Understanding exposure (pt2)

In my last post Understanding more about exposure I talked about learning to see in black and white and how images are made up of tones.

In this blog post I want to talk about how to assess a scene and select the best exposure and in Part 3 of this blog I will talk about what to do when a scene falls outside of the dynamic range of your camera. But first a recap and some notes on histograms. and camera settings

Why you should not rely on your histogram

Your camera’s histogram only shows you the distribution of tones in your scene and whilst it is informative it is not an accurate way to control exposure.

Camera settings

Most cameras come with built-in light meters but when your camera is in anything but manual mode you don’t get to see the light meter, only the exposure compensation settings and perhaps a histogram

Camera viewfinder in programme mode Lumix GX8

When you switch your camera to manual mode you are presented with a similar view but the exposure compensation value indicator turns into an expsoure meter with a number of stops shown.

Camera in manual mode and spot metering

In the shot above the camera is set to ISO 800 an aperture of F5 and a shutter speed of 1/40s. Spot metering has been enabled and the spot reading shows that the subject under the crosshairs is 2 stops brighter than the mean exposure.

Mean Exposure

What is mean exposure?

Tones ranging from pure black to pure white

Without getting too technical, scenes are made up of a number of tones. Cameras use reflective light metering to measure the brightness of a scene or subject.

When using matrix or evaluative metering it measures the overall brightness of the scene or subject and then sets a mean exposure (average exposure for the scene) based on how dark or how light the scene or subject is.

This value is represented as zero (0) on your camera’s exposure meter (see above). Zero (0) equates to what is commonly known as zone 5, 18% grey or midtone.

18% grey represents the average reflectiveness of most day to day objects that we photograph and light meters are calibrated to produce an 18% grey tone.

The main point here is that if your subject is very dark or black your camera’s meter will set the exposure to brighten the subject. If your subject is very bright or white, your camera’s meter will set the exposure to darken the subject. Making them conform to 18% grey.

Knowing where your tones lie, taking spot meter readings from the most important tones and adjusting your exposure manually, so that your subject falls on the correct zone is the key to optimising exposure!

Mean exposure 0 (zone 5)

Spot Metering & Matrix Metering

Below is an every day scene, it’s made up of a number tones. Because the contrast range of the scene does exceed the dynamic range of my camera’s sensor, I was able to take the shot knowing that my shadows and highlights would render properly and not be too dark (underexposed) or too light (overexposed).

But how did I choose my aperture and shutter speed combination to ensure optimum exposure?

Let’s look at the scene from a tone perspective. Its made up of highlights, shadows and midtones. Notice that there are very few bright tones in this scene, the boathouse (highlight tone 8) is a relatively small percentage of the the overall scene but important in my shot.

Evaluating scene

If I had left my camera on matrix metering mode and used the exposure suggested by my camera, this would have happened.

Image from suggested exposure reading (matrix metering)

The scene is a little washed out as the matrix metering has tried to average out the exposure giving more emphasis to the darker tones.

Because I knew this would happen, I used a handheld light-meter and took a spot reading from the boathouse as I did not want the boathouse to be overexposed.

Tones relate to stops, each zone is equivalent to 1 stop on your camera.

My spot-meter gave a mean exposure reading of F8 @ 1/1000s (zone 5). Remember whether your subject is dark or light, A reflected light-meter always sets a mean exposure. Knowing this, I set my expsoure to F8 @ 1/250s. giving my boathouse 2 stops more exposure, pinning it to zone 7.

When you put your camera in manual mode and combine it with spot-metering, you can assess the dynamic range of your scene, measure the important areas and set your exposure accordingly.

Here is the final image

The boathouse has been placed on zone 7 to ensure that there is detail in the boathouse and that it is not too bright and all of the other tones in the scene have been rendered appropriately. Giving me the optimum exposure for my scene.

Notice that working this way I have managed to retain the subtle detail of the clouds in the sky too.

More on Camera Metering Modes

Another benefit of working this was is that there is little, if any correction to be made when post processing as the exposure is spot on!

The better cameras come with three types of metering, Evaluative (also known as matrix or segment metering), Centrer weighted and Spot metering.

Matrix and centre weighted metering provide a general expsoure that may or may not be correct for the scene. But, the only way to truly evaluate your scene and set exposure accurately is to turn to spot metering.

In my next blog post I will focus on how I use the spot meter in my camera and the benefits of using a hand-held light meter. I will also talk about what to do when the scene falls outside of the dynamic range of your camera’s sensor.

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