Pubicado el : 2016-03-20 10:08:32
Since lately I've been working quite a lot with the Sony FS7 and even though I am not a director of photography, I do think that is mandatory for the AC to know the camera he is working with as good as he can, so he can advise the DoP even on matters such as the exposure. So I started diving in the web and I came up with a remarkable PDF document that, even though is a bit old, seems to me to explain very clearly how to get the best of this camera. If you want you can download it for free by clicking on the link below
There are two things I would like to stress here. First of all, what you are going to read is basically a resumed edition of the PDF (29 pages) with the subjects that I consider the most interesting. That means that the information, technical data and all the advicing are not mine, but as they are written from the autor of the PDF, mr. Alistair Chapman. Keep in mind that this man, who by the way seems to me to have done an outstanding job which is way I think you all should consider buy the man a beer or a coffee as you can do on xdcam-user.com (I did it anyway), is the one who makes official reviews for Sony Europe, so he is most likely right on most of camera related subjects.
Second, there is some moment where the sentence “correct exposure for any given log gamma or LUT” is used, and to prevent obvious commentaries, I would like to underline that with this sentence we are only referring to the exposure that grants us the higher performance of the gamma and allows us to use the whole dynamic range of the sensor. I know perfectly that the exposure too can be a creative tools, so there is no such thing as a “correct exposure”.
Well, if you have worked with the FS7 you will know, the camera gives us the possibility, likewise the majority of the rest of digital cinema cameras, to record the “baked” signal (already with the final look) which is the CUSTOM MODE; or to record using a logaritmic gamma (the Slog3 for instance) and output a LUT on the signal both on monitors and on board LCD. (CINE EI MODE). Even though to old school operator used to the XDCAM and Betacam Sony cameras the PAINT menu of the camera in the CUSTOM MODE could ring a bell, what mr. Chapman (and I agree) suggest is to always use the CINE EI mode, unless we are 110% sure that the footage wont pass by a coloring process. And even in this case, I would recommend to keep on shooting in the CINE EI mode, but with a LUT baked in, since the PAINT menu is extremely extense and difficult to totally master.
Before going any further I think it may be necessary a refresh on the ISO and EI values of digital cinema cameras. As we all know, the sensor of a camera only has ONE sensitivity, that for convenience we express in ISO or EI (and that it's supposed to be the one that the brand tells us is the native sensitivity). To the signal received by the sensor, we can apply more or less electronic gain. By doing that we amplify the signal, but remember that this process is applyed to the whole signal. A very useful example is made by Alistair Chapman: if we think about the video signal as it was an audio signal, the volume knob is our gain control. So if we raise the volume, we will ear everything MORE (carefull, not BETTER) noise and imperfections included. In the same way, if we raise the ISO of our camera, we are just “raising the volume” of our video signal equally so the noise will rise too. Oh boy, so the cameras cannot create light.... I knew we would get here, one day or another. Be carefull anyway: the ISO values do have effects on the dynamic range of the camera, but we will get there in a bit.
That said, to get in to the exposure subject, I think is better to start by recalling the functioning of logaritmic gammas, since the way mr. Chapman recommend to use the exposure is strictly related to that. While a non logaritmic gamma compress all the dynamic range of the scene into the broadcast standard, leaving a relatively small room to the highlights as we can see form the image below
a logaritmic gamma, even if has to compress the grey scale of the scene in to the same standard, record in a different way the signal for the highlights part of the curve, leaving the 90% white a little lower to give more headroom to the bright and very bright lights (91 to 110%) as we can see here.
That is way, the footage shot with logaritmic gammas usually look underexposed and lacking contrast. Following this lead, the author reccomends to set the 18% middle grey and the 90% white, with the help of the zebra or the waveform monitor, to values that change slightly depending on the gamma we are using as you can see here
It's very important to recall, since we are speaking about the tools for exposure of the camera, that these always measure the signal they are getting, so if there is a LUT applyed, the values will be according to that very LUT. Same as for the log gammas, for every given LUT the values change slightly, but we could say that the 18% middle gray should be somewhere around the 40% (the exact values for each LUT are detailled in the PDF).
So, as we were saying before, something you should always keep in mind when you are setting the exposure is that the ISO you set on the LUT you are monitoring or recordind, if doesn't change the dynamic range of the sensor, does affect the position of the middle grey and that means the usable stop you have on top of that and below it.
The explanation is plain and simple: if you raise the EI value, the camera “sees” more in the shadows, so we could say it gain latitude on the low lights. But since the dynamic range cannot change, what we gain below, we loose above. As a result, as the chart down here tells us, with higher ISOs, less usable stop above the middle gray. Doesn't make much sense, does it?
For all of this, the author reccomends always set the exposure by watching the LUT and set the ISO value on 1000 EI. This way, setting the aperture according to the LUT we would be costantly overexposing the log gamma by approximately 1 stop, so when the footage get to the post production pipeline, the colorist will most likely lower the exposure by one stop (6db), and doing so he would lower the noise too, achieving a cleaner and better quality image. If you want my two cents, I 80% percent agree with this, but I would keep a close eye on the highlights in the log gamma, since specially if we are shooting at a low EI, we don't have that much headroom.
Last but not least, it might be handy to recall that the camera gives us the possibility to load customized LUTs for on line on-camera monitoring, and it's pretty easy to do.
All you need is to download the free app SONY RAW VIEWER, ingest a clip that was recorded with the gamma you are going to use, create the LUT and export it as a .cube file to an SD card where you previously created the following structure
PRIVATE: SONY: PRO: CAMERA: PMWF55_F5
Then put the card in the SD slot of the camera, open the menu to FILE>MONITOR 3D LUT and select LOAD SD CARD to save the LUT on one of the 4 available positions.
Fabio Giolitti - Focus Puller