Beware Exposing To The Right With Log.

advertise-here-275 Beware Exposing To The Right With Log.

That may seem like quite a sensational headline – beware exposing to the right with log – but let me explain.

First of all, I’m not saying you can’t or shouldn’t expose to the right, all I am saying is beware – understand the implications.

First of all what is normally meant by exposing to the right? Well it’s a term that comes from the world of photography where you would use the cameras histogram to measure the exposure levels. Exposing to the right would normally mean setting the shutter speed and aperture so that the levels shown on the histogram are as far to the right as you can get them without going beyond the right side of the histogram. This would ensure a nice bright exposure with lots of light falling on the sensor, something that is normally highly desirable as you get a nice low noise picture once you have adjusted and processed it in your photo editing software.

You can expose to the right with a video camera too. However when shooting with Rec-709 or conventional gammas this can often result in nasty looking highlights thanks to the default knee settings, so it’s not normally a good idea for 709 and standard gammas.

With log or raw as there is no highlight roll off you can expose to the right and it should give you a nice bright exposure… or will it?????

The problem with exposing to the right is that you are exposing for the highlights in the scene. If shooting a low contrast or low dynamic range scene this isn’t going to cause any problems as exposing to the right will mean that everything in the scene is nice and bright.

But if shooting a high dynamic range scene, say an outdoor scene with bright clouds in the sky but large areas of shadow, the exposure will be optimised for the highlights. The mid range and shadows may end up too dark. On a sunny day if shooting a person with their back to the sun the sky could easily be 6 or 7 stops brighter than the skin tones. If you expose for the sky/highlights the skin tones will be 1 or 2 stops darker than the basic exposure level recommended for most  log curves.

(S-log2/3 has 14 stops. At the base exposure you have 6 stops above middle grey and 8 below. Skin tones are normally between 1 and 2 stops above middle grey. So if the sky/highlights are 6 stops above the skin tones, then exposing for the highlights will put the skin tones where middle grey should be, which is 1 stop under exposed and 2 stops below where I would normally like to see skin tones when shooting with log or raw).

The first thing a viewer will notice when they look at a scene with faces or people will be the skin tones. If these have been under exposed they will be grainy and less than ideal. The viewer will notice noise and grain and poor shadows long before they look at the brightest highlights. Shooting log and protecting the highlights or exposing to the right will often compromise the all important mid tones because you are exposing for the highlights, not the midrange. In addition exposing for highlights with a high dynamic range scene can often push the shadows down in level and they will end up noisy and grainy. The biggest issue with exposing to the right is that it’s extremely difficult to estimate how many stops there are between your mid tones and the highlights, so you never know quite where your mid tones are falling.

(Midtones – generally a white piece of paper or a 90% reflectivity white card would be considered to be the top end of the mid tones. Go down about 2.5 stops from white and you hit middle grey  (18% grey card). This range between middle grey and white is where skin tones, plants, most animals etc will be and it probably the most important part of most images).

An important consideration with log and raw is that there is no highlight roll off. Standard gammas (with the default knee found on almost every camera) , cinegammas, hypergammas etc all roll off the highlights. That is to say that as you approach the peak recording level the contrast is reduced as the highlights are squeezed together to try to extend the dynamic range. This reduction in contrast means that it is very difficult, if not impossible, to recover any nice, useable picture information out of anything close to the peak recording level. As a result with conventional gammas we tend to avoid over exposure at all costs as it looks nasty. This highlight roll off is one of the things that gives video the video look.

Log and raw don’t have this same kind of highlight roll off. The image gets brighter and brighter until it clips. With log the stop immediately below clipping contains just as much picture information as any other stop brighter than middle grey. With linear raw the stop just below clipping has more information than any other stop. As a result in post production there is a very large amount of data that can be pulled out of these highlights, even if they are a little clipped! So don’t worry about a few clipped highlights when shooting log. The other thing to remember is there is no TV or monitor that can show these highlights as they really are, so they will never look perfect anyway.

Another thing that happens when exposing to the right is that grading becomes harder than it needs to be. Because the separation between the mid tones and highlights will vary greatly depending on things like whether you are shooting into or away from the sun, when you expose to the right you mid tone brightness will be up and down all over the place. So in post production as well as adding the look that you want to your footage, you are also going to have to spend a lot of time matching the mid range exposure to balance skin tones etc from shot to shot.

Rather than exposing to the right what I recommend is exposing for the mid range. After all this is the important part of the image. To do this you need to use a diffuse reflective shade. The most commonly used shades are a 90% white card and/or an 18% reflectivity grey card – middle grey. Get the mid range right and in most cases the highlights will take care of themselves. Getting the mid range right might mean exposing the mid range  brighter than the recommended levels. But it’s the mid range we need to measure, not the highlights, this is the important part of the image.

90% white is an incredibly important level in the world of film and video. A typical piece of office paper reflects about 92-94% of the light falling on it. Office paper often uses brighteners and special chemicals to make it look bright and white. This white is the brightest diffuse surface you will likely ever see.  Anything brighter than this is normally going to be an actual source of light. The sky perhaps or a direct bounced reflection off a shiny, reflective surface such as the bodywork of a car. So anything brighter than 90% white would normally be considered to be a highlight and to us humans, highlights are visually less important than the mid range. This is why the knee on most video cameras kicks in at around 90%. Anything brighter than 90% is a highlight so the knee only effects highlights and leaves the all important mid range alone.

Middle grey is also very important because it’s a shade of grey that to most people looks to be half way between black and white. Skin tones fall roughly half way between middle grey and white. In addition if you average all the brightness levels within a typical scene the end result is typically very close to middle grey.  Light meters are calibrated to middle grey. The relationship between middle grey and white is fixed. White reflects 90%, middle grey 18%, no matter how bright the actual light source. So whether you are indoors, outside. Whether it’s sunny or overcast, white and middle grey will always be close to 2.5 stops apart. They are extremely useful fixed reference levels.

There are many ways to measure the brightness of a white or grey card. My preferred method is with a waveform display. But you could also use zebras (use a narrow zebra window if you can).  You can also use false colour. Unfortunately it’s very difficult to use a histogram to measure the brightness of a specific target. The histogram is a great measuring tool for photography, but less than ideal for video. If you can’t get a white/grey card out in front of the camera you could consider using a light meter. It’s also worth noting that skin tones sit just a little over half way between middle grey and white, so if you have no other reference you could simply place your skin tones a touch brighter than half way between the values you are targetting for middle grey and white.

Just to be clear: I do still recommend exposing Sony’s S-log2, S-log3 and raw between 1 and 2 stops brighter than the Sony base levels. But the key take-away is that it’s the mid range you need to measure and expose at this level. Exposing to the right using a histogram or waveform and just looking at the peaks and brightest parts of the image does not tell you what is happening in he mid range. Measure the mid range, not the peak brightness.

 

Advertisements

13 thoughts on “Beware Exposing To The Right With Log.”

  1. Dude I love reading this stuff from you and I am learning so much from it. Having said that its still confusing me when it comes to what it means to me on a job. From what you said and for weddings and events should I set the zebras to 70% and try and use them on the skin tones as I do not have time to use grey/white cards/light meters etc. I need a way to get good exposures quickly. Should I shoot Log and use skin tones? Should I slightly over expose the skin tones…. basically … Help !! 🙂 (I use Sony A7 range by the way). thanks again for the info …

    1. Yes, with log I still recommend you expose the mid range brighter than Sony recommend by between 1 and 2 stops. That puts white at approx 70% and skin tones around 55-60%.

  2. My recommendation to expose between 1 and 2 stops brighter than the base levels given by Sony still stands. But what’s important is to expose the mid range 1 to 2 stops brighter.

    1. Thanks mate … so if I use the skin tomes in most case and expose slightly brighter then I should be about right? Basically you don’t even have time to check the camera meters sometimes at weddings. I have tended to use Log during the day (Hight dynamic range) and switch to cine 4 custom for everything else to keep nice colour and reduce noise…. Does that sound about right? This is great as I think I was using the highlights and exposing them higher instead of the skin tones… Thanks

      1. @Andy, bear in mind that if you are monitoring a log image, you zebra values need to be different o what they would be if you are looking at a rec709 image.

        1. Blimey… ok yea never thought about that… whats your advice. I used to use zerbas a lot but with the run and gun it can essentially … get in the way and feel of the shot. If I did use them what do you think would be the best for weddings/events? 80% so its skin plus a bit? I love learning all this as it makes us better filmakers. This is the one area where I need to nail down as it stands as it costs so much time in post if you get it wrong (That and focus of course). Thanks Nez

  3. I’m a little confused by your use of “expose the right”. That implies that the exposure on the histogram is pushed to the highlight end, ie, somewhat overexposed. So this would not result in underexposed, grainy midtones and shadows. This is not “exposing for the highlights”, which would be underexposing a bit to give more latitude in the highlights. So although I totally understand and agree with the rest of your post, I can’t understand why you are referring to “exposing to the right”.
    Thanks!

    1. Is it over exposed? Exposure isn’t peak brightness, exposure is putting the correct amount of light on the sensor. Whether it’s over exposed, under exposed or not depends on what’s in the scene you are shooting if you expose to the right via the histogram. For example if there is a lot of bright sky, it is the bright sky that will be at the right end of the histogram. That sky could be many stops brighter than the mid range. So exposing with the sky or the brightest parts of the image over to the far right of the histogram does not ensure that the mid range is also bright. All that it ensures is that the very brightest parts of the scene are as bright as they can be without clipping. It tells you nothing about where the mid range is because you are exposing for the highlights and the relationship between the highlights and mid range will vary hugely from scene to scene, shot to shot so the mid range will bounce up and down in exposure all over the place.

      1. Confusion of “expose to the right” meaning:

        a) shifting the exposure to the right of a histogram, thus everything get bright, midtones exposed higher, highlights more likely to clip.

        or

        b) exposing “for” the right, so the right of the histogram is given more weight, more protection.

        ?

  4. so basically its not a good idea to expose each shot to the right.. as then rather than a uniformed 1 or 2 stop over exposure for everything you shoot..EI mode when you go to grade your mid point is all over the place.. and its hard to keep a consistent look over a scene.. ? is that right..

  5. Thank you for this great article!!!

    How do you use your greay card or white paper when you are shooting into the sun? Would you then use the shadow side as a refference? (The shadow side is facing towards you when you are shooting against the sun…)

    Or would you “tilt” the grey card or white paper to use the reflections as a reference?

    Thank you Chris

    1. That’s always a tough situation and it will depend on how backlit you are prepared to have any foreground objects. The “correct” exposure would be obtained by pointing the camera away from the sun and metering off a grey card facing the sun. But this will often result in foreground objects that are too dark. So you may then to point the camera in the direction you wish to shoot with the grey card facing the camera and take a measurement from that. Then split the difference between the first and second measurement to get a middle ground compromise. Alternately use a 709 LUT and look at your monitor or VF and make your own assessment.

Leave a Reply

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

*