HDR Photography: Understanding Exposure

Other Posts in this Series "High Dynamic Range (HDR) Photography"

Previous Post: HDR Photography: Introduction
Next Post: HDR Photography: What is Dynamic Range?


This tutorial series is written for those just learning about their cameras.  If you have been using your camera for a number of years, then this tutorial isn’t really geared for you, but please feel free to read, we all learn something new even when reading about things we know.  

I decided to write this for those who really don’t understand or know what exposure is, or that their cameras have a built in light meter, or most other camera functions or photography basics.  This is for those who have used their cameras in the automatic mode and want to learn enough about the art and craft of photography to begin to use their cameras in the other modes available.  

I will have links to those sites that I have used to learn these concepts myself, and many of these sites will typically be a lot more in-depth than I will get.  These sites would be of interest to those with more knowledge and understanding.  Enjoy!

In order to understand HDR photography, we need to review our understanding of what is a proper exposure. This next section will be new to some, and to others will be just a review of what is meant by exposure and how the camera understands exposure. These are a necessary foundation to understanding how HDR photography works.

Middle Gray

Let’s say that our world has no color, just black and white and various shades of gray in-between. If I was to ask you about gray, you know, the color between black and white, the color which seems to have equal amounts of black and white, you would assume that it has exactly 50% black and 50% white. Makes sense doesn’t it?

And so this gray is often called middle gray and is exactly between black and white. So, for example, let’s say that you have a bucket that can hold exactly 256 balls (it doesn’t matter what kind of balls, lets say, golf balls!). Black would be 0 balls. And white would be 256 balls. What is half way between 0 and 256? Why it is 128! And this would be middle gray, and in sRGB (Standard Red-Green-Blue color scale of most computer monitors, hence sRGB), middle gray is 128,128,128. These numbers are, to put it simply, the amount of red, green and blue required to make middle gray.

So let’s try something other than balls, lets go with paint, cups of paint. So if you took 128 cups of red paint, and mixed it with 128 cups of green paint, and mixed that with 128 cups of blue paint, you would end up with, yup, you guessed it, middle gray!

So far so good!

In Camera Light Meters

Now comes the tricky part. Remember the primary purpose of a camera is to capture light. And there are two kinds of light, what is called incident light and reflected light.

Incident light is quite simply the light ray from the light source that hits the surface of an object. Another way of putting this is that incident light is the amount of light that is illuminating the surface of the object.

Reflected light is the light ray that is reflected off the surface of the object. And another way of putting this is that reflect light is the amount of light that is bouncing off the surface of the object being lit.

All modern cameras have built in light meters. And all in camera light meters are reflective light meters, meaning that they measure the amount of light that is falling on the sensor that is being reflected from the surface of the object being photographed. What is really being measured is not necessarily the amount of reflected light, but rather the brightness of the scene, called luminance.

Viewfinder of a Canon 5D Mark II, the Exposure level indicator is the indicator for the built-in light meter.


Now, we know that middle gray is halfway between black and white, at 50% of each. But luminance works on a geometric scale, where a previous value is doubled.

In the early days of photography, it was determined that soot, about the blackest thing photographers could find, had an average luminance of around 3%. Snow, about the whitest thing that a photographer would photograph (other than the sun), has an average luminance of around 96%. Now check out how we get from 3% to 96% in the scale below:

SOOT                                                                 SNOW
3%        6%        12%        24%        48%        96%




You will quickly learn in photography that the scales used to determine exposure operate on a geometric scale – everything from ISO settings to shutter speeds to aperture settings – all work on a geometric scale. You are doubling values going up the scale, or halving values going down the scale!


Pretty neat how that works. As it a geometric scale, the number doubles each time from the 3% all the way up to 96%. Now consider where the middle of the scale would be? Perhaps somewhere between the 12% and the 24%, right?

SOOT                                                                SNOW
3%        6%        12%        24%        48%        96%

Well, that is where middle gray sits, at 18% reflectance.

The Question of 18% Gray or 12% Gray?

There is a debate among photographers whether or not their cameras measure luminance at 12% gray or 18% gray. If you search the internet, you will find this debate has been going on for quite a number of years.

The reality is that cameras, which have reflective light meters, do tend to measure in a range of anywhere from 12% to 18%. And this is based on the ISO standard, ISO 2720-1974, “Photography – General purpose photographic exposure meters (photoelectric type) – Guide to product specification (First edition – 1974-08-15), that allows for the range depending on what certain constants, called K and C, are determined to be by the manufacturer.

For those who are curious, the standard states “The constants K and C shall be chosen by statistical analysis of the results of a large number of tests carried out to determine the acceptability to a large number of observers, of a number of photographs, for which the exposure was known, obtained under various conditions of subject manner and over a range of luminances.”

It is really an almost meaningless debate and one that you shouldn’t get all wrapped around the axle. For most photographers, it is almost imperceptible the difference between 12% and 18% gray, only around 1/2 stop depending on your camera’s light meter. So I would suggest using the idea of 18% gray as that is the industry standard description of the reflectance of middle gray, and it just makes life easier. Just note that if you use a Canon or a Nikon, your in-camera light meter will NOT most likely be exactly calibrated to 18%, but rather will be closer to 12% gray. So again, you can compensate by increasing exposure by around 1/2 stop. But no matter, the effect is mostly the same for most photographers.  For those who are more advanced, you can increase your exposure, for those just learning and trying to understand, don’t worry about it at this point in your journey.

FYI – Some of us actually test our cameras to determine what our light meters are calibrated for – I am guilty as charged 😉 – but that is way beyond this tutorial, and will be covered in another blog article at a later time.

In the next blog article we will take this understanding and apply it to the camera and how the camera understands exposure.



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