How We Judge Exposure Looking At an Image And The Importance Of ViewFinder Contrast.

This came out of a discussion about viewfinder brightness where the compliant was that the viewfinder on the FX9 was too bright when compared side by side with another monitor. It got me into really thinking about how we judge exposure when purely looking at a monitor or viewfinder image.

To start with I think it’s important to thing understand a couple of things:

1: Our perception of how bright a light source is depends on the ambient light levels. A candle in a dark room looks really bright, but outside on a sunny day it is not perceived as being so bright. But of course we all know that the light being emitted by that candle is exactly the same in both situations.

2: Between the middle grey of a grey card and the white of a white card there are about 2.5 stops. Faces and skin tones fall roughly half way between middle grey and white. Taking that a step further between what most people will perceive as black, something like a black card, black shirt and a white card there are around 5 to 6 stops and faces will always be roughly 3/4 of the way up that brightness range at somewhere around about 4 stops above black . It doesn’t matter whether that’s outside on a dazzlingly bright day in the desert in the middle East or on a dull overcast winters day in the UK, those relative levels never change.

Now think about this:

If you look at a picture on a screen and the face is significantly brighter than middle grey and much closer to white than middle grey what will you think? To most it will almost certainly appear over exposed because we know that in the real world a face sits roughly 3/4 of the way up the relative brightness range and roughly half way between middle gray and white.

What about if the face is much darker than white and close to middle grey? Then it will generally look under exposed as relative to black, white and middle grey the face is too dark.

The key point here is that we make these exposure judgments based on where faces and other similar things are relative to black and white. We don’t know the actual intensity of the white, but we do know how bright a face should be relative to white and black.

This is why it’s possible to make an accurate exposure assessment using a 100 Nit monitor or a 1000 Nit daylight viewable monitor. Provided the contrast range of the monitor is correct and black looks black, middle grey is in the middle and white looks white then skin tones will be 3/4 of the way up from black and 1/4 down from white when the image is correctly exposed.

But here’s the rub: If you put the 100 Nit monitor next to the 1000 Nit monitor and look at both at the same time, the two will look very, very different. Indoors in a dim room the 1000 Nit monitor will be dazzlingly bright, meanwhile outside on a sunny day the 100 Nit monitor will be barely viewable. So which is right?

The answer is they both are. Indoors, with controlled light levels or when covered with a hood or loupe then the 100 Nit monitor might be preferable. In a grading suite with controlled lighting you would normally use a monitor with white at 100 nits. But outside on a sunny day with no shade or hood the 1000 Nit monitor might be preferable because the 100 nit monitor will be too dim to be of any use.

Think of this another way: Take both monitors into a dark room and take a photo of each monitor with your phone.  The phone’s camera will adjust it’s exposure so both will look the same and the end result will be two photos where the screens will look the same. Our eyes have iris’s just like a cameras and do exactly the same thing, adjust so that the brightness is with the range our eyes can deal with. So the actual brightness is only of concern relative to the ambient light levels.

300x250_xdcam_150dpi How We Judge Exposure Looking At an Image And The Importance Of ViewFinder Contrast.

This presents a challenge to designers of viewfinders that can be used both with or without a loupe or shade such as the LCD viewfinder on the FX9 that which be used both with the loupe/magnifier and without it. How bright should you make it? Not so bright it’s dazzling when using the loupe but bright enough to be useful on a sunny day without the loupe.

The actual brightness isn’t critical (beyond whether it’s bright enough to be seen or not) provided the perceived contrast is right.

When setting up a monitor or viewfinder it’s the adjustment of the black level and black pedestal which alters the contrast of the image (the control of which is confusingly called the brightness control). This “brightness” control is the critical one because if the brightness adjustment raises the blacks by too much then you make the shadows and mids brighter relative to white and less contrasty, so you will tend to expose lower in an attempt to have good contrast and a normal looking mid range. Exposing brighter makes the mids look excessively bright relative to where white is and the black screen surround is.

If the brightness is set too low it pulls the blacks and mids down then you will tend to over expose in an attempt to see details and textures in the shadows and to make the mids normal.

It’s all about the monitor or viewfinders contrast and where everything stits between the darkest and brightest parts pf the image. The peak brightness (equally confusingly set by the contrast control) is largely irrelevant because our perception of how bright this is depends entirely on the ambient light level, just don’t over drive the display.

We don’t look at a VF and think – “Ah that face is 100 nits”.  We think – “that face is 3/4 of the way up between black and white” because that’s exactly how we see faces in all kinds of light conditions – relative levels – not specific brightness.

So far I have been discussing SDR (standard dynamic range) viewfinders. Thankfully I have yet to see an HDR viewfinder because an HDR viewfinder could actually make judging exposure more difficult as “white” such as a white card isn’t very bright in the world of HDR and an HDR viewfinder would have a far greater contrast range than just the 5 or 6 stops of an SDR finder. The viewfinders peak brightness could well be 10 times or more brighter than the white of a white card. So that complicates things as first you need to judge and asses where white is within a very big brightness range. But I guess I’ll cross that bridge when it comes along.

7 thoughts on “How We Judge Exposure Looking At an Image And The Importance Of ViewFinder Contrast.”

  1. Funny how mobile phones solved this problem years ago with a simple auto brightness feature. The detect ambient light and raise or lower the screen brightness accordingly. It’s not rocket science. Saying you can judge it if the contrast is right is also a bit hit and miss. After all some screens have much higher contrast ratios than others eg OLED. What you really mean is you can judge exposure fairly accurately from skin tones off the monitor once you are familiar with your particular monitor/viewfinder.
    Personally I still prefer zebras or better still a waveform because it takes monitor brightness and contrast out of the equation.

    1. I meant what I wrote, which is that we asses whether faces etc look right based on where they are relative to white and black. Because we know what those relationships are – irregardless of the brightness. That’s how the world we live in is. Outside on a sunny day or indoors, faces are always 75% of the way between black and white, regardless of how bright they are or how much ambient light there is. Indoors faces may be extremely bright, indoors they may be very dim but they are always 75% of the way between black and white.
      In SDR the contrast ratio of an OLED should be little different to any other half decent SDR screen. The OLED may have better blacks in the very darkest parts of the image, but that will only make a difference in a darkened viewing environment.

  2. Hi Alister,

    Many thanks for this.

    Care to share how you have setup the Sony supplied FX9 viewfinder? Are your personal contrast and brightness settings left at the factory 0 or have you adjusted?

    I believe Paul Ream posted somewhere -50 for brightness and +10 for contrast … might be wrong about that.

    Curious to see your settings.

    Thank you for an informative website.

    1. After much experimentation I have left it at the defaults. If you try to use the built in ARIB bars to calibrate the VF I find it ends up brighter than it should be, if you use the SMPTE Bars it ends up close to the defaults. I don’t understand why this is.

      Paul set his up by comparing it against another monitor. This is dangerous as different screens have different brightness back lights, unless you know exactly what the back light brightness of each device is this can be miss-leading.

  3. Hi Alister,

    Thank you very much.

    I have also struggled with setting up the viewfinder – which is why I asked.

    I do like the viewfinder – it is an improvement — but have been a bit lost at sea when trying to calibrate with bars.

    Thank you for your time.

  4. Thank you Alister all makes total sense and explains why I find viewfinders and monitors seem to be ‘lying’ to me so often! I’m still learning the technical side of cinematography (I just find the creative part far more interesting for obvious reasons) but something I’ve always done from my photography days is to ignore the exposure that a screen is telling me and always use the histogram. Make sure the blacks and whites aren’t clipping (or if unavoidable, hardly any are) and if I’m being careful, set zebras at 75 and see if there are any on my subject (if there is a human subject large enough in the shot). Probably quite slapdash from you point of view but seems to work for me! Thanks for all the tips 🙂

    1. But the issue with exposure isn’t whether blacks or whites are clipped, it’s whether you have sufficient data and low enough noise in the all important mid range. Audiences rarely notice clipped highlights, but if the mid range is noisy or of poor quality they will notice it instantly and it really won’t matter how good the highlights are, all they will notice is a poor mid range. So it’s not enough to check blacks and highlights as these are the least important parts of the image. It’s the all important mid range that must be right. As for zebras at 75 – well it depends on the gamma curve you are using and the brightness of the faces you are shooting, all of these vary greatly, for example 75% zebras on any face with something like a hyper gamma or S-Cinetone would be greatly over exposed.

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