1 October 2018
GETTING THE EXPOSURE JUST RIGHT

Our previous few blogs have each looked at one of the three elements that help to determine an image’s final exposure, namely:

  • the aperture,
  • the shutter speed,
  • and the ISO.

In today’s blog, we are going to delve a little bit deeper into how we can take finer control over the exposure when the camera’s internal metering systems don’t quite get it right in trickier lighting conditions.

The idea of the “correct exposure” has been floated around a few times in the preceding blogs, but what exactly is the correct exposure, and how is it determined? Firstly, the correct exposure is simply that exposure which best captures the light of the scene in front of us so that it is recorded how we want it to be recorded. The problem is, how we want it to be recorded is not always how the camera “wants” it to be recorded. Remember, today's camera's are nothing but computer's that record light, and it is programmed to capture “average”, mid-toned (18 percent grey) scenes and present these as the images that we see. In order to assess the scene, and thus determine the exposure, the camera makes use of a variety of metering modes to read the light, and then set the relevant aperture, shutter speed and ISO to record it as such. In the not so distant past, the metering systems were not nearly as accurate as they are today, and without the luxury of LCD screens on the back of film cameras to review your result, it was sometimes a hit or miss affair; if you found out that the images were over- or under-exposed once they were developed, well, then it was too late!

With the advent of digital cameras, one of the many game-changing developments has been the ability to immediately review images on high quality screens (with the aid of detailed histograms) to assess their exposure, and if any errors exist, changes can be made on the camera and another image can be taken! Oh how easy we have it as photographers these days!!!

What has also helped is improved metering systems that do much of the “thinking” for us. Still, they don’t always get it right in, as again, cameras do not “know” what they are looking at; they simply evaluate the light in the scene and expose for a mid-toned scene; when scenes are not mid-toned, the camera gets it “wrong”, we end up with images that are either too light, or too dark.

Now, the way that the camera evaluates the scene depends greatly on the metering mode to which it is set. Generally speaking, there are three metering modes that cameras make use of:

  • evaluative or matrix,
  • centre-weighted,
  • and spot metering.

The most advanced of the metering systems is the evaluative or matrix metering system whereby the entire scene presented to the camera is divided up into segments, and each of these segments is then evaluated, and using metering algorithms, the exposure that best suits the whole scene is used. Depending on the make and model of camera, sometimes more weight is given to the area surrounding the selected focus point (based on the assumption that the subject is at point of focus, and is thus the area most critical for an accurate exposure). Evaluative metering works well for front-lit scenes, and general images of landscapes where no tricky back- or side-lighting is present.


The red area indicates the parts of the frame from which evaluative metering has been measuring the light to determine a balanced, overall exposure for the scene

Centre-weighted metering evaluates the scene, but places more weight and emphasis on making sure that the centre of the image is correctly exposed. This is useful for centrally composed images, and has its best application in portraiture work, and is perhaps not the most suited metering mode to be used in wildlife photography, and as a result, it is not one that I personally find any value in.


The central placement of a subject could necessitate the use of centre-weighted metering where more importance is given to the camera’s metering readings taken off the central part of the image; it can work well with backlight, central subjects

The last metering mode is spot metering, whereby the metering is recorded at the centre of the frame, metering from only 1,5% to 5% of the scene depending on the camera.What you are essentially doing is telling the camera to expose correctly for the small, selected spot, and ignore all other lighting in the scene. This metering mode works particularly well in difficult lighting situations such as back- or side-lighting, or where the background of the subject is either very light or very dark (such as a spot lit animal in the dark of night).By telling the camera to expose the subject correctly, you are able to keep the background as the dark or light background that it is meant to be rather than letting it interfere with the cameras metering, leading to an incorrectly exposed subject.


Here, spot metering is shown as the small red area from which the metering of the entire scene has been set; in this instance the camera was told to meter the baboons correctly whilst ignoring the very dark background that would otherwise have caused the camera to meter “incorrectly” for the scene we wanted to capture

To illustrate the differences between the two most suitable metering modes for wildlife, let us look at how evaluative and spot metering work in a few practical examples.

First, let us imagine a scene with a herd of impalas feeding on summer’s green grasses under a clear blue sky, with the sun behind us (front lighting). With our camera set to evaluative metering, we point the camera at the scene, and it will meter off each part of the scene to come up with a final exposure (remembering of course, that this all happens almost instantaneously!).If we examine each of the components of the scene; the blue sky, the green grass and the front-lit impalas, we will see that all are roughly mid-toned elements, and with the simple front-lighting of the scene, the camera’s evaluative metering will not have any issues in exposing this scene correctly.If we preferred to use spot metering instead, we would take an exposure reading off the green grass (a handy, mid-toned element for us to lock the exposure on), recompose the image and snap away with a very similar result to the first metering effort.


A great example of where a scene full of mid-toned subjects and backgrounds allowed the evaluative metering to correctly meter the entire scene

Now, imagine a similar scene, but with dark rain clouds in the sky. The camera doesn’t know that the sky is full of dark clouds – in fact, it doesn’t even know it is looking at a sky.It is looking at tones.If the sky dominates the scene, it is going to see that the darker-than-mid-tone clouds make up a large portion of the frame, and it will attempt to expose the scene so that the darker-than-mid-tone clouds appear as mid-toned. As a result of this, the whole scene is going to be lightened up, leading to a slightly over-exposed image. Spot metering off the green grass would give the scene a better exposure and keep the dark storm clouds as dark clouds.


Due to the rain clouds being darker than mid-toned, and due to their prevalence in the scene, the evaluative metering determined that the scene was too dark, and thus lightened it up resulting in a slightly over-exposed image


By telling the camera to spot meter off the grass, the overall exposure has improved by leaving the rain clouds as the dark-toned objects that they are

The same thing would happen if you were photographing any dark-coloured animal.A mud-covered rhino, a large herd of cape buffalo or an elephant bull emerging from the water.The camera doesn’t know that these objects should be rendered as dark as our eyes see them, so instead, the metering system tries to make them mid-toned, and exposes them as such.If the image has to be lightened up to the point where a dark, wet elephant appears mid toned, then it means that all parts of the image will get lighter too, and will result in an over exposed image; a bright, washed out image.Again, if one were to spot meter off a mid-toned object in the same scene (and in the same light), then the camera would meter for only that spot and ignore the wet elephant, leading to an image with a correctly exposed surrounds, as well as an elephant that appears as dark as it actually is.

Some slightly wet elephants in the water were seen by the camera as being too dark, so in attempting to expose them as mid-toned objects, the entire scene has lightened up leading to an over-exposed image


Very wet elephants are even more difficult, but by metering off a mid-toned element in the photo (such as the water), the overall exposure has remained correct and rendered the wet elephants as the dark-toned animals that they are


Buffalo are notoriously difficult to photograph because in metering off their dark hides, the camera sees a darker-than-average scene and attempts to lighten it up resulting in a washed-out image.


Spot metering off the more neutral background will allow the camera to ignore the dark tones of the buffalo in its metering calculations and in so doing, ensure a correctly exposed image

A last example of a scenario where the camera’s evaluative metering will get very confused by the scene in front of it is when photographing nocturnal predators with the aid of a spot light. Our eyes have no problem interpreting and seeing the scene surrounded by darkness, but for the camera’s “eye”, it appears to be a very dark scene with one very bright patch where the spot light is illuminating the animal. In attempting to correctly expose the dark background as a mid-toned element of the photo, the camera tries to lighten it up (by using a long, slow shutter speed to allow more light in).There will only be one result; the blackness of the night stays gets a little lighter, and the area highlighted by the spotlight becomes horribly overexposed (and blurry) in the process.By spot metering off the illuminated subject, the camera will then correctly expose for only the highlighted area - making that mid-toned – and give you a correctly exposed subject surrounded by darkness that stays black.


In letting the camera assess this night scene, it got horribly confused by the black background and the bright spotlight; in trying to balance things and lighten the scene up, the longer shutter speed has resulted in an overexposed (and blurred) subject and a dark background with some detail in it


If we spot meter off the bright spot-lit part of the scene, the camera will ensure that this part remains correctly exposed and the dark background of the image stays as a dark background

Although encountered less frequently, the converse situations do occasionally show themselves in wildlife photography where, rather than being presented with darker-than-mid-toned subjects and backgrounds, we find lighter-than-mid-toned subjects and scenes.If one has a white subject, like a pure-white cattle egret for instance, that takes up a good portion of the scene, the camera will meter off that and think that the area is much too bright, and as a result, it will try to expose the pure white area as mid-grey, and the entire scene will be under-exposed, and much darker than desired.The same thing would happen with a less obvious environment, like the light-coloured sands of the riverbeds that one often finds in the bush.Similarly, the camera will attempt to meter off the sand, think that it is too bright and expose it as a mid-toned surface instead of the lighter-than-mid-toned surface that it is.Again, this will result in dark, under exposed subjects if they are photographed on the bright riverbeds – to avoid this, use spot metering, and take the light reading off the mid-toned subject (or something else mid-toned in the same scene) and this will keep the bright surfaces bright.


Here the camera has tried to expose the pure-white cattle egret as a more mid-toned bird, not “knowing” that it should be rendered white; this results in a darker, under-exposed scene


In a similar manner, the light coloured sands of the dry riverbed were deemed to be too light by the camera’s metering system and thus exposed to be captured as mid-toned sand – in doing so it resulted in a darker than intended scene


If spot metering is done off a mid-toned object in the scene, the egret can still be captured as a white bird


Likewise, by metering off the leopard, the camera will correctly expose the riverbed sand as the light-toned element that it is

Spot metering is best used in conjunction with your camera’s AEL (auto-exposure lock) button at the back of the camera (AEL in Nikon, and an asterisk “*”in Canon).In spot metering mode, you point the centre of the camera over the area that you want to be spot metered and push your AEL button to lock the exposure.You can then focus on the subject, recompose the image, and press the shutter to click a correctly exposed image! 

Hopefully this has helped you to more clearly envisage the way that the camera sees and interprets the world around us, and demonstrated how by making use of the spot metering function of your camera, you can better control your exposures. But before we leave the exposure element of photography behind, we need to look at another very useful way of controlling your exposure, and that is through the use of the exposure compensation dial of the camera.


The basic exposure compensation dial and the effect that changing it has on the exposure of an image; the top image is a correctly exposed image of a mongoose with no exposure compensation (set to 0); the middle image has an exposure bias of -1 and has resulted in a dark, under-exposed image; the bottom image has been over-exposed by +1 stop using exposure compensation leaving us with a brighter image

The exposure compensation dial goes from -3 through 0 to +3; 0 represents what that camera’s metering system determines to be the correct exposure.If you move to the minus-side of the dial, you will be under-exposing the image, whilst a move towards the plus-side will cause an over-exposure.Personally, this is my preferred way of exposing images; I typically leave the exposure compensation set to 0, and after taking a shot, have a quick review of it on the display screen; if the image is too bright (over-exposed) and I need light to be taken away from the image, I move the exposure compensation dial to the minus-side, to remove light from the scene.Conversely, if the image is too dark (under-exposed), and I need to “add” light, I move the exposure compensation dial to the plus side, to brighten it up.

In terms of assessing the exposure, one can use the display screen on the camera, but do bear in mind that these are not always of the best quality, and depending on screen brightness, ambient light and viewing angle, how you see your image will be a more or less accurate view of what the camera has actually recorded.For this reason, it is a good idea to learn how to use the histogram feature on the camera.

The histogram is a very useful tool for accurately assessing the spread of pixel values across a photo.Looking at the horizontal axis, you have the brightness values of the pixels, with the left edge having a value of 0 (or pure black) which increases to a value of 255 on the far right (pure white).The vertical axis shows how many pixels have been recorded for each brightness value, so this will let us know two thing: firstly, by showing where the majority of the pixels fall, it tells us whether the image is under-exposed (more pixels to the left of centre) or over-exposed (more pixels to the right of centre), and secondly, if there are lots of pixels at either end of the horizontal axis, it warns us if we have blown out our highlights (collection of pixels on the right hand side of the graph) or have not got any detail in the shadows (pixels gathered on the far left hand side).


The histograms for this correctly exposed image show a concentration of pixels around the middle of the graph


This under-exposed image has the histogram leaning to the left, indicating a prevalence of darker pixels in the image


This over-exposed image has the histogram leaning to the right, indicating a prevalence of lighter pixels in the image


This intentionally under-exposed image of a buffalo herd shows a big grouping of pixels to the left end of the histogram; if the shadows warning indicator is turned on, it shows which pixels in the image have been clipped and contain no information (i.e. pure black)


The opposite is true for this image that has been over-exposed and recorded no information in the highlights that are shown as pure white (represented as red where the highlights warning has been turned on)

If you take and image and see that the histogram is leaning to the right, it means that the image is overexposed, and you can use your exposure compensation to correct this and dial in to the minus side to reduce the exposure.The further the histogram leans to the right, the more you need to under-expose to the minus side.The opposite is true if the histogram indicates an underexposed image and leans to the left; you then need to adjust your exposure compensation to the plus side to give more light.One point to remember is to always set your exposure compensation back to 0 after you have finished photographing a scene, otherwise the next thing you photograph could be horribly over or underexposed depending on your previous adjustments!

The more you play with these adjustments, the quicker you will learn how to get the correct exposure; over time, I have instinctively learnt how scenes will be recorded by my camera, so I will pre-emptively dial in my exposure compensation to over or underexpose a scene. An example would be when I automatically set my exposure compensation to +2/3 when I am photographing birds against a blue sky to prevent the camera from underexposing the bright sky.


If photographing anything against a pure blue sky, the camera will usually under-expose this by about 0.67 of a stop, leaving you with a darker-than-desired image


To counter this, whenever I photograph a bird against a blue sky, I set my exposure compensation to +0.67 to ensure that it lightens up the image sufficiently to leave with with a correctly exposed scene

The good news is that, until you learn to perfect this in the field, with the right post-processing software, even if you get it a little wrong you are able to make corrections with post-production to end up with a good image – that shouldn’t make us lazy photographers however, as there is no substitute for a good, correctly exposed image!Just do know that all is not lost if you do get it a little wrong.

So there we have it; all you need to know about getting the correct exposure to conclude our discussions on what is arguably the most difficult – but absolutely essential – element of understanding photography, and an element that will no doubt help you to get so much more out of your photography.

Happy clicking until next time!

 

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