To understand exposure we need to learn about a few concepts in photography.
Metering in photography means measuring the available light. We know objects reflect only 18% of the light they are exposed to. All the metering devices, including the one inside the camera, assume this basic concept. When the subject is too dark or too light, however, the metering device may make a mistake; it may think they are middle grey-see below. In these situations, by looking at the histogram-also, see below- the photographer may override the metering system, and get a more accurate exposure by overexposing a snowy scene and underexposing the dark scene.
In general, there are two distinct metering tools, evaluative and reflective.
Evaluative measures the light present in the scene, reflective, almost all camera metering system, measures the light reflected from the object.
There are three mechanisms by which the camera measures the amount of light on the subject and determines the Exposure Value-EV.
Icons of the various metering systems
Evaluative or Matrix
In this mode, the camera averages the entire scene. This is often the factory default mechanism unless the photographer changes it.
Similar to the matrix, it evaluates the scene, but only a small central portion of it, and ignores the surrounding area.
Like the Center Weight, it limits the area in which light is being measured. Although in this case as the name suggests, it measures the reflected light from a single point photographer has focused on the object.
The most useful indication for spot metering is back-lit subjects. With a bright background, using a matrix metering, the background dominates the scene and will render the subject underexposed. The options are either use matrix metering and overexpose the image or try spot metering and correctly expose the image.
Histogram charts the distribution of the light within an image. Dark tones will be of the left side, light tones on the right and mid-tones in middle. The histogram is the most sensitive index of how the image has been exposed. The image on the back of the camera is not a good indicator of exposure since it is a Jpeg image, which means it has already processed by the camera. The dynamic range of histogram goes from 0 to 255. Anything around zero will be dark with no details and around 255 will be white with no details. These values can not be printed. Normally one wants the image to be in the range of 10-240.
Looking at the histogram the photographers see whether the luminosity values have reached beyond the range, called clipping. On the left means, dark is too dark and there is no detail in dark area, on the right, it means the highlights are more than the camera could capture.
Histogram demonstrates tones from dark,left, to highlight, right.
The histogram is a concept in digital photography, in analog photography, Ansel Adams came up with the idea of the zone system. He divided all the luminosity values into eleven zones, from zero to 10, ranging from darkest to lightest. Although the zone system still could be used in digital photography, essentially histogram has replaced it as it provides more information. In analog photography since there is no access to the histogram, when in doubt whether the exposure is correct, the photographers used to capture more images with higher and lower EV, which is called bracketing. Although bracketing still is being used in digital photography, the objective is different, ie the photographer feels a single capture is not able to register the entire dynamic range.
A histogram displaying all channels overlapping.
Highlighted the triangle on the top left suggests dark tones have been clipped.
Cameras have a feature which could be activated to show the clippings, various processing programs also have this tool, both called “blinky” since areas of over and underexposures tend to blink to draw viewer’s attention; blinking red areas, show highlight clipping, and blue, dark values. To look at the histogram on the back of the camera on the menu one has to go to highlight and shadows. Histogram can be demonstrated in a single graph of all three channels of red, green and blue, or one can opt to see individual channels. With DSLR to see the histogram, an image should be already in camera-captured, unless the camera is in live view mode, mirrorless cameras and smartphones are able to show the histogram of the scene which yet to be captured-live histogram.
Three channel histogram reveals red tones (highlight) have been clipped.
Although looking at luminescence values with a single histogram might be adequate, checking three-channel histograms provide more detailed information about the individual colors, red, green and blue. At times it is extremely important for a specific color not to have been clipped.
Another subject is the dynamic range. It essentially means the spectrum of light from dark to light that the camera could capture with details. Cameras' dynamic range has improved in the recent past, nonetheless, compared with the dynamic range of human eye remains limited. Dynamic range of human eye is about 18-20 stops, while the DSLR camera is about 12-14. Such a significant difference creates a problem for the camera to capture a wide spectrum of contrast like back-lit subjects; where camera averages the entire scene and renders metering which is too dark.
There is a function in most cameras called program shift- manufacturers may call this by various names-which allows the photographer to change the mode in order to get the same exposure by different means. Going from a shutter speed of 1/125 to 1/250 while changing the aperture from 8 to 5.6 and the same ISO. The program shift uses the information from the camera metering system.
Looking at the exposure parameters would demonstrate whether they are within the acceptable range-if one is using the program mode or one of the semi-programs modes like aperture priority. In program mode or aperture priority as camera will decide on shutter speed, it is extremely important to look at your shutter speed while shooting; shutter speed decided by the camera might be too slow for the scene, for instance, subject might be moving or even in still subjects, dropping shutter speed below accepted range for the lens, 1/focal length; in both situations the end results might be a blurry image.
The photographer should be familiar with the camera’s handling of ISO. By and large, DSLRs can handle high ISO very well. Although you may want to check images with high ISO and decide where your comfort level is with high ISO, 6400, 12500, 25000, or even higher. Floating ISO is another mode requires intimate knowledge of ISO handling of the camera. While setting up the auto-ISO, you need to determine how high ISO may go. Some of the cameras have feature for floating ISO to let the photographer decided the lowest shutter speed acceptable for the scene, for instance in wildlife photography you want a shutter speed not less than 1000 or 1250, anything below may render blurry images Cameras tend to have a nature ISO, between 100-200. When the light is adequate, you should use the recommended nature ISO.
Depth of Field
Although the issue of depth of field mostly related to composition, as the aperture is one of the factors influencing depth of field, it should be briefly discussed. Certain compositions require a deeper depth of field and vice versa. In portrait photography, to draw attention on the subject, the background should be blurred, in the landscape photography, on the other hand, one wants a deep depth of field. Wider aperture, creates a shallower depth, as the aperture closes off, depth increases. For landscape, the recommended aperture is 8-11, sometimes 13. The tighter aperture may increase the depth but will be associated rarefaction which blurs the image. Other factors influencing the depth of field are the size of the sensor, larger sensor creates shallower depth, the distance to the object, shorter distance decreases the depth, and, lastly focal length, the longer the focal length, telephoto lenses, the shallower the depth of field, as it compresses the field. One main reason a lens with a 100 mm or 120 mm focal length is used for portrait photography is to create a soft background.
An example of Depth of Field chart
To preview the depth of field there is a button next to the lens, pressing on in while looking through the viewfinder, the photographer gets a sense of the depth of field. Simply looking through the viewfinder after setting up the aperture does not give you the depth of field. In DSLRs, regardless of the chosen aperture, the camera will keep the aperture wide open to provide adequate light for focusing, setting up the composition and so forth, aperture will not close to the designated number until the shutter is pressed all the way-pressing the shutter halfway will not do it either. There are several apps to calculate the depth of field.
There are situations where the photographer feels metering of the camera is not accurate, to override the camera metering, one option is to use exposure compensation. With EC one may increase or decrease the exposure normally up to three stops. In the Back-lit situation, as noted previously, camera often has difficulty metering the subjects, one has to increase the exposure to overcome the overwhelming background, similar situation happens when the scene is too dark; by looking at the entire scene the camera often overestimates the darkness and tends to overexpose the scene, with EC one may decrease the exposure to have a more natural outcome.
There are other tools in the cameras these days that further help both to control the exposure and focus. To shoot a panorama with DSLR, you need to take several images from the scene, normally with ⅓ rd overlap between the images. There are several programs that can stitch the images into a single panorama. At times the scene may not have similar lighting and stitching these images will create an uneven exposure for the final image. In this situation, you can expose one image properly and then lock the exposure, and capture the remainder of the scene. It is often necessary to modify the exposure of the images in post-processing to make them look more natural.
Light has a color, daylight, incandescent light, fluorescent light all have contrasting colors that will impact the scene. An experiment in physics called black body radiation is the basic concept in color of the light. If a black mass gets heated, after some time starts to radiate red, and as the temperature rises, yellows and blue and finally white will be radiated. This is the reason the color of light is called color temperature which is measured by kelvin. 10,000 kelvin is blue, and as the temperature decreases, we see a spectrum of colors. Although in practice we call yellow a warm color and blue cold. This has to do with the wavelength of cooler colors creating more energy and more heat. Warm colors emit more heat than cooler colors. Although cameras have distinct settings for white balance, daylight, cloudy, shade, incandescent and, flash, in reality, each of the above setting has a defined kelvin number. Daylight often sets around 5200, the midday sun is a bit higher, about 5700.
Below is a single image with four different standard white balance.
White balance is particularly important if you are shooting in Jpeg mode. If you are shooting in raw, the white balance may be changed in the post-process. Jpeg is a processed image, so it does not lend itself to post-process editing, without digital artifact.
Although auto-white balance may be used
often, there are a few conditions a camera may have a problem with finding the right white balance. Photography in shade is one of
them. Most cameras tend to make scene colder, with a bluish tinge. Using a specific white balance for a scene, shade in this case, the camera tends to compensate for the coolness of the image and makes it warmer. If a scene is lit with various light sources, when you are using another source of light in ambient light, the white balance should be checked carefully. In situations like these, one can use a grey card. Grey card is made of middle grey reflecting 18%. You capture an image of the grey card in whatever light the photography will be done; various post-processing programs have a tool to identify the card as neutral grey and subsequently convert the while balance of all the other images to neutral. As noted earlier, the images should be captured in raw format to use the grey card.