You’ll find many people using the terms dynamic range and latitude (more properly exposure latitude) interchangeably. In reality though, they mean two different things. This article will attempt to explain both in the simplest way possible.
The fact that he’s holding a color checker is purely circumstantial.
What is dynamic range?
Imagine you’re looking out a bright window, but from the far side of the room. You can see the interiors of your room (lights are turned off) and also the cityscape outside.
Or can you? If you’re like most humans, you only hold a dynamic range of 20 stops. If you focus on the bright cityscape outside the window, the room will go dark. If you concentrate on the room to see everything clearly, the outside will blow out. Just like a camera.
The limit of what you can see, from bright to dark, within the same exposure is the dynamic range. There are two kinds:
- The dynamic range of the scene
- The dynamic range of a camera
Look at the image above. The seat is of a certain size. It represents the camera. The baby is of a certain size. It represents the scene. No matter how big the baby, the size of the seat is fixed, and the baby must compromise. If the baby is a 30-year old grown man, we have problems.
The different ways to measure dynamic range
How do you measure dynamic range? Videographers and engineers measure it differently, and this allows camera manufacturers to play around with numbers – by usually preferring the largest number. You see this happening with television sets and computer monitors as well.
Here are the different ways to measure dynamic range:
The engineering way – the only measurable and repeatable method
Science demands repeatability. Until an experiment can be repeated independently by other scientists the results of an experiment are not taken very seriously. The way to measure dynamic range is by calculating the signal-to-noise (S/N) ratio.
Traditionally in analog systems one measured this using voltage levels that formed the foundation of a video signal. In digital video cameras, it is more common to measure luminance, though one will have to specify what they really mean by ‘luminance‘:
- Luminance in nits
- Luminance as Luma
- Luminance as IRE normalized from voltage levels
The first is not possible because it depends on the scene and not the camera. The second doesn’t take into consideration RAW and RGB images, so I’m not in favor of it. The third will apply to all video, and is my candidate.
The signal-to-noise ratio is usually notated in decibels (dB) but might also be expressed as:
- Contrast ratio: The simple ratio between the peak signal and the noise floor (if you know a bit of engineering, you can see how some fudging can take place here)
- Stops (which is possible because dB is in the logarithmic scale)
Do not concern yourself too much about the engineering dynamic range. Now that you know it, forget it.
The cinematographer’s way – stops of light
What could be easier? The only thing we care about when exposing a scene is to make sure the information we want is being recorded by the sensor. If I’m shooting my room and the window overlooking the cityscape, and I want both, I know my camera can’t give me both.
I’ll have to either sacrifice the dark shadows in the room or the details of the cityscape in the window. One will have to give. To measure this, all I need is a camera meter or a light meter. I measure the brightest portion and the darkest portion (and all important parts in between) in whatever units I fancy:
- Lux or footcandles
- F-number or f-stop
They all tell me the same thing. The most common system is taking readings in f-stops. If the brightest part of the scene is at f/128 and the darkest is at f/0.7, that’s a difference of 15 stops. If my camera can only read 10 stops (in the real world! Don’t use the marketing number!) then I must sacrifice five stops.
One tackles this problem by using lights to brighten up the scene (called the ‘fill light’) or by sticking gels to the window to darken it down. In this way, one tries to squeeze in the larger dynamic range of the scene to the smaller dynamic range of the camera. Fun stuff.
Now, what if the dynamic range of the scene is lower than the dynamic range of the camera?
What is exposure latitude?
What if our baby is smaller than its seat? In that case, it has some room to play around in. This ‘wiggle room’ is called exposure latitude.
E.g., if you’re shooting a foggy morning scene or a high-key sitcom that has a dynamic range of 7 stops; with a camera that has 12 stops of dynamic range, then you have the freedom to place your exposure in this fashion:
If you decide to expose towards the blacks, making the scene darker and moodier, you can move five stops (12 – 7) to the left. If you decide to expose towards the whites, making the scene brighter and more cheerful, you can move five stops to the right. You can also expose anywhere in between.
This five stops of wiggle room is called latitude or exposure latitude. In many ways, this is more important than dynamic range, because it tells you how much creative control you really have with your camera in terms of exposure. Here’s the formula that relates dynamic range and latitude:
Exposure latitude = Camera dynamic range – Scene dynamic range
If you have a lot of latitude, you can control the look of the scene, especially its mood, in-camera. If the dynamic range of the camera is lower, then your exposure latitude is negative, which means you’ll need some kind of fill lighting or filtration. If the latitude is zero, then you’re a tight fit and there’s only one way to go (unless you add lighting or filters).
This is the fundamental difference between latitude and dynamic range. And it’s also an overly simplistic one. Measuring the real latitude is slightly more complicated, because the gamma curve for most cameras are not linear. To follow up, I suggest you read Alister Chapman’s excellent article on the subject.
Terms like ‘camera latitude’ really don’t make sense, because you can’t know latitude unless you know the scene you’re shooting. Today, many cameras have usable dynamic ranges of 10-12 stops, which is great for most scenes. Yet, with HDR scenes (like windows, or shooting at the sun, specular highlights from reflections, etc.), there is no camera that can capture that entire dynamic range.
Not even our eye can do that.