Lesson Seven - Available Light / Exposure
The word "photograph" comes from the Latin for "light" and "art". It is then, The Art of Light. With traditional film cameras, a light sensitive film is loaded inside the camera and carefully exposed to enough light to imprint the artful world of light that surrounds us. A digital camera, which uses no film, has instead a sensor chip that captures a digital representation of the colors of light around us.
Without light we would have no imagery. Our eyes see light emitted from artificial sources, from natural sources (fire and sunlight), and light that is reflected back off of most surfaces. Light is actually ALL we see. We never actually "see" a flower - what we see are the colors of light that reflect off the flower. What we DON'T see are the colors of light that the flower absorbs. Items which look "light" to us (white, silver etc) are items that reflect a lot of light. (To our eyes, all the colors of light blended together appear white.) Items that appear "dark" to us are actually absorbing more of the light rays, reflecting less light back to our eyes (this is why black is a hot color to wear on a sunny day). So by definition, light is extremely important to photography.
There are many types of light, natural and artificial, and it comes in a spectrum of colors. We will focus in this lesson on available light, whatever type or color it may be. In order to get a properly exposed photograph, your film or digital sensor needs to receive enough, but not too much light.
While all the colors of this light are around us on a sunny day, in order for us to see the colors, they need to be refracted because when blended together they appear white to us.
An overexposed photo will appear white and washed out (too much light entering camera) and an underexposed photo will be dark (not enough light entering camera). So you can see it is important to allow only the right amount of light into the camera in order to capture the scene as you see it. You can control this with aperture, shutter speed, and ISO (film speed). So to visualize this with a familiar concept, imagine that you have a glass and you want to fill it from the tap. Aperture is how much you open the tap. Shutter speed is how long you leave the tap open. And ISO is the size of your glass. A full glass represents a properly exposed photograph. Sound confusing? Lets sort it out.
We learned in lesson one that a lower numbered F/stop gives a smaller depth of field and vice versa. What aperture actually refers to is the width that the back of the lens opens when you depress the shutter button. This width is measured in F/stops. Here is where the whole thing gets a bit tricky. Smaller numbered F/stops (shallow depth of field) are actually larger openings because F/stops are actually fractions. F4 is 1/4 and F29 is 1/29, so the opening at F4 (1/4) is greater than F29 (1/29). Because the opening is bigger, the light floods onto the film (or digital sensor) faster (imagine a tap open fully rather than a trickle) and a proper exposure takes less time. This is ideal in low light situations when you would prefer not to use a flash (to maintain the natural quality of light, to avoid redÃ‚Â¬eye, not to distract your subject, no flash is allowed etc.). Because with lower numbered F/Stops the shutter opens wider, it takes less time for adequate light to enter your camera. An added bonus is that you will also avoid the likelihood of camera shake.
Shutter speed refers to the amount of time the camera shutter is open, allowing light to enter your camera. (With our water glass scenario, this would be the amount of time you leave the tap open, allowing water to pour into your glass.) The longer you allow the shutter to remain open, the more light enters your camera. When you are unwilling or unable to use flash in a low light situation you can allow enough light to reach your film (or digital sensor) to obtain a proper exposure by allowing the shutter to remain open longer.
Long shutter speeds increase the chance of camera shake though, so it's advisable to use a tripod or otherwise stabilize your camera and perhaps even use the self-timer to avoid the small shake that comes from depressing the shutter button. Because the shutter will be open longer, a moving object would be captured in movement, so this technique is more for photographing still subjects (unless you want creative action blur). A perfect example of a long shutter exposure is photos at dusk or night.
ISO refers traditionally to film's speed or its light sensitivity. Smaller numbers (100, 200) are slower films that are less light sensitive and take longer to reach a proper exposure. Larger numbers (400, 800, 1600) are faster films that are more light sensitive and will be properly exposed in a shorter time. (This is the size of your glass - how much water is needed to fill it?). Digital cameras have a similar function, also called ISO, that changes the image sensor's degree of sensitivity to light. While film users have to load a different canister of film in order to change ISO, with digital cameras ISO can be changed on the fly, picture to picture. When choosing ISO, film users need to plan ahead what type of photos they will take (lower ISO for brightly light scenes, higher for low light situations), but digital users can change ISO for each situation.
So now you are thinking.. ."great, faster is better!" Sometimes yes, but faster does come at a price. With both film and digital, using a higher ISO increases film grain or digital noise in your photographs. While some people like film grain (particularly in black and white photos), I see it as a degradation in image quality. Nothing annoys me more than forgetting to reset the ISO on my camera back down to 100 after having it set high for a low light shoot.
I often use a higher ISO in combination with a wide aperture (small F/Stop) to avoid using flash in very low light situations. While this increases the digital noise in my photographs, I can maintain a more natural light tone and avoid glares and shadows that I would get from using a flash. So it is a trade-off.
One way to automatically set the aperture and shutter speed on your camera for the part of a scene you want exposed properly is by spot metering. Normally, your camera uses center weighted metering to measure the light in a scene. This means that it measures the light across the scene placing an emphasis on the center (where you likely will focus on your subject before recomposing). But what if you want to manually select where the camera measures the light?
Many cameras have a button to control this called Auto Exposure Lock (AEL). You simply position the camera so the area you want to measure the light from is in the center and press the AEL button. One situation where this is handy is with back-lighting. For example your friend is sitting in the shade of an umbrella on a very bright day. When you compose the picture, the light behind your friend is enough to fool the camera into using a shutter speed too slow to capture the details in your friend's face because it measures an average across the frame. You can force it to measure the light falling on your friend's face with the AEL feature. You may notice then though that the exposure is set for the shady area beneath the umbrella and the scene outside the umbrella will be over-exposed (appear very bright).
For a proper exposure in low light with no flash:
* Wider Aperture (smaller F/Stop #)
* Slower Shutter Speed
* Tripod and perhaps the self-timer (with slow shutter speeds)
* Higher ISO (will add grain or noise, but less time to expose properly)
*Combining high ISO with either wide aperture or slow shutter speed will intensify their effect, but add some grain or noise to your photo.
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