Arri have a little trick in their cameras when shooting log to ProRes that the Sony Log cameras don’t have. When you change the Exposure Index in an Arri camera they modify the position of the exposure mid point and the shape of the Log-C gamma curve. There is actually a different Log-C curve for each EI. When you take this into post it has the benefit that the brightness at each EI will appear similar. But as the curve changes for each EI a different LUT is needed for each exposure if you want something shot at say 800EI to look the same as something shot at 200EI.
With a Sony camera the same S-Log curve is used for each Exposure Index and the LUT brightness is changed so that you end up altering the mid point of the recording as well as the highlight and shadow range. In post each EI will appear to be a different brightness. You can use the same LUT for each EI provided you do an exposure correction prior to adding the LUT or you can use dedicated offset LUT’s for each exposure.
But what you need to remember is that you are always working within a restricted recording range with either system. You can’t go darker than the black recording level or brighter than the highest value the codec can record.
If you do it in camera, as Arri do and change the log curve, at a low EI you seriously constrict the recording range (at 200 EI peak only reaches around 78IRE). This happens because at a low EI you put more light on to the sensor. So to keep the mid range looking a normal brightness in post it must be recorded at at a level that is offset downwards compared to normal. So with all the levels now offset downwards to compensate for the brighter exposure you end up recording your entire capture range with a reduced or compressed recording range. In addition to avoid clipping the blacks at a low EI the shadows are rolled off so you lose some detail and textures in the shadows. You can see the different Log-C curves in this Arri White paper.
Most people choose a low EI for 2 reasons, better signal to noise ratio and improved shadow range. The Arri method gives you the better SNR but while the dynamic range is preserved it’s recorded using less data and in particular the shadow data decreases compared to shooting at the base ISO.
Shoot at a high EI, you put less light on to the sensor. So to maintain similar looking mids in post everything has to be recorded at a higher level. Now you have a problem because the highlights will extend beyond the upper limits of the recording range so Arri have to add a highlight roll off at the top of the Log-C curve. This can present some grading challenges as the curve is now very different to regular Log-C. In addition the highlights are compressed.
Most people choose to shoot at a high EI to extend the highlight range or to work in lower light levels.
The latter is a bit of a pointless exercise with any log camera as the camera sensitivity isn’t actually any different, you are only fooling yourself into thinking it’s is more sensitive and this can result in noisy footage. If you using a high EI to extend the highlight range then really the last thing you want is the extra highlight roll off that Arri have to add at 3200 EI to fit everything in.
One thing here in Arri’s favour is that they can record 12 bit ProRes 444. 12 bits helps mitigate the compressed recording range of low EI’s provided the post workflow is managed correctly.
The beauty of the Sony method is the recording range never changes, so low EI’s and brighter recordings deliver better shadow ranges with more data in the shadows and mids and high EI’s with darker recordings deliver better highlight ranges with no additional data restrictions or additional roll-offs giving the cinematographer more control to choose the exposure mid point without compromise to the data at either end.
But it does mean that post need to be awake and that the shooter needs to communicate with post regarding the brighter/darker looking images. But to be honest if post don’t understand this and recognise what you have done either buy just looking at the footage or checking the metadata what chance is there of post actually doing a decent job of grading your content? This should be fundamental and basic stuff for a colourist/grader. For a colourist/grader to not understand this and how to work with this is like hiring a camera operator that doesn’t know what an ND filter is.
The Sony FS7/FX9/F5/F55/Venice cameras can do something similar to an Arri camera by baking in the S-Log3 LUT. Then in post the exposure will look the same at every EI. BUT you will lose some highlight range at a low EI’s and some shadow range at a high EI’s without gaining any extra range at the opposite end. As a result the total dynamic range does reduce as you move away from the base ISO.
In addition on the Venice, FS7/F5/F55 (and I suspect in a future update the FX9) you can bake in a user LUT to the SxS recordings. If you create a set of S-Log3 to S-Log3 LUT’s with EI offsets included in the LUT you could replicate what Arri do by having an offset and tweaked S-Log3 User LUT for each EI that you want to shoot at. You would not use the cameras EI control you would leave the camera st the base ISO. The LUT’s themselves will include the exposure offset. These will maintain the full dynamic range but just like Arri they will need to roll off the shadows or highlights within the LUT.
But monitoring will be tricky as you won’t have the benefit of a 709 type LUT for monitoring so you you may need to use an external monitor or viewfinder that can apply a LUT to it’s image. The good news is the same LUT would be used in the monitor for every version on the offset S-Log3 LUT that you are baking in as the exposure brightness levels will be the same for each offset.
So here you are a set of 4 S-Log3/S-Gamut3.cine offset LUT’s for those Sony cameras that will take a user LUT. I have named the LUT’s – 2S Down SL3C, 1S Down SL3C, 1S UP SL3C, 2S UP SL3C.
The name means (Number of Stops) (Down or Up) (Slog3.Cine).
So if the cameras base ISO is 2000 (F5/FS7 etc) and you want to shoot at the equivalent of 1000EI, which is 1 stop down from base you would use “1S Down SL3C”.
As always (to date at least) I offer these as a free download available by clicking on the links below. But I always appreciate a contribution if you find them useful and make use of them. I will let you pay what you feel is fair, all contributions are greatly appreciated and it really does help keep this website up and running. If you can’t afford to pay, then just download the LUT’s and enjoy using them. If in the future you should choose to use them on a paying project, please remember where you got them and come back and make a contribution. More contributions means more LUT offerings in the future.
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