Saturday, July 28, 2012
Through advances in health-care technology, as well as healthier lifestyles, many consider age 80 to be the new 65. There’s a similarity in photography, where 1600 ISO may be the new 800 ISO.
If you don’t know your ISO from your AOL, I’ll explain.
In the days before digital, film was given an ASA number. It was a numerical indication of the sensitivity to light of a particular film.
The classic slide-film, Kodachrome 64, was known for its fine grain and remarkable color rendition. It was National Geographic magazine’s film of choice. The down side is that it limited a photographer to well-lit environments or forced them to use a flash.
In comparison, the higher the light sensitivity of a film, like Fujichrome 800 ASA, enabled a photographer to shoot in much lower light, but the resulting images would lose color quality and contain more noticeable grain — a degradation of the silver particles in the film.
In digital photography, ASA is now known as ISO and is the numerical indicator of the light sensitivity of a digital camera’s sensor.
Sensors, being the digital equivalent of film, capture and collect photons to create an image. Low ISOs (100-200) maintain excellent image quality, while larger ISOs (800-1600) decrease image resolution and increase noise — the digital version of grain.
The good news is that camera sensors are improving, and some higher ISO images can be acceptable. But because all camera exposures are based on the combination of three controls — shutter speed, aperture and ISO — you should be aware of how and when unintended increases in your ISO can occur and affect the quality of your image.
One of the worst offenders for this increase in ISO is the Program (P) mode. If you choose this mode on your camera, you are allowing the camera’s exposure system to adjust any combination of the three controls to determine a correct exposure.
In low-light conditions, the camera may determine that a shutter speed of 1/30th is as slow as it should go. The camera — actually the manufacturer — has figured that below that speed, people can’t hold a camera steady enough for sharp photographs. So if the camera still requires more light for the correct exposure, it seeks a larger aperture or a higher ISO. Most maximum apertures on point-and-shoot cameras are not large, so inevitably this leaves ISO as the control to get adjusted. The loss of image quality can be the unintentional result.
My suggestion is to avoid (P) mode. Instead, stick to the auto-exposure modes of aperture priority (AV) or shutter-priority (S/TV) and lock in manual selections of an appropriate ISO. You can manually set your ISO in your camera’s control menu. I suggest 200 ISO when outdoors in good light and 800 or higher when indoors in dim light. In this way you can gain some image quality control and ensure that your ISO remains A-OK.