Workshop Measuring Light
Writing started 28-08-2023. Mainly text at first, illustrations to follow.
In the basic part of this workshop you will be learning how your camera measures light and how to interpret that measured value and, most important, how to correct it into the value you need it to be for the exposure you have in mind.
The advanced section will go deep into contrast ratios and colour handling.
Why even bother?
Light is just light, an energy of a certain wavelength and intensity. It has no colour, our eyes retina give it colour. It is the intensity that we need to measure in order to get the desired exposure. If you have no clue of how much light is entering the camera you are blind in that light. It is obvious that to much light doesn't get you a picture other than a white screen or no light at all leaves it all black. But... there is way more to it. Between those white and black images lays a track of contrasts and intensities that have to built a detailed image. The closer you are to the ideal and optimal exposure the more contrast and colours and thus details you will get. Specifically when your camera can not record images in the RAW-format or you can not process that RAW format or don't know how to process RAW, you have to get as close to that ideal point as you can to enable the camera to built a good JPG-file for you.
The Key and Holy Grail: Mid Grey.
To explain what needs to be measured we will go back in time to the old days of the film and the darkroom to reveal the mechanics behind the technique. The principle is quite simple. Something black in the subject stays blanc on the film and something white becomes black on the film. When the films gets printed the blanc turns out black on the paper and the black on the film stays white on the paper. That is why the film is called the 'negative'. With this in mind dark grey becomes light grey on film and dark grey again on paper. And light grey obviously becomes dark grey on film and light grey again on paper. So... following this logic there is this grey shade that won't change on film or paper as it is exactly the same on both. All the darker and lighter shades revolve around this mid grey. It is this mid grey that we are looking for. It is this pivoting point of Mid Grey that clears things up and is much harder to explain (if at all) in the direct way that digital imaging does. Even the old film slides use a 'negative'.
So why this Mid Grey?
The camera has no clue whatsoever where the light that it sees is coming from, it just sees a quantity of light. This light can be coming from any source in any intensity. It can be mid grey but also white or black. In fact... it could be poorly lit white or brightly lit black both looking like grey. In fact very poorly lit white could look dark grey or... when really poorly lit it can look like... black... to the human eye at least because the camera can make this white again if we just tell it to handle it that way. Telling it how to interpret the measured value is the correction to invoke in 1/3 or 1/2 stop increments in either positive or negative value. How much simply depends on how much the measurement deviates from the desired shade. So, the camera doesn't know anything and will always give you a reading that will by definition produce Mid Grey.
For white subjects correct it with 'over'exposure so the light burns into the film making it more black resulting in a whiter shade on paper.
For black subjects correct it with 'under'exposure so the light doesn't burn into the film leaving it more blanc resulting in a darker shade on paper. Remember to always select a correction as described here under that preserves the details and prevents whites from washing out and shadows to burn black.
For something white like a snowy slope against a cloudy sky with a person in a white skiing suit you will need about 2 extra stops of light over the measured value whatever that shows. Remember: the only thing the camera can do is assuming the light it measures is Mid Grey. Following the measured value will deliver the described scene as Mid Grey. Look at your old Skiing photos... anything familiar? So... we have to tell the camera to 'over'expose with about two stops and white becomes white again. Every scene needs its own correction. For white this can be somewhere between +1 2/3 till +2 1/2 stops. If the measured values were 400 iso, 1/125, F5,6 and the scene is as described you would most probably choose 400 iso, 1/60, F4.0 to keep both speed and depth of field (and lens faults) acceptable.
For something black like someone in black clothes against a night sky you will need about 1.5 lesser stops of light over the measured value whatever that shows. Remember: the only thing the camera can do is assuming the light it measures is Mid Grey. Following the measured value will deliver the described scene as Mid Grey. Look at your old outside evening photos... anything familiar? So... we have to tell the camera to 'under'expose with about 1.5 stops and black or very dark becomes wblack or very dark again. Every scene needs its own correction. For black or very dark this can be somewhere between +1 1/2 till +2 1/2 stops. If the measured values were 400 iso, 1/125, F5,6 you would most probably choose 400 iso, 1/125, F11 to increase your depth of field while you're at it.
The grey card and its white side
Grey cards come in different sizes which is convenient if you have different bags but they all do the same. The grey side reflects 18% of the light that hits it and therefore it is exactly Mid Grey. The backside is 90% reflective white which means that when this side is measured you can add 2 stops of light to get bright white while preserving the details. Both sides are also useful to have an eyedropper point for your image processing software to tell the white balancing tool which part of the photo is absolutely neutral. So you take the photo twice: one with the card for the eyedropper and one without as 'real' photo to which you copy the values found by the eyedropper in the previous photo. I will refer to this from the paragraphs about white balancing and fluorescent light sources
'Faulty' measurements and how to correct them
Not every measurement is easy to correct. There are circumstances that are more complicated, there is of course no faulty measurement as the camera doesn't know how the light it measures is composed. Specifically when there is a quite average scene with an element disturbing that equal pattern.
But what is average in this case? An average scene consists of many colours and shades of about the same intensity. For instance a market in a city. All added together it will be like neutral noise except for the dots being people and stands and products and bags and so on.
When there is a lantern shining from a stand somewhere in this average scene its light dominantly disturbes the neutrality of the scene. The camera sees light, it doesn't see darkness. Let's say we have a white background with a black circle of 50cm across and the camera is at a distance of 3 meters with a 50mm full frame. A flood light lights the scene. A camera will practically ignore the black spot on the white background. When this scene is reversed to a black background and a white spot, that white spot will most certainly reflect a lot of the flood light and change the measurement much more substantially than the black spot did on the white background. Try it and make measurements with and without the spots.
Methods to measure light
So now you know what to measure and how to read and correct the values. But how do you measure the light? Which method is best?
There are different ways to do the job.
- Integral with emphasis on the center
- Matrix in combination with a database of known situations
- Intelligent or smart spot
These four are known to me as I use Sony. It is possible that other brands have more or different variations to numbers 1, 2 and 3.
Integral with emphasis on the center is just what it says. It takes the whole field of view into the measurement and takes the center part double or triple or something like that. Whichever the brand has chosen there is an emphasis on the center as that is most commonly the subject of the photo.
Matrix in combination with a database of known situations brakes down the field of view into pieces like a honeycomb. Each cell is evaluated and results in a pattern of intensities and colours. This pattern is held against a database of known patterns and results in an advice how to deal with the specific situation in the best way possible. It is not more than an advice as you can still override unless you are in the full P-mode of your camera. The amount of known patterns can go up into several tens of thousands depending on brand and model.
Spot lets you take a small (mostly 12%) area of the field of view and measure only that part and ignore everything around it. In my earlier example with the light shining in an average scene you can block out the light that disturbes. But also you can pick out any detail in a scene to get exactly that detail right or you can pick out that one detail to concentrate on the right correction as you have confidence that you can measure, read, understand and correct that reading for the whole scene. A few brands give you the opportunity to define the spot area as normal or super spot by letting you select an even smaller part like 8% or something like that of the whole.
Intelligent or smart spot is also known as 'Focussing point link' and links the measuring spot to the point where you focus on. It is a function in my Sony A99ii so I cannot tell if other brands have this feature as well or even other features which is quite imaginable of course.