At its core, a photograph is a recording of light on a medium (film, sensor etc.). When we make an image we try to manipulate the amount of light that strikes this medium. Some of this can be controlled on the camera while others are external to it; selecting the time of day to shoot, using reflectors and strobes to control contrast etc.
In this post we'll talk about three basic controls on a camera that every photographer needs to understand to get the most out of their exposures; aperture, shutter speed and ISO. Together these three variables make up the exposure triangle. Develop and understanding for how they work and interact and you're off to good start.
The information discussed below is relevant to all types of cameras, DSLRs, point-and-shoots and mobile devices but the controls on point-and-shoots and mobile devices will be more restrictive. Here's an image of the info screen as seen on a Canon EOS 40D. There's lots of information here but in this post we'll focus only on the items that are circled.
Figure 1. Info screen
Aperture refers to the size of the opening on the lens and it controls the volume of light that strikes the sensor. The aperture setting on camera is indicated by the letter 'f'', followed by a number eg. f/4.0, f/5.6, f/11 (item circled in green in Figure 1). Assuming all other controls remain the same, a wider opening will allow more light to enter the camera and therefore make a 'brighter' exposure.
As the f-number increases the size of the opening decreases and reduces the amount of light entering the camera. An aperture setting of f/4.0 will allow more light into the camera compared to an aperture setting of f/11. When you move from one f-stop to the next you halve or double the amount of light entering the camera. For example, changing your aperture from f/4 to f/5.6 will halve the amount of light. Changing the aperture back to f/4 from f/5.6 will double the amount of light.
Older cameras only allowed changes of full f-stops. Modern day cameras can support half f-stop and one-third f-stop increments or decrements.
Standard full f-stop numbers: f/1.0, f/1.4, f/2.0, f/2.8, f/4.0, f/5.6, f/8.0, f/11.0, f/16.0, f/22.0, f/32.0
In a DSLR camera system the aperture is an attribute of the lens, not the body.
Now that we've covered the amount of light, let's talk about the duration. The duration of time for which the sensor is exposed to light is controlled by the shutter speed. A faster shutter speed means the sensor is exposed to light for a shorter duration of time. Assuming all other controls remain the same, a slower shutter speed will allow light to enter the camera for a longer time and therefore make a 'brighter' exposure.
The shutter setting on a camera is typically indicated by a fraction. Referring again to the image above the shutter speed is 1/250s, which means the shutter is kept open for a mere 1/250s. Now the principle of doubling or halving the duration of light works the same with the shutter speed as it does with the aperture. Going back to the example above, if we double the shutter speed to 1/125s we've doubled the time for which the sensor is exposed and increased the exposure by one stop. Conversely, if we decreased the shutter speed to 1/500s we halve the sensor exposure time and decrease the exposure by one stop.
Depending on the type of camera shutter speeds can be varied from 1/8000s to 30s. DSLRs typically also include a 'bulb' setting for the shutter speed. When the camera is in bulb mode an external device is used to open and close the shutter. In this way the shutter can be held open for any amount of time. Bulb mode is typically used when shooting in low-light conditions and photographing at night. Shutter speed works on the idea that the camera record light cumulatively.
Now onto the third element in the triplet, ISO. ISO is the equivalent to the ASA number used with film. In a digital camera ISO behaves like a volume knob that affects the sensitivity of the sensor. When light levels are low, ISO can be increased to increase the sensitivity of the digital sensor. Most DSLRs have ISO numbers ranging from 100 to 12800. Newer cameras (circa 2012) go to even higher numbers. Check out ISO range supported by the EOS 6D.
Like everything else in life the ISO feature has its tradeoffs. In low-light conditions, it becomes difficult to expose every pixel evenly and while increasing the ISO helps amplify the signal captured, it also tends to accentuate the fact that the different pixels were not illuminated evenly. In the final capture this tends to show up as noise and can be observed clearly in the darker areas of the image.
In the upcoming posts I will discuss each of these controls in detail and describe how they interact with each other. I'll also be posting a few exercises for you to try to develop an understanding of these concepts.
Post any questions you have in the comments section.
No comments:
Post a Comment