Mini Professional Photography Course: Aperture
Understanding aperture is essential to comprehending exposure, as well as depth-of-field, an important compositional tool. This mini-course will cover basic aperture concepts as well as suggestions for putting those concepts into practice.
Aperture refers to the diameter of the camera lens opening that lets light pass through to the sensor or film. Aperture is expressed as a ratio between the diameter of that opening and the lens' focal length. Larger focal length numbers or f-stopsindicate a smaller opening (which means less light hits the sensor). Conversely, smaller f-numbers indicate larger openings.
The standard range of full f-stops is f/22, f/16, f/11, f/8.0, f/5.6, f/4.0, f/2.8, f/2.0, f/1.8. Each full stop up (wider) doubles the amount of light coming through the lens and vice-versa.
The f-numbers also indicate relative depth-of-field, which refers to the range of the photograph that will be acceptably sharp. Larger numbers indicate greater depth of field (meaning that more of the picture will be in focus), while smaller numbers indicate a shallower depth of field.
Proper exposure is achieved with a specific combination of aperture, ISO, and shutter settings ensuring that the right amount of light hits the sensor. That combination is possible with a variety of aperture, ISO, and shutter settings, but the total amount of light reaching the sensor must be the same.
For example, if proper exposure required an aperture of f/2.0 and a shutter speed of 1/500s at ISO 200, a smaller aperture of f/2.8 would require a twice as much time (1/250s) to get the same exposure. And, although the exposures would be identical, depth of field would be different in each photo.
Putting it into Practice
Practice is the best way to understand the relationships between time, aperture, and depth-of-field. The following steps will help you get the most out of your test shots:
1. Mount your camera on a tripod near a table.
2. Place three objects on the table at different distances from the camera.
3.Manually focus your lens on the object that is farthest away from the camera, and open the aperture to its widest setting.
4. Using either the camera's internal light meter or a handheld meter, set the shutter speed for correct exposure and ISO and take a picture. Although DSLR's allow you to change the ISO from shot to shot, this exercise will be more effective if you stick with one. If you are using film you will need to write down your settings with each shot in order to analyze your results.
5. Decrease the aperture one full stop down (which will cut the light in half) and double the exposure time. Take another photo. Continue adjusting the aperture and shutter speed throughout the aperture sequence.
6. Because the camera is on a tripod, it should be possible to stop the aperture all the way down and leave the lens open for much longer without any noticeable camera shake, so when you review your photos, contrast the differences in depth-of-field.
7. Use this same procedure to experiment with aperture and exposure as well.
Once you understand the basics, experiment in the real world. Try a wider aperture and shallower depth-of-field to isolate your subjects. Use smaller apertures to capture all the detail in your composition.