Why Manually Adjust Your Exposure?
Getting the correct exposure for your photos is only half of the battle. The other half is getting your photo to look like what you pictured in your head. What happens when your subject is moving very fast? Or, you want a certain amount of depth of field? Or, both! What then? This is where things get tricky. You can set your camera to aperture or shutter priority and hope you get the picture you envisioned in your mind. But, your camera doesn’t know what you are hoping to achieve. This is where full manual control comes into play.
Modern digital cameras give you a wide range of adjustments and settings for achieving correct exposure. They are all based on adjusting aperture, shutter speed and ISO. If you learn the basics of exposure adjustment you will get the shot you envisioned.
Lets take a look at the elements of exposure.
Simply put, aperture is an adjustable hole in a lens through which light travels. You can read more about aperture here. For our purposes we’ll just look at the aperture values (f-stop) for obtaining proper exposure and desired visual effects. A quick look at depth of field shows us that the larger the aperture, the less is in focus and the smaller the aperture, more is in focus. This is one effect of aperture that needs to be considered when adjusting exposure. How much do you want in focus?
Aperture Value – f-stops
f/1.4 – large lens opening allows more light through, but has a small amount of depth of field (less is in focus).
f/8 – medium lens opening allows less light through, has more of depth of field (a bit more is in focus)
f/32 – small lens opening allows little light through, has much more depth of field (a lot more is in focus)
Each f-stop value decreases or increases the amount of light allowed through the lens depending on if you are closing down or opening up the aperture. This change in f-stop represents one stop in your exposure. More on that later!
In its simplest terms, shutter speed is the time duration of light allowed to reach the imaging surface. More on shutter speed here. A slow shutter speed allows a longer duration of light to reach the imaging surface, but moving objects are blurry. A fast shutter speed allows a shorter duration of light to reach the imaging surface and freezes fast moving subjects. Shutter speed is used to stop action or create a sense of motion. This is something to think about when adjusting your exposure. The other thing to consider is camera shake. When your shutter speed is too slow (usually 1/30 sec. or longer) it is harder to hand hold your camera without getting blurry images due to camera shake.
Shutter Speed – time values
1/8 sec. – long time value allows more light to reach the imaging surface, moving objects are blurry. Camera shake noticeable.
1/60 sec. – shorter time value allows less light to reach the imaging surface, slow moving objects are frozen, but fast moving objects still blurry. Camera shake not noticeable
1/1000 sec. – very short time value allowing very little light to reach the imaging surface, fast moving objects frozen. No camera shake.
Every time you increase your shutter speed by 2X or decrease your shutter speed by 1/2 of your current shutter speed you are changing your exposure by one stop. We will bring this all together after we discuss ISO.
ISO is the measurement of an imaging surfaces sensitivity to light. More on ISO here. A low ISO setting is less sensitive to light, and produces less noise or grain. A high ISO setting is more sensitive to light and has more noise/grain. The noise/grain is something that effects your final image and should be taken into consideration when adjusting your exposure.
ISO 100 – Is less sensitive to light and has very little visible noise/grain.
ISO 800 – Is more sensitive to light and noise and grain is visible.
ISO 6400 – Is very sensitive to light and there is a considerable amount of visible noise/grain.
If you double or half your ISO from your current setting you are changing your exposure by one stop. Now, lets put this all together.
Adjusting Your Exposure
We have looked at three elements of exposure individually, ISO, Shutter Speed, and Aperture. Lets look at them together and how they make up your exposure:
Example 1 – It’s a beautiful day and you want photograph your friend on a beach. the exposure your camera is giving you is f/11 at 1/125sec at ISO 100. You take the picture and your friend is perfectly exposed and in focus, but so is everything in the background. You want your friend in focus, but not the background. The solution – adjust the aperture to f/2.8. By doing that you have increased the amount of light (+4 stops) reaching your imaging surface. To compensate you need to decrease the amount of light or light sensitivity by -4 stops. The easiest way is to increase your shutter speed to 1/2000 sec. allowing -4 stops less light to reach the imaging surface. This will maintain the correct exposure.
In the mean time you can download the new Exposure Adjustment Chart as well as watch the video on how to use the chart.