. Technicolor worked directly with Canon and got access to the chip further up stream in the image processing pipeline that ever before, Canon worked with Technicolor.

But that’s just me.

Re the ARRI Alexa: Let me just quote from Nick Shaw, Workflow Consultant London, UK, who’s opinion I trust.

I have seen a lot of problems caused by people not understanding the difference between “legal” and “full/extended range” signals. If in doubt, stick to “legal”. You may have slightly fewer code values to play with, but you do not risk your image data getting clipped at some
point in your workflow.

As for Charles Poynton – I thank him for his work with us last week. The great news is that he is teaching all of this at fxphd this term in his advanced level course, DCT301 – Camera Tech and Colour Science.

By Mike Seymour