Exposure and Metering
Photography is all about light and shadows. An image is often called as an exposure. So what is exposure?
There are many explanations for exposure in photography. The summary of those explanations is below. When an image is said to be properly exposed?
Answer: when there is no clipping in highlights and shadows. It means there are detail in all the areas of the image/frame.
Highlights: The brighter areas of the frame
Shadows: Darker areas of the frame Grayscale: Grayscale will have/give luminance information 0 to 255 at any pixel. (It is a wide topic, let’s keep it as above for our understanding)
If it is 255 it will be pure white. If it is 0 it will be pure black
Clipping: If the luminance value at a pixel reaches 0 or 255 then the clipping occurs (which means no details)
In a frame, camera will measure all the light reflects from all the colours in grayscale. A properly exposed image will be perfect 18% Gray (which is the midpoint between 0(black) and 255(white)
But in practical it is not possible, as each colour will have different reflective property. Hence the camera will arrive at an average and will preserve the detail on highlights and shadows
Below are two examples of how camera meter works, based on the metering mode you select. Just observe the images will discuss about these images below. They are straight out of camera without any post processing
Image 1: Shot with Evaluative metering
Image 2 : Shot with Spot metering
Every digital camera in the world will have built in light meter, with which it will calculate the required amount of light to get the proper exposure for the frame in front of it. If it is advanced camera with manual settings (all DSLR and Mirrorless and Few Bridge Cameras) will have options to change the metering mode. If I explain the metering modes, you will easily understand what is metering. In any DSLR or Mirror less these are the metering modes available
I am a Canon user I will use canon terminology (excuse me guys) 1. Evaluative Metering 2. Centre-Weighted Average Metering
3. Partial Metering 4. Spot Metering
Above image shows the metering modes available in Canon and Nikon Cameras and its icon respectively. As you can see every icon is like a Frame or like shape of sensor/film. Among these metering modes most used metering mode is Evaluative Metering (Matrix metering in Nikon). Next mode which is widely used is Spot metering. Will discuss about these two modes But usage is based on the need and scene. Evaluative Metering/ Matrix Metering:- In this metering mode camera will take all the reflected light from entire scene/frame in front of it to calculate the required setting or will give the meter reading to achieve the proper exposure. This will be widely used for event photography, indoors etc.
lets discuss about image 1 - It was shot with evaluative metering, in which camera has brought down the exposure to get the details in highlights in the background(the details of the building are visible)
(All digital cameras will work to expose the highlights properly)
But at the same time our main subject is way under exposed. This is because the camera considered the light from entire scene and averaged the exposure to keep details in highlights and shadows
Spot Metering:- In this metering mode camera will consider the light from (in most cameras) centre focus point to calculate the exposure. The local area at centre focus point will be properly exposed. If there is any highlights clipping or shadow clipping at other areas camera will not worry about it. This mode is used when there is heavy backlit that means heavy light from back of the subject towards camera, to shoot a flower etc. For example:- Consider the image 2, It was shot with spot metering. The red warning areas are highlight clipping (it means details were missing). Camera took the light values from the centre of the frame and arrived at the required exposure. So the subject is properly exposed and back ground is blown (over exposed), The details in the buildings are not clearly visible.
Best performing 35mm sensor in current mark have dynamic range of around 14 stops. Human eye can read about 20 stops.
Stops – that is how light calculated in photography, will discuss in detail in coming topics
So metering (calculating the light) is what camera do to arrive at proper exposure. Metering modes are there to customize the camera meter for various shooting conditions.