Getting the correct exposure of an image is easy. Or is it? When you talk about ‘Exposure’, people will have differing opinions regarding the matter. Most often when the subject is brought up with new photographers, the opinions will always be subjective in nature. In a way, the subject of exposure really is subjective and it leaves a lot of room for the artistic interpretation of the photographer. But if we delve deeper into the topic, exposure is more than that. ‘Correct Exposure’ is capturing the image with all the relevant details found in both the shadows and the highlights.
How many times have we taken an image and all we see are all black shadows and not a hint of a detail hiding in them? Or perhaps washing out the details of a cloud and just producing a white sky? Sometimes an image may turn out alright and look good visually. But in most cases, if the image was exposed properly, showing tiny details in the shadows or revealing more information in the highlights, it turns out to be outstanding!
For us to be able to come up with beautiful images containing both details in the shadows as well as the highlights we need to understand some of the challenges and limitations we have to live with as well as understanding the tools and techniques available.
Most cameras, especially DSLRs, have several metering modes to choose from:
a)Evaluative, Zone or Matrix Metering This is the most popular default metering mode in cameras used by photographers. The camera’s meter divides the viewfinder into several zones where the light intensity is measured. The results are then combined with that of the area where the autofocus point is located, which will be given more weight than the others. This mode is favorable for use in most subjects.
b)Center-Weighted Metering The camera meter’s sensitivity, which occupies about 60 to 80 percent of the viewfinder area, is concentrated at the center. This is ideal for subjects that occupy most of the frame and are situated at the center of the composition, such as portraits.
c)Spot and Partial Metering The meter measures the light intensity falling on the 1 to 5 percent (for Spot) and about 8 to 13 percent (for Partial) of the viewfinder area. The spot where the sensitivity is measured is tied to the focus point and can be moved around in some cameras. This metering is ideal for high contrast or complexly lit subjects where you want to be precise on the exposure of your main subject.
2. Camera’s contrast range
This is the breadth at which the brightness values of an image can be recorded by the camera. Our eyes can see in a range of around 10 f-stops. Most cameras can only record 5 f-stops' difference in lighting between highlight and shadows.
As shown in the diagram above, our eyes may be able to discern the full spectrum of brightness, from very dark to very bright, as denoted by the blue ink. However, the camera will not, notice the range of brightness your camera is able to capture as shown by the yellow ink. By varying the camera setting (aperture, shutter speed or ISO values), the camera range can slide within this full range of brightness but the area that the camera can capture remains fixed, it can never grow larger.
When exposing high contrast subjects (e.g. portraits or landscapes containing highlights and shadows), there is a limit to what you can capture. You will either sacrifice some details or use one of the techniques mentioned below.
All light meters including those in your camera are calibrated to give an 18% grey reading. This is because a uniform surface will reflect 18% of the light that will fall on it. So shooting in black and white, you get 18% shade of grey. If its color, it will have a color equivalent of 18% reflectivity. This becomes apparent and a problem when shooting scenes with brightness approaching white or black. For example, white snow will turn into a muddy colored snow instead of white. A black ball will be recorded as a grey ball. To correct this you will need to compensate by over-exposing or under-exposing the frame.
Over-expose - increasing the exposure by +1 EV or more from the baseline reading of the meter. This makes the image brighter.
Under-expose – decreasing the exposure by -1 EV or more from the baseline reading of the meter. This makes the image darker.
Subjects with a majority of white or black area will not be exposed properly by the camera. You will need to compensate in order to achieve the right exposure you are looking for.
Techniques to overcome these exposure challenges
To expose an image properly, the first thing you should do is to learn how to evaluate your subject; will it will be affected by the limitations we mentioned above? Is it well within the contrast range of the camera? Does it have a majority of either black or white which can throw off your camera meter? By understanding the subject we can apply the following techniques to address whatever exposure issues may arise.
Bracketing for safety
Bracketing is a technique that has been in use since the days of film is to shoot the same subject in different exposure settings. Usually the first shot is exposed for the ‘correct exposure’ (EV 0) based on the camera meter reading, and 2 additional exposures are taken, 1-stop above (EV +1) and 1-stop below it (EV -1) taking a total of 3 exposures. For example, if your camera meter reading gives you an exposure setting of f5.6 at 1/60 sec. (EV 0), shoot the subject in that setting first. In the same position, for the second and third exposures, adjust the shutter speed for 1/30 sec. for 1-stop above (EV +1) and 1/125 sec. for the 1-stop below (EV -1). This works if your camera is on a tripod. Some cameras have a bracketing mode, setting your camera on this mode will automatically adjust the settings for you.
Using the exposure compensation
You may come across situations where the subject has a large expanse of light area such as a snow scene. Or you might find a background that is brighter than your subject, such as people at the beach. In these instances the result will be an under- exposed image. If you are using Manual Mode in your camera, it will be easy to make the adjustment in the exposure. But if you are using the camera’s auto exposure (Aperture Priority, Shutter Priority, Program or Auto Mode), the adjustments can be made using the Exposure Compensation button. Using this button allows you to over-expose (EV +) or under-expose (EV -) your settings.
Using the histogram to check the exposure
This is a graphical representation of the light levels found in an image. The histogram shows all the brightness levels of an image from the darkest to the lightest. A digital image has 256 brightness levels ranging from absolute black to absolute white, also referred to as the dynamic range. An average scene of an average subject will be exposed and displayed somewhere in the middle of this graph. A well-exposed image will have details found in the entire range, from the dark, mid-tones and the light areas of the graph. If the details go beyond the boundaries of the graph and are clipped, it means the image may contain areas of pure black or pure white. This also shows that the details were no longer recorded in those areas. If this happens, try to adjust the camera settings and re-take the shot (adjusting either the aperture or the shutter speed) until the details in the histogram improve by spreading across the graph.
a)Shooting in RAW File Format A RAW file image has greater information than a JPEG file. This is because the information gathered directly by the camera sensor contains the RAW and complete file. In a JPEG file this RAW image is processed by the camera to incorporate the White Balance, Sharpening and other post-processing parameters then the resulting image is converted into a JPEG file.
However, to access the RAW image file, you’ll need suitable software. This RAW image file is then converted by your computer and allows you to process the file yourself, letting you decide how the black and white areas should be exposed (within reason of course).
b)Use a graduated ND (Neutral Density) filter A graduated neutral density filter is a piece of glass, one edge of the filter is dark and the other clear with a gradual change between them. Placing this in front of the lens, the clear portion allows the light intensity coming from the subject to pass freely and hit the lens. The shaded portion is positioned upwards to block and lessen the intensity of the bright sky from reaching the sensor. This reduces the overall contrast range of the scene, allowing it to meet the 5 f-stop contrast range limit of the camera. This filter will work well when shooting landscapes with bright sky.
These are just some of the techniques you can use to expose for an object. The next time you encounter a complicated scene, try doing one of these and you’ll come away with far more ‘keepers’.
Most people think this post is Awesome. What do you think?