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Exposure Learning with MUNNS - January 2019

We’ve covered all the important areas in controlling exposure, but what is it, why does it matter and what does it change? How do we know what is the correct exposure?

Before we move on, I will repeat something I once read and hope it gives you as much to think about as it did me;

“there is no such thing as over or under-exposure”

In photography, exposure is the amount of light that reaches your sensor or film. It is what determines how light or dark the image will be.

Now, although there are 3 points to the exposure triangle, there are only two factors that can change this; shutter speed and aperture.

But you said ISO is a major factor!” True, but ISO controls the sensitivity (or reaction) of the light reaching the sensor.

So does more light always mean better pictures?” Good question; let’s take a look and see.

Part 1 – Metering

Older film cameras often had no light meter and the photographer would rely on a hand held meter to check the optimal exposure settings.

The light meters used throughout the 60’s, 70’s and 80’s were not too different to the hand held meters we use today, albeit a little less electronic.

Every modern day camera will have built-in light metering.

Interchangeable lens cameras (ILC’s) will usually have more than one type.

The 3 common types of metering are:

  • 1. Matrix, Pattern & Evaluative Metering
  • 2. Centre-weighted Metering
  • 3. Spot Metering

We’ll go through these in a bit.

Metering is the camera’s way of deciding on the optimal shutter speed and/or aperture to use for an evenly lit shot. If ISO is set to ‘Auto’ it will also evaluate he best ISO value for the lighting conditions.

When I say “evenly lit” I mean that the camera will provide the results needed to show an even 18% grey image.

18% grey is the mid-point between shadows and highlights.

Metering will only calculate the light (brightness) of an image. It will not adjust for colour.

So why isn’t it 50% is it’s the ‘mid-point’?

18% is not a measurement of shade, it is a measurement of reflection and so 18% reflective is 50% grey (roughly) or colour #7C7C7C on the RGB scale.

Regardless of how technically advanced a digital camera is, they are not yet able to determine the requirements of the photographer and so, like any tool, it takes a little knowledge and expertise to judge exactly how best to take a shot.

I’ll talk about exposure more in part 3.

Metering methods:

Matrix, Pattern or Evaluative metering.

This type of metering is excellent for shots where light is evenly distributed without much difference in light and dark areas.

The camera will evaluate points from across the entire frame to determine how bright the subject matter is.

The amount of the image metered will depend on the age, make and style of camera. This can range from 8 points to 95% of the frame. Each camera and system will meter this differently.

Centre-weighted metering.

When shooting something specific that doesn’t take up the whole frame, (portrait for example), we want to disregard the background and only concentrate on the lighting of the subject.

For this we only want to use a selection around the centre of the frame. Typically centre-weighted metering will use around 20-50% of the image from the centre (like a bullseye).

Spot metering.

This is used when the subject matter is very small or if we’re tracking a moving object. It allows you to meter a very small area to correctly light it against a contrasting background (like a bird in the sky or the moon at night).

The usual area for spot metering is around 2% of the image.

On some modern cameras the spot metering can be set to the same place as the focus point. This provides additional control of your image in automated modes.

Part 3 – Over, under & ETTR

So now you understand what the camera does, and from where it measures the light, we can go over a few circumstances where the photographer will ignore the camera’s advice and decide for themselves how to expose the image.

In the image, you can see why the photographer may need to adjust the exposure to obtain the correct light. This example is showing an extreme but gives a good view of why metering cannot always be right.

Over-exposure:

Simply put, this is an image that is considered to be lighter/brighter than expected.

I use the word ‘expected’ because the only ‘correct’ exposure is that which you ‘expect’ to achieve.

With some images, it’s easy to see if the settings were incorrect and the expected exposure was over-shot, take these for example.

Under-exposure:

In contrast to over-exposing an image, under-exposing the image makes the final product darker than expected.

In exactly the same way, sometimes it’s easy to see from the final image if the exposure was not as expected.

ETTR – Expose to the right:

For years and years, people have stretched the limits of film and camera alike to obtain as much detail in an image as possible.

With film cameras people would be able to over expose and then adjust images in the darkroom using the best chemicals with the best paper and knowing exactly when and for how long to dodge and burn, gaining detail in the shadows without making the noise more visible.

In digital photography, ETTR is the technique of adjusting the exposure of an image as high as possible, without causing unwanted clipping (blowing out highlights or loss of detail in the shadow), to collect the maximum amount of light and thus get the optimum performance from the image sensor.

With digital, it’s a lot easier, it is always better to expose the image to the right of the histogram to capture and retain as much detail as possible whilst maintaining the minimum amount of noise (as long as you post process).

There is a big myth that states ETTR is used because more information is held in the lighter end of the scale.

Although there is an idea behind it, technically the statement is incorrect.

There is the same amount of data acquired by the sensor throughout the light scale (you can test this for yourself using a light to dark gradient card).

However; in post process, it is easier to maintain a high quality image by reducing the exposure, keeping the detail, than if you try to recover detail by increasing exposure.

Increasing exposure post process will normally increase the noise seen in the final image.

It all gets very technical, so let’s just say “if the sensor doesn’t ‘see’ it properly, it’s not going to be clear on the final image”.

With both film and digital imagery, ETTR will require some advanced post processing to obtain a good final image.

Part 4 – Colour

A lot of what you see with your eyes can be different in camera when adjusting exposure values. Colour is an area that is directly affected by exposure and metering.

Take a sunset for example; you go out and see a perfectly red sky at sunset with nice blues and purples. You need to know how to set up your camera to obtain the same in the final image. Exposure can be tricky if you don’t know what you’re doing.

If you pay attention to the metering suggestion and you use ESP (or pattern) metering, you might end up with poor rendering as the sky is still very bright. This is because your camera is trying to even out the light across the whole frame.

Depending on where the horizon is, your images may be under- or over-exposed.

The colour of the sky could be really washed out, or really dark.

A slight change in exposure can give a completely different effect.

The other example is shooting at the beach, or in the snow.

If you use the in camera metering, you are most likely to obtain a dull grey colour in the white sand or snow.

This is again because the camera is seeing white, and trying to make it grey (18% Grey / Mid-point Grey). To overcome this you will need to over expose your image by up to 2 stops.

Part 5 – Visual assistance

There are many visual aids available to overcome exposure issues.

External Light Meters:

There are two main types of light meter for still photography; Incident and Reflected light meters. Some hand held meters will have dual function providing both reflected metering and Incident.

Reflected light meters are similar in function to those found in-camera. They measure the amount of light that is reflected by the subject.

Incident meters are those that you hold in the shot and measure the amount of light falling onto it.

To aid the light meter and provide an easier reference for colour, it is not unusual to see grey or colour cards held in front of the subject for a test shot.

I’ll explain these in a later tutorial.

In-camera assistance:

Light meter.

As above the modern digital cameras will all have a light meter and small exposure meter. Normally this is presented in the viewfinder or on screen with an easy to read bar of between 3 and 5 stops either side of exposure.

If you like to use exposure compensation, this is usually where you can see it.

Histogram.

Another useful tool is the histogram. As I explained above, the histogram can show in a linear graph the distribution of light to dark in the image.

An exposure with even light will show a nice even line of tones. Low-key (darker) images will be pushed to the left and high-key (lighter) to the right.

It is important to keep all your tonal data between 0 and 255 as this is the range for all visual media.

Zero as pure black and 255 as pure white.

You may also be able to select RGB colour although this is usually only available after the image has been shot and processed by the camera.

Highlights & Shadows (Blinkies).

This is an option on most modern digital cameras and allows the photographer to see instantly which areas of the image are over exposed and which areas are under exposed.

It does this by flashing a coloured overlay on the image where the highlights and shadows have been clipped (the extreme ends of the histogram).

Highlights will normally show in a bright colour (such as orange or yellow) and Shadows will normally show in a dark colour (like Dark Blue or Dark Green).

As always, these are only guidelines. Now that you know how Exposure works, you can experiment yourself and have some fun!
Look out for more tutorials coming soon.
Created By
David Munns
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