So, I’m late with my May article because I decided to change my topic last minute. I had prepared a 4 series article on Action Photography. I recently taught a class on Action Photography to some of our varsity baseball moms and realized – before I can get these ladies off auto mode they MUST learn the importance of exposure. We started the class with exposure and there were several “ah ha” moments and “the light bulb just went on” comments. THEN, we were able to move on to Action Photography. Stay tuned for the Action Photography articles next month!
In one word – “exposure” – is what photography is all about. Exposure is the total amount of light allowed to fall on the image sensor. In Greek, Photography means writing with light. Exposure is the combination of three factors (ISO, Shutter Speed and Aperture) that determine what the light writes… hence the Exposure Triangle. ISO measures the image sensor’s sensitivity to light. Aperture determines how much light enters the lens and is registered by the image sensor. Shutter Speed determines the amount of time the level of light enters the lens and is registered by the image sensor.
Why Does Exposure Matter?
Overexposed Images = loss of highlight detail, that is, when the bright parts of an image are effectively all white, known as "blown out highlights".
Underexposed Images = loss of shadow detail, that is, the dark areas that are indistinguishable from black, known as "blocked up shadows".
These terms are technical rather than artistic judgments; an overexposed or underexposed image may be "correct", in that it provides the effect that the photographer intended.
The Exposure Triangle – ISO, Shutter Speed, Aperture
Each one of the three elements of the triangle relate to light and how it enters and interacts with the camera. The combination of these three elements results in a given subject’s exposure value (EV). What is critical to remember is that any change in any one of these elements will cause a predictable impact on the other and consequently impact the final image (i.e. by changing the Aperture, you change depth of field; by changing ISO rating, you change the amount of light required to obtain an image, and by changing the Shutter Speed, you effect how motion is captured). You will never be able to independently control a given element, because you have to take into account how the other two elements will interact for the final exposure. Fortunately, the mathematics of photography just so happen to work in such a way that each element in the Exposure Triangle has a relative “stop of light” value. If you increase the light by one stop by reducing the Shutter Speed, you can regain the original EV by either decreasing the Aperture by the same stop value and/or adjusting the ISO rating accordingly.
A Little More Detail
ISO. The measure of a digital camera sensor’s sensitivity to the amount of light present.
- The lower the number the less sensitive your camera is to light and the finer the grain. The higher the ISO, the more sensitive the image sensor and therefore the possibility to take pictures in low-light situations.
- Higher ISO settings are generally used in darker situations to get faster shutter speeds (for example an indoor sports event when you want to freeze the action in lower light) – however the cost is noisier shots.
Aperture.
The size of the opening in the lens when a picture is taken. The larger the hole the more light that gets in – the smaller the hole the less light. The aperture is like a pupil.
Aperture is measured in ‘f-stops’. You control the aperture by setting the "Aperture Opening", also known as an F-Stop i.e. f/2.8,f/4,f/5.6,f/8,f/22 etc. Moving from one f-stop to the next doubles or halves the size of the amount of opening in your lens and therefore the amount of light getting through.
Smaller F-stops = larger openings. Larger openings = more light
Shutter Speed. The amount of time that the shutter is open.
Changing each element not only impacts the exposure of the image but each one also has an impact upon other aspects of it (ie changing aperture changes depth of field, changing ISO changes the graininess of a shot and changing shutter speed impacts how motion is captured).
Using Automatic or Manual Exposure?
Automatic Exposure (AE) mode automatically calculates and adjusts exposure settings in order to match (as closely as possible) the subject's mid-tone to the mid-tone of the photograph.
Aperture Priority mode gives the photographer manual control of the aperture, whilst the camera automatically adjusts the shutter speed to achieve the exposure specified by the TTL meter.
Shutter Priority mode gives manual shutter control, with automatic aperture compensation. In each case, the actual exposure level is still determined by the camera's exposure meter.
Manual mode is where the photographer adjusts BOTH the lens aperture and shutter speed to achieve the desired exposure. Many photographers need to control aperture and shutter independently because opening up the aperture increases exposure, but also decreases the depth of field, and a slower shutter increases exposure but also increases the opportunity for motion blur.
Keep in mind that Aperture controls Depth of Field, Shutter Speed controls
focus/blur and ISO controls graininess.
So what is correct exposure? When the camera effectively reproduces a subject on the image sensor where the most uniform amount of picture information is visible in the highlights, midtones and shadows. Most dSLRs today have an EV meter in the viewfinder that provides an EV on the subject that you are metering. An effective way of ensuring a correct exposure is to employ Exposure Bracketing. This is a technique in which you’ll be taking at least 3 exposures – one at the designated exposure value (EV), one 1/3 of an f/stop above, and one at 1/3 of an f/stop below. On some features-laden cameras, you set the ISO, f-stop and shutter to acquire an exposure value (provided by the TTL meter), and press the shutter release. The camera will automatically shoot the upper and lower bracketed exposure. When you review the bracketed exposures, you’ll be able to see subtle, but key differences in the images – most specifically if there is any over- or underexposure.
The wonderful thing about digital photography is that you can continue to experiment at no cost to you as you learn and master the three elements of the Exposure Triangle, going from semi-automatic to full manual. It takes a certain amount of practice and storing a great deal of information in your head… but master it you can!
Mary M.
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