Understanding Log and Exposure Levels (also other gammas). PLEASE READ and understand.

Please, please read this and try to understand how shooting with a high range gamma curve such as a cinegamma or hypergamma or log recording works. The principles are not well understood by many, even highly experienced DP’s and DIT’s get this so horribly wrong.

Why do so many get it all wrong? Because we are brought up used to looking at a monitor or viewfinder and seeing a picture that looks correct.

Why doesn’t the picture look right when we shoot log (or other extended range gamma)? It’s simply because the monitor does not have the right gamma curve (unless you have a log monitor), so there is a miss-match between the camera and monitor.

So what does this mean? DO NOT USE THE MONITOR TO JUDGE YOUR EXPOSURE unless you have a well calibrated Look Up Table between the camera and monitor!

For many people this takes a huge leap of faith. To shoot with a picture that looks wrong goes against everything most camera people are taught. Directors and producers will look at the monitor and not like what they see, perhaps encouraging you to adjust your exposure, because it looks wrong. In the end many give in and instead of exposing the Log or other gamma correctly they will adjust the exposure to something they are more comfortable with, something that is a bit brighter. But this is a mistake, an easy one to make but one that may mean your pictures just won’t look as good as they should. Please see this article on exposure with extended range gamma curves.

Some more things to consider before I go further:

Most TV and film production monitors are based on the REC-709 standard. The input into these monitors will normally be digital, either HDSDI or HDMI.

A digital signal contains a range of data values. For 10 bit video we have a total range of data bits from 0 to 1023. Our monitor will show data bit 64 as black (the values below this used for super blacks and sync) and data bit 1019 will make the monitor show the brightest level that it can. Normally data bit 940 is considered “white” and anything above this is brighter than white. It may be 8 bit not 10 bit, 8 bit uses values from 0 to 255. For this article I will use 10 bit values, but the principles are exactly the same whether 10 bit or 8 bit. Also I’m only considering brightness here, not colour.

A typical LCD monitor or TV set has a very limited contrast range and can only display about a 6 or 7 stop dynamic range. OLED’s are a bit better.

Thanks to the Rec-709 gamma curve in the monitor, when we send data bit 940 to the monitor we see what appears to be white. Send bit 64 and we see black, send bit 440 (approx) and we see a shade of grey that appears to be half way between black and white, also known as middle grey.

Middle grey is approx 2.5 stops darker than white (as in a piece of white paper or similar) and if we go around 2.5 stops darker than middle grey we will see something very close to black. So we can see that using bits 64 to 940 we will get around a 5 stop dynamic range on the monitor with a bit of extra range from bit 940 to 1019, so overall there’s our typical 6 stop monitor range.

Now, what happens then if we have a camera with a much greater dynamic range than 6 stops? Well, the monitor can never show the cameras bigger range accurately as it can only ever show 6 stops, if we feed say 14 stops into the monitor the brightness range on the monitor will still only be 6 stops. So now the contrast of the picture is reduced as we are squeezing the cameras large contrast range into the monitors much smaller contrast range.

Now lets consider the camera.

Lets consider a Rec-709 camera. If I shoot a white card, I record it using bit 940, if I shoot a grey card I record it using bit 440, that way the white card looks white and the grey card looks grey on my monitor which uses those same levels for those same shades, then I have a little bit of extra space above 940 for a little extra dynamic range. Remember, near black to white is approx 5 stops of dynamic range.

But what if I want to extend my range beyond 5 stops? If white is bit 940 and my top limit is bit 1019, I really don’t have a lot of data space to record a load of extra range, so I have to do something else.

What do the camera manufacturers do to record a bigger dynamic range? They shift the data values used down. Taking SLog2 as an example, instead of using bit value 940 to represent white, they now use bit 600 (approx) and for middle grey, instead of bit 440 we now use bit 347. This now gives us a large amount of spare data from bit 600 to 1019 to record a greatly extended range beyond our original 5 stops.

This shift downwards of our data levels does not just happen with log recording it also happens when you use almost any non-standard gamma curve. For example Sony’s Hypergammas and Cinegammas also lower the bit value for white down to between bit 700 and 800 and middle grey can go as low as bit 320 (depending on the curve used). Again this then frees off extra data above bit 800 to extend the dynamic range beyond our Rec-709 6 stops.

But this now gives us a problem. If I am using SLog2 and expose CORRECTLY and as a result record middle grey at bit 347 (32%), when I send bit 347 to my Rec-709 monitor it will look dark because a rec-709 monitor will show bit 347 as darker than bit 440 so darker than the normal middle grey displayed by the monitor.

It’s very, very important to understand that just because the picture looks dark, you are NOT under exposed in any way. It is just the miss-match between the camera an monitor that is making the picture LOOK dark. IT IS THE MONITOR THAT IS WRONG NOT YOUR EXPOSURE.

Now the next common mistake is the thought that: “OK, my picture looks dark, so when I take it in to post production and raise the levels, it’s going to get noisy”. Well, this is to small degree true but it is not nearly as bad as many assume. The reason it’s not as bad as many assume is that you must remember that YOU WERE CORRECTLY EXPOSED. You are not trying to lift an under-exposed image. Remember what I said at the beginning: “The noise in a digital camera comes almost entirely from the sensor”.

So, with the same camera, if we expose any given gamma correctly then as the amount of light falling on the sensor is the same, the ratio of sensor noise to signal coming from the sensor does not change. So taking a face as an example, exposed correctly (ie. with middle grey at the correct level for the gamma curve in use) the amount of noise on that face will remain constant across all the different gamma curves. Do note however that some cameras may have different ISO ratings for different gammas and this might have a small impact on noise levels (but that’s the subject for a different article).

Now consider what happens when we go into the edit suite. If the gamma you are shooting with is quite close to the gamma curve of your target display device, which in most cases will be Rec-709 for TV or the Web. Then a small level change in post will bring middle grey and your whites up to the level the monitor is expecting and won’t add any significant noise, after all we are working with digital images and digital processing and don’t forget – you were not underexposed, just using different data bits to represent different brightness levels.

But what about a more aggressive gamma curve like SLog2 or any other log gamma. This is going to need some big level changes, surely this is going to get noisy. Again, no, not if you handle it correctly. You really should be using a dedicated grading tool for any log material as this will apply corrections that are designed for log and this will minimise any added noise. But the other thing to consider is that this is where you should be using a LUT or Look Up Table on your output to convert you data values from Log values to Rec-709 values.

By placing a LUT on the output of your project you shift your data levels from one range to another. Your grading is done to the original material in it’s original range so that you can retain that full range and then your LUT is used at the end of your grade (on the last node) to then convert your data values from log values to 709 values. When you do this you are simply moving your data values. So if the original input value for a part of the image is is bit 347, SLog2 middle grey for example. On your output you just use bit value 440 (709 middle grey) instead. Your just transposing data from one range to another and this does not add noise in the same way as adding gain does.

Now, looking at Log and the way it works. You should note that in order to squeeze 14 stops of dynamic range into our normal recording codec you use a lot of compression in the brighter stops. Remember, every time you add a stop of exposure, to record everything in that additional stop you should be recording the new stop with twice as much data as the previous. But that’s impossible with conventional recording, the amount of data required is simply too big. So log records every stop using roughly the same amount of data. This means that the brighter stops are very highly compressed, so it’s very important not to over expose log to get the best results.

So in summary: When you shoot and expose correctly with a gamma curve with a large dynamic range (cinegamma, hypergamma, log etc) it will look darker on your conventional monitor or viewfinder. That is how it should be, that is correct exposure, you are not underexposed, so the picture will not be noisy. The dark looking picture is because your monitor gamma does not match the cameras, it is the monitor that is wrong, not your exposure. The picture will not be noisier than any other correctly exposed picture, even though it looks dark because of the monitor miss-match. So have the confidence to shoot with these slightly dark looking images. Especially if your shooting log where over exposure can seriously compromise your end results.

This entry was posted in Uncategorized. Bookmark the permalink.

22 Responses to Understanding Log and Exposure Levels (also other gammas). PLEASE READ and understand.

Leave a Reply

Your email address will not be published. Required fields are marked *

This site uses Akismet to reduce spam. Learn how your comment data is processed.