Cameras these days provide good to great photos if you take the time to learn a few camera basics. As I demonstrated on my first blog post, shooting in “auto everything mode” is great for a vast majority of situations. This is particularly convenient for folks who don’t want to think much about the technical details of photography. Indeed, cameras these days are pretty smart, and only a handful of situations get new photographers in trouble like shooting in heavily back-lit situations, in low light, or in weird artificial lighting (fluorescent or other metal vapor bulbs, for example).
This post will look at ISO and its direct relationship with camera noise (in a forthcoming post, I’ll focus on white balance). With film (yes, there are film shooters out there), ISO is simply the sensitivity of the film. ISO 100, daylight film, has less sensitivity to light than an ISO 800 film that someone would use indoors or other low-lit spaces. As anyone who has shot with film can tell you, ISO 100 film is relatively free of grain (noise) and can be enlarged with details in the photo remaining intact. High ISO film, like ISO 800 or above, on the other hand would increase your probablity of getting a decent low light image, but at a cost. Grain (noise) is a lot more noticeable and when enlarged, and details fall apart.
For a majority of photographers today who practice digital photography, a working knowledge of ISO is still very important. In fact, ISO in the digital world is fairly similar to ISO in the film world. With digital cameras, ISO relates to the sensitivity of the sensor to light. The digital camera has a base ISO where the sensor performs at its best: providing clear, noise free images with excellent color representation. As digital ISO increases, the camera processing starts amplifying the output of the sensor; and in this amplification process, noise is also amplified. The higher the ISO is set, more noise starts to creep into a photo until it gets to the point that photos are only really useable for small web shots and not for printing.
To demonstrate the impact of increasing ISO on image noise, I set up a little test, photographing my wife’s stack of books that are all over the house (she’s in grad school). To keep things consistent from shot to shot, I had the Nikon P7000 compact camera on a tripod, and shot all shots at f5.6 only adjusting shutter speed and ISO. I also set a custom WB with an ExpoDisc and used an incident light meter to determine exposure. The first shot below is how the whole photo looks.
The following are 100% sectional enlargements from the photo above with the Lightroom exposure info in the upper left. Look carefully at the ISO 100 enlargement details. In the enlargement, you can clearly see the texture on the red and aqua book spine as well as the texture in the blue spine print. Also notice the absence of noise in the dark space between the red and aqua books.
At ISO 200, things are pretty much the same. Noticeable differences at this ISO are hard to detect.
At ISO 400, the camera’s noise reduction is starting to become noticeable. This camera doesn’t have the capability to completely turn off noise reduction when shooting JPEG. You can start to see detail getting smudged, but just a bit. The texture on the book spines mentioned in the ISO 100 photo is starting to lose its regular pattern. This is most evident in the red book spine.
At ISO 800, more detail is getting “smeared” away, and colors are starting to noticeably lose their vibrancy. Camera noise reduction algorithms are getting better year by year but are still far from perfect. The difficulty lies in a program differentiating random noise specks from photo detail. When lots of noise is present in a photograph, the challenge of targeted noise reduction becomes more difficult, and the end result is lots of detail from the image gets removed along with the noise.
At ISO 1600 and ISO 3200, most of the texture is gone due to noise reduction.
At ISO 6400, the photograph looks like a watercolor painting at this point. Colors are off, and color separation between different colors is poor. For example, look at the white letters on the red book spine. The edge of the characters are not well defined at this point. The original blue colors of the letters on the white spine are almost dark grey with random splotches of purple and green. The shadows are a mess, looking like the inside of a browny.
Just for comparison, I took the RAW version of the ISO 6400 shot and processed it in Lightroom with very modest sharpening, luminance, and chroma noise reduction. The texture is still gone but the photo doesn’t look like a water painting anymore. Luminance noise reduction eliminates the noise that appears as “grain,” and chroma noise reduction removes the random color splotches.
Everyone’s threshold for what is acceptable when it comes to noise is different. For a landscape photographer who will end up enlarging and selling a photo greater than 8″x10″, any JPEG image above ISO 100 might be unacceptable. For the average person wanting to print a photo of their kid blowing a birthday candle, an ISO 3200 JPEG might be acceptable for an 8″x10″. For me, a carefully exposed and composed (no cropping) JPEG photo from this camera at ISO 800 would probably be acceptable for an 8″x10″ print. If I wanted to go bigger, I’d process from RAW and limit myself to ISO 200.