This is Part 1 of a four-part series about understanding the exposure triangle.
A photographer who shoots with a digital SLR may not remember the term ASA/ISO as applied to film speed ratings, but certainly will have noticed a camera setting called ISO. ASA and ISO relate to the light sensitivity of film, or for the purpose of this article, the digital camera’s sensor.
What is ISO and ASA?
Film speeds tend to range from 100 to 800 (higher ASA being available) but modern digital SLR cameras have ISO ranges of 100 to as much as 12000 and higher. Higher numbers equal more sensitivity to light. Effectively, setting the ISO is how to set the camera sensor’s sensitivity to light.
Higher-ASA films are nicknamed “fast” films because they come in handy in sports photography and other applications where it was important to freeze motion. Keep in mind, that the moniker “fast” is due to the fact that using more sensitive film allows the user to keep up a fast shutter speed (to freeze motion) while still allowing plenty of light to enter the equation.
The one caveat of a higher ISO, just as with high-ASA film, is a greater propensity for grain–or its digital equivalent, “noise.” With the digital SLR, this noise is exacerbated by under-exposure, sometimes yielding images that are unusable due to detail loss and noise.
Because of this, the most important factor when successfully shooting at higher ISOs is to take care not to underexpose. Depending on the camera, at ISOs greater than 800-1600, there often will be significant digital noise. Software is available to reduce this noise, such as Imagenomic’s Noiseware. It is important to remember that software is not a substitute for proper exposure and cannot always “save” a poorly exposed high-ISO image.
It will be the photographer’s decision in some situations whether to “embrace the noise” or try to reduce it. Sometimes noise works, especially with black and white or moody images. Noise can be used to make a statement. Noise is often part of available light photography in low-light situations, such as a wedding cerremony in a dimly lit churck or lifestyle photography in a darker home. It is not always a terrible thing, but is something that should be used with purpose and/or to make an artistic statement.
A good rule of thumb when choosing camera settings is to choose the lowest possible ISO that will allow use of the desired aperture and shutter speed. One benefit of the Phoenix area is that most days, there is plenty of light. Outdoors, this means an ISO of 100 or 200 will often be the ISO of choice, even in shade. Indoor available light photography will require higher ISOs (remember to shoot at a large aperture, such as f/1.8, to keep ISO in a reasonable range).
Here are some examples of starting ISOs in the valley:
Daylight, Outdoor – ISO 100
Daylight, Open Shade – ISO 100-200
Daylight, Indoors – ISO 400-1600+
Late Day, Outdoors (sun below horizon) – ISO 200-400
Dusk or Sunset, Outdoors – ISO 400-800
One Last Tip
While it is recommended to always use the lowest ISO possible, it is important to remember than a well-exposed image at ISO 1600 will have less noise and better color, contrast and detail than an under-exposed image shot at ISO 800. Don’t be afraid to raise the ISO when needed, and give the sensor plenty of light to produce a beautiful image.