What we see with our eyes is reflected light, which is essentially light bounced off a surface. This light then travels as wavelengths to the receptors in our retinas, or sometimes the sensor in our digital cameras. Understanding how we see is invaluable for understanding how we shoot.

Our eyes have two light receptors, rods and cones. The rods in the retina operate in dim light. They respond to dark tones and are found towards the edge of the retina. The cones respond to normal levels of light, see colour and detail, and are clustered towards the centre of the retina. So it’s no wonder that our eyes are attracted towards ‘light’, since our eyes are designed for daylight. Cameras, on the other hand, can photograph in all levels of light, and even see into the dark.

To be able to pre-visualise a photograph it’s important to know the difference between how our eyes ‘see’ and how a camera ‘sees’. The iris controls the light that enters our eyes. When it’s dark the iris opens up and as a consequence the light now reaches the more sensitive rods. The rods don’t see colour very well, so when it’s dark we are almost seeing in black and white. Conversely, the opposite happens when there’s plenty of light: the iris becomes smaller, triggering the cones into action. This makes it easy to distinguish objects, see colour and focus.

Our dSLR cameras can be set to detect light from the centre of the frame (centre-weighted or spot) to all over the frame (matrix). There is no right or wrong way to use your metering systems, it’s more about what’s appropriate for a particular situation. If the subject is small in the frame and you want to specifically meter it, adjust your settings to use the spot meter; if you’re happy to get an average reading from all over the frame, use the matrix metering.

Unfortunately, there are no rods or cones in your dSLR, just light-sensitive receptors that become more sensitive to light as we dial up our ISO. But like rod receptors, as we dial up our ISO, we lose detail and colour. An interesting fact about the eye is that even though the centre of the retina (the fovea) is for fine vision, it is less sensitive to light. So if you want to detect a dim light in the night sky, it’s best to gaze slightly to the side of the star in order to project the image to the more sensitive rods. Camera lenses, on the other hand, are sharper towards the middle and are at their very sharpest at f8.

Light enters our DSLR camera in three ways:

1) through the aperture in the lens, which can be a big hole or small hole, read in ‘f’ stops — f22 represents a small hole, f2.8 is a large hole
2) through the shutter, which is a kind of blind that opens and closes allowing more or less ‘light/time’ to pass through to the sensor (it is read as fractions of time, an example being 1/60, or one-sixtieth of a second). The slower the shutter, the more light/time is recorded. A slow shutter records a blur, compared to a fast shutter, which can freeze motion
3) a control called ISO that determines how sensitive the sensor is to light (the higher the ISO number, the more sensitive the sensor). The higher ISOs (above ISO800) lose detail, particularly in the highlights and dark tones.

The light meter in your dSLR camera calculates the combination of aperture + time + ISO and is calibrated to a tonal value called ‘middle grey’, or 18 per cent grey. That’s why most photographs you make on auto settings are reasonably well exposed, as most of the world is a mixture of midtones (with some whites and some blacks).

On average, people’s skin is a midtone, a blue sky is midtone, the grass is midtone. To be able to shoot high-key (bright) or low-key (dark) images you will need to know how to control exposure. You can achieve this by either adjusting the exposure through the exposure compensation dial or using manual mode.

Manual mode (M) is by far the best way to have full control of your photography. in M mode it’s up to you to set the controls. See your camera manual if you need more help but, put simply, once you turn on your camera and half press the shutter button the light meter will be activated. It’s best to look through the lens and recognise your exposure guide; it may look like a scale going from -2 -1 0 +1 +2 or it may be just a positive or negative number indicating how many stops your exposure is away from grey.

Whatever the case, if it reads in the negative your exposure is dark, and if it reads as a positive then your exposure is bright. With Canon cameras, if the number +2 or -2 on the scale is blinking, this indicates that your exposure is ‘off the scale’. In this case you need to turn the dial until the sign stops flashing, then count how many clicks you make with the dial to know how far away from grey you have gone. Remember that each click of the dial equals one-third stop of light and ‘grey’ is represented by the ‘0’ sign.

Exposure compensation is used when shooting in all other modes. In most dSLR cameras you can adjust your exposure compensation by between two stops (Canon) and four stops (Nikon). The button commonly looks like a plus and minus sign: +/-. With a Canon camera, moving your main dial to the right  (+) will lighten an image (because it slows the shutter or opens the aperture) and vice versa when you move the main dial to the left. Which way does your main dial work? Test and find out.

Again, remember that each time you move the dial one click, it is equivalent to one-third of a stop of light. So it takes three clicks of the wheel to change the exposure by one stop, which effectively is doubling the amount of light or halving the amount of light (depending which way you go). A high-key image is most likely to be in the range of two stops +ve, and low-key image two stops -ve.
Av mode stands for aperture priority or aperture value. You choose the aperture by moving the main dial and the camera will select the shutter speed.

Tv is for time value and is the same as S for shutter. You select the shutter speed and the camera will select the aperture. If the aperture ‘f’ number is flashing (Canon) or a word ‘low’ or ‘high’ flashes (Nikon), it means that the camera can’t achieve the correct exposure. Either the aperture in the lens can’t open wide enough to let enough light through or conversely it won’t go small enough to cut back the light.

In P mode (Program), once you point your camera at your subject and half depress the shutter release button, your camera selects a combination of aperture and shutter speed, much like Auto. The difference between Auto and Program is that if you move the main dial while the light meter is still active, you can then change this combination of aperture and shutter (the exposure remains the same). The change only lasts for that one shot then goes back to initial combination.

The best way to judge your exposures is by referring to your histogram. The histogram is a graph that represents the distribution of tones within an image. The horizontal axis represents black on the left-hand side and white on the right-hand side, with midtones in between. The vertical values represent the number of pixels in the images that have that tonal value.

Check your camera manual to find your histogram.

A successful high-key image is one where the tones represented in the histogram are predominantly in the right-hand side of the horizontal scale. Aesthetically, these tones also express a feeling — words like ‘ethereal’, ‘happy’, ‘angelic’ and ‘clean’ come to mind.

Generally, the main subject matter should retain detail where other parts of the scene may completely ‘blow out’ or be white (as long as they don’t distract from the subject). The peak in the vertical axis of the histogram will be high, because the image is full of light tones. An example of a high-key subject in nature could be a snow scene or cloudscape.

Low-key images are emotively at the other end of the scale. I would use words like ‘sombre’, ‘brooding’ and ‘mysterious’. There should be detail in the dark tones. It’s this detail that captures our eye and stimulates our subconscious memories. We reflect on past experience and relive it, if only for a moment.

The level of lighting and the tonal range you select as a photographer are your emotional communication tools; use them at the time of capture and explore how you ‘feel’. The advantage in making your images either high or low key at the time of capture, instead of later when you are back with your computer, is that you are reminded of the experience and the detail will be where you need it. I have a rule: expose correctly for the subject, make the subject the most obvious part of the frame and always keep it simple.

Susan Sontag once claimed that photography “teaches us a new visual code, photographs alter and enlarge our notions of what is worth looking at and what we have a right to observe”.

A successful photograph should communicate something, be it a simple record of a moment in time or a whole story. One thing is for sure: we visually read the emotion of the image by its tone. Photographic images that exist in the high keys or the low keys are meant to convey a feeling first and an object second.

Key points:

• In high-key images, the midtones are high on the exposure scale, making them lighter and brighter.

• Low-key images emphasise the dark parts of a shot.

• Cameras will have problems focusing in dim and/or flat light, on bright shiny surfaces or when the tones in the viewfinder are very close together (like a sky or at night). The lens may go ‘searching’, rolling in and out, looking something contrasty to latch on to. To resolve this problem I think it best to turn your camera to manual focus. This button is usually found on the lens (use a tripod and Live View if you have it, because the image is greatly magnified and easier to see). Use a torch while you focus, or focus on an object that is a similar distance away from the camera, then re-frame before making the exposure.

• Be wary of contrasty light on your subjects and/or backgrounds because this light will make dark shadows in high-key images or light edges in low-key images.

If your shutter speed gets slower than the focal length of your lens, you will need a tripod to stop camera shake.

Words: Jackie Ranken

This article is from D-Photo issue 32.

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