Scene luminance

THE BRIGHTNESS OF THE SCENE YOU WANT TO PHOTOGRAPH

Scene Luminance in a nutshell

Scroll down for a more detailled description.

What is scene luminance?

For those of you who don’t want to get a doctor in photography, but rather want to quickly learn photography: all you need to know is that the scene luminance describes how bright the scene is that you want to photograph.

Since photography is all about light – remember, it’s Greek and means “the light paints”, scene luminance is the most important component of image brightness.

Exposure values

In photography scene luminance is basically measured in exposure values or EV. Here is a graphical representation of certain exposure values for scenes with different brightness.
What is important to understand is, that one exposure value will double the scene brightness and therefore also double image brightness if you don’t compensate with any camera settings.
So 1EV is twice as bright as 0EV, 2EV will already quadruple the brightness compared to 0EV, and 3EV will octuplicate it (what a word!🤣)

AVAILABLE LIGHT SCENARIOS

Looking at the graph above, you already understand that a scene gets darker really quick without you even realizing.
So you quickly lose a lot of scene luminance, when you compare a very well lit daylight scene to less than perfect scenarios. Here are a few examples.

1. Slightly cloudy with sun hiding behind clouds

Let’s start from the right side, a scene lit by the sun at noon has 15EV.
If the sun hides behind light clouds, you already lose one EV, so you end up with half the light.

2. Overcast sky outdoors

In a scene outdoors with an overcast sky at noon, you will already get down to 12EV.

Let's calulate

  • 14EV is half as bright as 15EV
  • 13EV is half as bright as 14EV (1/2 of 1/2 is 1/4)
  • 12EV is half as bright as 13EV (so 1/2 of 1/4)

The result is 1/8

3. Indoor gyms (e.g. kids basketball)

If you continue that to EV7, which is the brightness level of most gyms where your kids play basketball, you will be at 1/256.
So outdoor in the bright sunlight, you have 256 times more light available, than in your kids gym.

That loss of light creates massive problems in photography. Continue reading and you will suddenly start to understand photography.

4. Night scene lit by stars only

Want more? What about a scene lit by the light of the stars (so without the moon present)? A scene lit by the sun is more than 16 million times brighter. That’s beyond massive!

"stops" in photography

Before we continue with the camera settings that help us compensate scene luminance to get a correct exposure,
let me quickly tell you what “stops” are. Photographers usually talk about stops, and not about exposure values. basically, that’s the same. One EV is one stop.

So if someone says: “expose your image one stop brighter” he means to make it twice as bright. And making your image one stop darker means exposing it half as bright.

That doubling and halving of brightness is fundamental in photography.

If you are a hobby or beginner photographer, that was all you need to know and you better click that button “continue to aperture” above. Because what follows is the nerdy stuff. But if you are here for a rather “scientific” approach, here is

Scene Luminance in detail

What is luminance?

According to Wikipedia, “Luminance is a photometric measure of the luminous intensity per unit area of light traveling in a given direction. It describes the amount of light that passes through, is emitted from, or is reflected from a particular area, and falls within a given solid angle.

In photography, it’s mostly the light that is reflected from something (your scene) and (in case you photograph light sources like the sun or other lights), the light that is emitted. Either way, the lens you have attached to your camera will project that light onto your camera sensor that will then measure the luminance for every pixel in your scene. In reality, it will measure the illuminance though, because the light is bundled by the lens (and is, therefore “brighter” similar to a reading glass that will bundle sunlight to help you set your books on fire) but don’t let that confuse you.

The unit for luminance is candela (cd) per m2. Candela is Latin for candle, which also gives us a hint for the “brightness” of 1cd/m2. A common candle emits light with roughly 1 cd luminous intensity. This is very likely the reason why it is the base (0) of our exposure value scale. You sure have seen the candle in my graphic.
What might be a more familiar unit for some (but is in reality the same) is “nit”. That is often used to describe the brightness of e.g. smartphone screens, which btw. helps understand luminance a little better.

Consider photographing your smartphone. The higher the nits, the “brighter” the smartphone screen, and obviously the brighter the picture of your smartphone with given camera settings. The word brightness, however, is more the term for the subjective impression, while luminance is an objective photometric measure.

If you are a person who wants to know it all in the finest detail, let me recommend checking the formula of luminance, illuminance, and other stuff on Wikipedias luminance page.

How does scene luminance affect exposure?

When photographers talk about exposure, they are merely talking about image brightness. Scene luminance is the base of image brightness, because as you have learned above, the scene can get darker pretty quick without you even realizing by how much.

The exposure value takes that influence into account. It represents any combination of a camera’s shutter speeds and f-numbers that create the same exposure (or image brightness if you will) at a given scene luminance.
So for example these two images, despite taken with totally different settings, create the same brightness (or in other words exposure). Both have the same Exposure Value of 14EV (if you are interested in how these are calculated, here is a link to an Exposure Value Calculator). So as long as the image has the same image brightness as well as the same scene luminance and ISO stay the same. Rremember, we said ISO is not a part of exposure, therefore it doesn’t affect the EV, but being the gain, it of course affects image brightness.

Camera settings: 1/125sec | f/16 | ISO200

Camera settings: 1/1000sec | f/5.6 | ISO200

Exposure thereby of course only refers to the brightness of the image, not to the look (which can be different in regard to depth of focus, etc.).

Read more about exposure value on wikipedia.

 

Do you want to shoot images like these?

These images were all shot by me. My name is Wolf Amri, I’m a professional photographer and film maker.

On my youtube channel, I explain not only exposure, but everything else you need to know about photography. Explaining in a video is much easier than written because I can show you images, animations and examples on the way. So let me recommend watching my youtube beginner photography course.