S-Log, Latitude, Dynamic Range and EI S-log. Or how to modify your exposure range with EI S-Log

workshops-275 S-Log, Latitude, Dynamic Range and EI S-log. Or how to modify your exposure range with EI S-Log

The big issue most people have when working with log and exposing mid grey at 38 is that when you look at it on a standard monitor without any lookup tables it looks underexposed. The assumption therefore is that it is underexposed or in some way too dark to ever look right, because that’s what people used to working with conventional gammas have become programmed to believe over many years from their experience with conventional gammas.

So, for confidence you add a lookup table which converts the log to a Rec-709 type gamma and now the image looks brighter, but as it now has to fit within Rec-709 space we have lost either some of our high end or low end so we are no longer seeing the full range of the captured image so highlights may be blown out or blacks may be crushed.
It’s important for people to understand the concept of gamma and colour space and how the only way to truly see what a camera (any camera) is capturing is to use a monitor that has the same gamma and colour space. Generally speaking lookup tables don’t help as they will be taking a signal with a large range and manipulating it to fit in a small range and when you do that, something has to be discarded. If you were to take an F3 set to S-log and expose mid grey at 38 and show that on one of the nice new Sony E170 series monitors that have S-log gamma and place that next to another F3 with Rec-709 shooting mig grey at 45% and a similar but conventional 709 monitor the lower and mid range exposures would be near identical and the S-log images would not look under exposed or flat. The S-log images however would show an extra 2 stops of dynamic range.

Furthermore it has to be remembered that log is log, it is not linear. Because of its non linear nature, less and less brightness information is getting recorded as you go up the brightness range. As our own visual system is tuned to be most accute in the mid ranges this is normally fine provide you expose correctly putting mid tones in the more linear, lower parts of the S-log curve. Start putting faces to high up the S-log curve and it gets progressively harder to get a natural look after grading. This is where I think a lot of people new to log stumble. They don’t have the confidence to expose faces at what looks like a couple of stops under where they would with a standard gamma, so they start bringing up the exposure closer to where they would with standard gamma and then have a really hard time getting faces to look natural in the grade. Remember that the nominal S-Log value for white is 68 IRE. Part of the reason for this is that above about 70 IRE the amount of compression being applied by log is getting pretty extreme. While there is some wriggle room to push your exposure above or below the nominal mid grey at 38 it’s not as big as you might expect, especially dealing with natural tones and overexposure.

If you do want to shift your middle grey point this is where the EI S-log function and a light meter comes into it’s own, it’s what it’s designed for.

First something to understand about conventional camera gain, dynamic range and latitude. The latitude and sensitivity of the F3 is governed by the latitude and sensitivity of the sensor, which is a little under 13 stops. Different amounts of gain or different ISO’s don’t alter the sensors latitude, nor do they alter the actual sensitivity, only the amount of signal amplification. Increasing the camera gain will reduce the cameras output dynamic range as something that is 100 IRE at 800 ISO would go into clipping if the actual camera gain was increased by 6db (taking the ISO to 1600) but the darkest object the camera can actually detect remains the same. Dark objects may appear brighter, but there is still a finite limit to how dark an object the camera can actually see and this is governed by the sensor and the sensors noise floor.

EI (Exposure Index) shooting works differently, whether it’s with the F3, F65, Red or Alexa. Let’s consider how it works with the PMW-F3. In EI S-Log mode the camera always actually outputs at 800 ISO from the A/B outputs. It is assumed that if your working with S-Log you will be recording using an external 10 bit recorder connected to the A/B outputs. 422 is OK, but you really, really need 10 bit for EI S-Log. At 800 ISO you have 6.5 stops of over exposure and 6.5 under when you shoot mid grey at 38 or expose conventionally with a light meter.
Now what happens when you set the camera to EI 1600? Understand that the camera will still output at 800 ISO over the A/B outputs to your external recorder, but also note that 6db gain (1 stop) is added to the monitor output and what you see on the LCD screen, so the monitor out and LCD image get brighter. As the cameras metering systems (zebras, spot meter, histogram) measure the signal on the monitor side these are also now offset by +6db or + 1 stop.
As the camera is set to EI 1600 we set our light meter to 1600 ISO. If we make no change to our lighting the light meter would tell us to stop down by one stop, compared to our original 800 ISO exposure.
Alternately, looking at the camera, when you switch on EI 1600 the picture gets brighter, your mid grey card would also become brighter by one stop, so If we use the cameras spot meter to expose our grey card at 38 again we would need to stop down the iris by one stop to return the grey card to 38 IRE (for the same light levels as we used for 800). So either way, whether exposing with a light meter or exposing using the cameras built in metering, when you go from EI 800 to EI 1600 for the correct exposure (under the same lighting) you would stop down the iris by one stop.
Hope those new to this are still with me at this point!
Because the cameras A/B output is still operating at 800 ISO and you have stopped down by one stop as that what the light meter or camera metering told you to do because they are operating at EI 1600, the A/B output gets darker by one stop. Because you have shifted the actual recorded output down by one stop you have altered you exposure range from the original +/- 6.5 stops to + 7.5 stops, -5.5 stops. So you can see that when working at EI 1600 the dynamic range now becomes + 7.5 stops and -5.5 stops. Go to EI 3200 and the dynamic range becomes +8.5 stops and -4.5 stops.
So EI S-log gives you a great way of shifting your dynamic range centre while giving you consistent looking exposure and a reasonable approximation of how your noise levels are changing as you shift your exposure up and down within the cameras dynamic range.
EI S-Log doesn’t go below 800 because shifting the dynamic range up the exposure range is less beneficial. Lets pretend you have an EI 400 setting. If you did use it, you would be opening up the iris by one stop, so your range becomes +5.5 and -7.5 stops compared to your mid grey or light metered exposure. So you are working with reduced headroom and you are pushing your mid range up into the more highly compressed part of the curve which is less desirable. I believe this is why the option is not given on the F3.

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11 thoughts on “S-Log, Latitude, Dynamic Range and EI S-log. Or how to modify your exposure range with EI S-Log”

  1. Hi Alister,
    This was a great post, very informative as usual.
    I am still a little confused about EI mode though…. when you said:
    “So EI S-log gives you a great way of shifting your dynamic range while giving you consistent looking exposure and a reasonable approximation of how your noise levels are changing as you shift up and down the dynamic range.”

    …Can I ask you about the ‘shifting’ of the dynamic range… Does EI Mode actually changes the dynamic range of the camera (i.e. does it actually shift the sensor’s mid point, and if so how?).

    I thought the range of the camera (i.e. where is starts and when it ends) would be fixed in the H/W of the sensor and EI Mode just allows you to estimate what the recorded signal would look like when pushed or pulled in post but otherwise does not affect the recorded signal. If it does actually alter the mid point of the sensor (as opposed to showing you an approximation of a post process) then this would be a great advantage to have.

    Many thanks again for your great insights to this camera..
    Darragh.

    1. Hi Darragh.

      The dynamic range of the camera remains constant in EI mode. It’s a hard concept to explain. You have to think of what happens if you use a light meter to expose the scene. If you set the light meter to 800 ISO, the cameras native S-Log ISO you have 6.5 stops over and 6.5 stops under. Now what would happen if without changing anything on the camera you set the light meter to 1600 ISO? The meter would tell you to close the aperture by one stop (compared to 800). So the amount of light reaching the sensor is reduced by one stop. As a result you gain one stop of extra headroom, but because the image is now darker you loose one stop of under exposure, so your exposure range has become +7.5, -5.5. But, if you make no changes on the camera the picture looks dark and under exposed. When you turn on EI mode and set the ISO to 1600 the monitor out (not the A/B out) is made brighter by one stop, so the monitor out no longer looks under exposed, but will be a little noisier.

      So what you are doing with EI S-log is shifting your exposure range up and down within the cameras total range. The total range remains at a little over 12.5 stops, but you can shift from your exposure being centred within that range to being centred lower in the dynamic range. So the dynamic range that you use shifts from being centre weighted +/- 6.5 stops to +7.5/-5.5 or +8.5/-4.5. All the time though the total range remains at just over 12.5 stops.

  2. Thanks Again Alister,
    put it another way then – EI mode does not have any effect on the limits of what the camera is capable of recording – i.e. the brightest highlight that can be captured by the camera is the same regardless of what the EI is set to? (ditto for the lowest shadow). And hence EI only effects what you see in the viewfinder (assuming you’re recording from the SDI-A/B and not the SDI out)

    Is that correct?

    many thanks
    Darragh.

    1. Yes but you have to be careful using terms like “brightest” because brightness alone is a function of exposure and if you need to capture a brighter object you just stop down.

      The dynamic range of the camera never changes. It remains constant.

      So when you use EI1600 or EI3200 correctly and expose accordingly, you are under exposing relative to 800 ISO, so you are able to capture a one or two stop brighter highlight before you reach clipping, but this comes at the expense of loosing one or two stops off the bottom end, but from darkest to brightest is still 12.5+ stops.
      Yes, it is essentially only a change to the monitoring output, but when used as designed it has the effect of increasing the over exposure headroom at the expense of low end exposure. The same effect could be achieved simply by under exposing, but then you don’t have the benefit of a monitor output that shows you the relative noise increase that will occur in post when you correct the exposure.

  3. That really has cleared things up for me.
    Now I get what EI is for.
    You are right of course – brightness of the subject is proportional to exposure (aperture/ISO/Shutter speed) and not an absolute. I think that’s where I was getting muddled.

    Thank you Alister.

  4. Help me out Alister, I have an F3, S-Log and recently a Sekonic “cine” meter. If I test the exposure on a gray card and set my aperture to what the meter says, it will read the gray card at 38% and a white card at 68% like you said in your article. Now, when I grade the footage, add back the contrast and bring the levels up to white at 100%, I need to add about a stop (+1) to get a full range and the proper image. To me this means that I am already underexposing by a stop since middle gray is at 38% and not 50%. In addition, if I use EI 1600 on the monitor it puts middle gray back to 50% and white at 100% which is another indicator that at ASA 800 with these settings the image is really underexposed by a stop.
    What I have found is that this is enough when the darker or skin tones are what is most important. If I then add another stop of underexposure, this helps with bright highlights like the setting sun, but it makes for significant noise in the shadows and less pretty skin tones in my limited experience.
    So my question is — with setting the exposure to what the meter tells me to set it to for “cine mode” at 800 ASA, when shooting S-Log, am I not already underexposing the captured image by at least a full stop, and in many cases, isn’t this enough?

    Best,

    Jon Gordon

    1. What you are seeing is correct. Your footage, on a conventional monitor or in a conventional edit suite should appear about 1 to 1.5 stops under exposed. Simply consider this: If your monitor has a maximum brightness point it is obviously impossible to show something brighter. So the only way to capture a greater brightness range (since we can’t show/capture anything brighter) is to make everything darker, and it is this that gives us the greater range.

      In S-Log your camera is using a different gamma to your monitor and your edit system (unless you have log gamma on them). Because of this miss-match the picture will look dark, but it is not underexposed, it’s just your monitoring tools are not displaying it correctly. I can’t stress this enough, the correct (text book correct at least) exposure for S-Log will look underexposed on a non S-log monitor.

      Using the EI1600 monitoring output will of course bring your levels up 1 stop unless you re-set your exposure. When you re-set you middle grey for 1600 ISO then again it should look under exposed if your using a standard gamma monitor. In the grade when you lift things, it will get noisier, but then so would any camera using +6db of conventional gain.

      S-Log will appear underexposed when you shoot with it and use normal 709 gamma monitors, it’s important that it does. I hope you are using a 10 bit recorder because trying to use the 8 bit internal recordings will introduce a lot of compression noise into the shadows when you try to lift them in post production. It is very important to understand that the S-Log gamma curve is a type of compression. As the image gets brighter more and more compression is applied to the image. Because of this accurate exposure is very important because if you place mid tones and in particular faces etc too high up the exposure range they will be in the compressed part of the curve and it will be very difficult to then grade these mid tones so that they look natural. In addition unless you are using log grading tools such as those found in Color or Resolve you should be de-logging the footage with either a LUT or S-Curve prior to grading to move the image to the linear space used by most conventional grading filters, especially those used by edit software. If you don’t do this you will find that adding conventional filters will greatly alter your highlights, have much less effect on mids and little effect on shadows. As a result trying to lift the mid and low range quickly results in blown out highlights or just shifting the mid and low range introduce extra stretch and press that can introduce noise and odd effects that should not really be there.

      In my opinion the best tool for metering exposure with the F3 is the cameras built in spot meter as this is actually metering what the camera is seeing and as a result takes into account shutter speed, lens losses, ND filters and f-stop.

  5. Thanks Alister!
    I am using a 10 bit recorder (Samurai) and understand what you are saying. I am de-logging with Magic Bullet Looks (adding in the S-Curve) or Pomfort plugin (not so pretty in the deep shadows). At the end of the day, the footage needs to play on conventional monitors, and so even after doing that, there needs to be +1 stop added for final editing/viewing/distribution. All I am saying is that based on this, it seems that recording correctly for ISO 800 which looks properly underexposed on my monitors but comes up nicely with +1 on conventional monitors we all use, why would I want record at an even lower exposure where I would have to add +2 to be viewable on a conventional monitor unless there were extreme highlights to protect? Viewing at EI 1600 with some delog LUT just gives me an idea of how my flat recorded footage will look after I add that stop and delog it in post, if I understand things correctly. I think that many people think that they need to record at an even lower light level than what is necessary, and I am questioning whether that is correct. If I am using the in camera meter for the center value and it shows middle gray at 38% (and not at 50%) and white at 68% then am I not recording at the correct level? Why push these even lower? I realize this gives an even number of stops above and below, but for skin tones that are 1 stop above middle gray (which with these settings come in around 45%) shouldn’t this be optimal? If skin tones were up around 70% then there is bad compression, but I am not suggesting that.
    I am certainly not an expert at this and greatly appreciate your help in hopefully reaching a better understanding.

    Respectfully,

    Jon Gordon

  6. The only way to gain more headroom is to record at a lower level. That’s why you would do it. Yes there will be a noise penalty, but it’s really not that bad when you use a dedicated grading package like Resolve. Magic Bullet is notoriously noisy. When using EI1600 middle grey should be 38%, if you use EI1600 and set grey at 50, then you may as well just use EI800, the exposure is the same, but EI1600 gives you one stop more headroom when exposed correctly.

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