This is a question that keeps popping up:
What do the zebras measure when shooting S-Log3 using the CineEI modes in the FX3 and FX30?
The convention for zebras with the majority of cameras is that zebras are a viewfinder applied measurement. As such they almost always measure the “viewfinder” image. As the LCD on the FX series cameras is in effect the viewfinder, the zebras measure what you see on that screen. So, when you have a LUT on, the zebras measure the LUT, not the S-Log3.
Common ways to use the zebras include measuring skin tones, which for the default s709 LUT will be somewhere in the region of 60% depending on the face brightness. You could also use the LUT’s to measure the brightness of a white card or white piece of paper which should be around around 81% for a proper white card or 83% for white paper.
You could also use Zebras to indicate when you are close to clipping Depending on the LUT that you are using the peak LUT output will typically be at 100%, so a common usage would be to have Zebra 2 (which measures from the zebra point and everything above) set to a touch below 100 to act as a clipping indicator. BUT it must also be remembered that depending on the Exposure Index in most cases the LUT will have a lower highlight range than the S-Log3 recordings. So, when your highlights hit 100% on the LUT there may still be available headroom in the S-Log3 recordings. If you end up backing off your exposure every time the LUT clips you may be missing out on the full recording range and un-necessarily bringing the mids and shadows down. So, my preference is to measure the exposure of a white card or skin tones and to get the mid range and shadows right, rather than obsessing over small amounts clipping.
The s709 LUT does fit the full highlight range of the S-Log3 into it’s output. But as there is only a tiny difference between +5 and +6 stops (approx 1.5%) it is very difficult to determine what is clipped and what is 1 stop below clipping. +4 stops above middle grey is output at 93% and +6 above middle grey is at 98% so it becomes very difficult to see what is really going on in the highlights via the LUT when the top 2 stops are crammed into just 5% of the recording range. Zebra 2 set to 95% (for example) would appear over 1.5 stops below clipping, even if set to 97% zebras will show almost a full stop below clip.
It is one of the frustrations of the FX3/FX30 that there is no way to monitor via the LUT and measure the S-Log3 at the same time.
Tag Archives: zebras
Zebras and Log – Use a narrower window/range.
If you are using Zebras to measure the exposure of a log gamma curve you should consider using a narrower Zebra window.
Why?
From middle grey to white (50% to 90%) in the world of standard dynamic range Rec-709 each stop occupies approximately 16% of the recording range. Typically the default zebra window or zebra range used by most cameras is 10% (often +/- 5%). So, when Zebras are set to 70% they will appear at 65% and go away at 75%. For Rec-709 and most conventional SDR gammas this window or range is around 3/4 of a stop, so less than 1 full stop and generally reasonably accurate.
But if using most Cineon based log curves, such as Sony’s S-Log3, between middle grey and white (41% to 61%) each stop only occupies around 8% of the recording range, half the range used by Rec-709. As a result if you use a default 10% zebra window, zebras will appear over a 1.2 stop range, this is excessive and introduces a large margin of exposure error. Compared to Rec-709 the zebras will only be half as precise, especially if you are trying to measure the brightness of a grey card or white card.
I recommend reducing the width of the Zebra window to 6% when using Zebras to measure skin tones within the S-Log3 image (if measuring a Rec-709 LUT there is no need to change the window). This will then give a similar range and accuracy to a 10% window in 709. If you are using zebras to measure a white card or grey card then consider bringing the zebra window down to 2% to gain a more accurate reading of the white/grey card.
The zebra window or range can normally adjusted in the cameras menu under the zebra settings. On the Sony Alpha’s and and FX3/FX30 you can adjust the range of the C1 and C2 custom zebras.
Setting BOTH zebras on an XDCAM EX, very confusing!
If you set zebra 1 to 60 and then set zebra 2 to 90, then go back to zebra 1 you will find that the displayed value of zebra 1 will now be 90, however provided you don’t make any changes to zebra 1 if you go down to BOTH zebra 1 will work at 60 and zebra 2 at 90.
The thing is that going back to Zebra 1 (or zebra 2) and make any changes you select selects Zebra 1 (or 2) only, ie a single zebra and in doing so sets the level to the last level set. It’s only by going to BOTH that you enable both zebras together.
So to set two indendent zebras first set zebra 1 to your required level, then set zebra 2 to your required level, final scroll down to both and select. Now you zebras will be working at the independent levels you set, even though this may not appear to be the case in the menu.
It’s not the most logical way to lay out the menu as it does not show you both settings together at any point, hence the understandable confusion, but you will find that both zebras will work at the independent levels you set.
