If you have a digital camera, the chances are that you’ve met a weird-looking scale before. It has numbers from -3 to +3, with a 0 in the middle.
It’s the exposure compensation scale.
Today I’ll show you how and why it works. Find out how to use exposure compensation to improve your photography workflow.
Exploring the Human Eye Dynamic Range for Exposure Compensation
Let’s start with at the very basics, at the thing that defines photography: light. You need to understand this in detail to understand exposure. And to understand exposure compensation, you need to know the concept of exposure.
So, light travels in waves, like sound. But, it also has its own substance, photons. It’s countable, measurable, and collectable.
Light starts to travel from the light source. It bounces off (reflects) from surfaces, and continues its way in straight lines. It finally ends its journey at the point where it gets fully absorbed.
Light is also a measurable energy. When something absorbs light, this energy transforms into other forms of energy, like heat or electricity.
This is why we can transform it into solar energy. And this is also why photography works.
How We Record Light
When light hits the camera sensor or film, its energy initiates specific processes. On film, this process is a chemical reaction. On digital sensors, the energy is converted into an electric current.
More light generates a higher current – less light generates less electricity.
What Is Exposure in Photography?
Exposure is the amount of light that a sensor or film detects. Three factors determine it:
- Scene luminance, which is the intensity of light present in the scene. A bright, sunny day presents a higher scene luminance than a moonlit landscape;
- Shutter speed, which is the length of time when the light can pass;
- And Aperture, which is the relative amount of light getting passing through the camera lens.
The above listed three factors determine the total light getting onto the sensor. This is an absolute way of expressing camera exposure, without considering the sensor type and size.
What Is Exposure Value?
Exposure value is the popular way of expressing aperture and shutter speed at once. It also reflects the transmission of exposure settings.
These two can be changed in such a way that the total amount of light doesn’t change. If you double the size of the aperture, you have to half the shutter speed.
If you give with one hand and take with the other, the total won’t change.
In the middle of the 20th century, a simplifying effort was launched. It later became the Exposure Value System.
The point is to combine f-stop and shutter speed into one number: the EV.
This can be illustrated on an infinite scale of combinations. In the middle, 0 EV is defined as 1 second at f/1.
Any other combination resulting in the same amount of light is also 0 EV. If you double the shutter speed to 2 seconds and decrease the aperture to f/1.4, you’ll still be at 0 EV.
The scale works in doubles and halves. 4 EV is a 24 times higher amount of light. -4 EV is a 24 times lower amount.
Using ISO to Increase Light Sensitivity
As we discussed, when light hits the sensor, it generates a current.
That current is then converted into image data using an analogue-to-digital (A/D) converter.
However, your image might not look as bright as you’d wish. You can find yourself in a situation where your other parameters are fixed. You can’t play with aperture or shutter speed.
Here comes in ISO. With a higher ISO setting, you can make your sensor more ‘sensitive’ to light.
Thus, ISO becomes the third parameter that you can control in-camera.
The exposure value scale still works: add ISO 100 everywhere in the previous diagram. You can also increase exposure using only the ISO.
Unlike aperture, ISO is linear. So 3200 is 32x more ‘sensitive’ than 100.
Understanding Dynamic Range
No camera film or sensor is capable of capturing everything.
A scene contains brighter and darker elements. Brighter things emit or reflect more light than darker ones.
EVs can also express the luminance differences between these bright and dark things. I’ll explain later how.
Dynamic range is the difference between these darkest and brightest parts. This range still contains valuable information.
Contrasty scenes have a wide dynamic range, while misty, mild scenes have a narrower one. So, a camera sensor will struggle more to capture the contrasty scene.
Midrange digital cameras today have a range of about 10-12 EV. If a scene is less dynamic than this, you have more room to ‘play’ with exposure.
If a scene has a wider range than that, you’ll either have to sacrifice data or bracket exposures.
Choosing the Correct Exposure
What Is the Correct Exposure?
Exposure is correct if you, the photographer think it is. Your camera doesn’t have an artistic vein like you. We need to teach the camera what a correct exposure is. No matter how bright or dark it is – if that’s what you want, it is correct.
There is a standard for the algorithmically correct exposure. It’s the following:
If an area that is 18% grey (middle grey) is exposed to the middle, the exposure is correct.Let me explain.
Calibrate Your Camera to Middle Grey for Correct Exposure
Middle grey is a shade of grey that our eyes perceive being halfway between black and white.
It’s not a color in a scientific sense. It has all the colors in it at the same intensity. So, it can be used with both monochrome and color cameras as a reference point.
When you average the luminance levels in a usual scene (say, a cityscape in moderate sunlight), they are very close to the luminance of middle grey.
So, if we ‘teach’ our camera that middle grey is a proper exposure, it will also expose correctly in most everyday scenes.
Hence, your camera’s built-in light meter is calibrated to expose for middle greys and anything that has the same luminance.
You can purchase grey cards to perfect your own exposures (and colors, too, to an extent). However, for most applications, it’s not necessary.
Measuring the Correct Exposure
Let’s photograph a subject with EV 0 settings. So, for example, at ISO 100, 2 seconds and f/2.
You measure how much brighter or darker the subject is than 18% grey.
With that difference, you can express the individual exposure value of a subject.
So, if your subject is 8 times darker than middle grey at EV 0 settings, you can say the luminance of the subject is -3 EV. (Note: If you see that a camera can autofocus down to -3 EV, it’s the same thing. It means that it can accurately catch focus on subjects brighter than -3 EV.)
Your modern camera can also do this for the whole scene.
Measuring Exposure Using Light Metering in Cameras
In old times, photographers had separate light meters. They would measure exposure by pointing the light meter towards their subjects. They would then read the measurement, set the correct exposure on the camera, and shoot.
This process is what happens is modern cameras too.
We have light meters incorporated in the camera bodies. They act as a very low-resolution, high-dynamic-range sensor.
The measurement happens after the light has passed through the lens, giving a more accurate result. This is called TTL, Through The Lens metering.
There are a few metering modes you can choose from. They use various algorithms and preferences that you can set.
These are the modes that almost all modern DSLR and mirrorless cameras have. No matter the brand or price point, you’ll find these in your camera, too.
- Matrix / Evaluative. This is the most complex one. It meters from the entire area of the scene. However, it tries to figure out what’s the actual subject and weighs it more. Often, it uses focusing distance, colors, tracking and relative size to improve accuracy.
- Center-weighted Average. A communicative name. Indeed, this mode meters the whole scene, weighing the center parts more.
- Partial. This mode only meters a part of the scene and ignores everything else.
- Spot. Similar to partial, but it meters an even smaller, point-like area. Old-school light meters are often spot meters.
When Should You Deviate from the Measured Exposure?
If your camera thinks that everything should be middle-grey-bright, some things will go wrong.
What’s the deal with bright winter landscapes full of white snow? Or with a nighttime cityscape? In these situations, your camera will probably miss the exposure by default. It will under and overexpose, in respective order.
You can also get into a situation where you have a well-lit subject before a dark background. The chances are that your camera will overexpose because the dark background occupies most of the frame.
But don’t give up! Here comes exposure compensation to save you.
What Does the Exposure Compensation Scale do?
Exposure compensation is a basic setting that you can adjust to change the final exposure.
You can do this in semi-automatic modes, and a few cameras have it in manual mode, too.
Where Can You Find Exposure Compensation?
You can find the exposure compensation scale on the main interface of your camera menu. In cameras, it occupies the biggest space in the Q (Quick) menu.
You’ll also find it on the top LCD screen if you have one on your camera.
How to Adjust Exposure Compensation
On cameras with two dials, one dial controls the priority setting. The other dial controls compensation.
On cameras with one dial, it’s a bit more complicated. For instance, on Canon’s lower-range DSLRs, you have to press the compensation button and turn the dial at once.Some cameras have separate compensation dials. This includes most of Fujifilm’s products, the Nikon Df, the Sony A7 series, and more.
Exposure Compensation in Semi-Automatic Modes
Semi-automatic modes are the following:
- P – Program mode, sets a combination of shutter speed and aperture
- Tv / S – Shutter priority mode, sets the aperture automatically
- Av / A – Aperture priority mode, sets the shutter speed automatically
In these modes, the camera measures light and sets exposure automatically, based on your preferences.
By default, it sets it to the middle. However, you can adjust it to be darker or lighter. So, it will compensate for darker or lighter conditions.
Let’s go back to our previous example. Suppose you have a beautiful, snow-covered landscape before you. Your camera is set to Av (aperture priority mode) because you prefer a narrower aperture in the situation.
By default, the camera will assume that the scene is middle-grey-bright. So, it will underexpose it, and indeed, the resulting image will be middle grey.
Now let’s set the exposure compensation to (around) +1.3 stops. If you take a shot now, it will be much closer to the real luminance of the scene.
You can use it in the nighttime cityscape situation too.
If you turn down exposure compensation by 1-2 stops, your exposure will give a better representation of the scene.
What Does Exposure Compensation do in Manual Mode?
Some select cameras, such as the Canon 5D MkIV, have compensation in manual mode, too.
You can have your aperture and shutter speed fixed. Just set ISO to Auto, and you can adjust the exposure compensation. I find this to be a practical feature.
When Should I Adjust Exposure Compensation?
You Need to Discard Highlight / Shadow Data
You can only do this with very steady hands (or a tripod), and where there’s not much movement in the scene.
Otherwise, you’ll have to sacrifice either highlight or shadow data and deal with clipping.
However, you surely have preferences on which to keep and which to discard.
In the previous portrait photograph, the artist chose to let go of some highlight data. In turn, the subject’s face is exposed bright enough to be clearly visible (and post-processable).
Set Exposure Compensation for High Key and Low Key Shots
There are other, artistic considerations, too. High key and low-key shots, besides careful lighting, also require over or underexposure (in technical terms).
If you want to photograph in a high-key style, you’ll need to set exposure compensation to overexpose slightly. Be aware of clipping highlights, though, if that’s not your intention.
For low-key, you’ll need to use negative compensation, often -3 stops or more. In such scenarios, I recommend using manual mode. You can easily manage it with exposure compensation, too.
Use Manual Mode for Full Control of Exposure
It’s worth learning to nail the exposure in manual mode. Exposure compensation, however practical it is, doesn’t give you full control over your camera.
In those situations, you’re much better off using manual, even with limited experience.
Exposure compensation is a powerful feature that allows you to eliminate bugging with the settings. It often gives you more time to concentrate on the actual photograph.
You must know how exposure compensation works, and when and how you should use it.
You should also know the limitations of using exposure compensation. Recognise when it’s time to switch to manual.
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