If you’ve read my previous posts about exposure, you’ve probably been wondering why i’ve been referring to film when discussing the parts of a camera. Who uses film nowadays anyway? I’ve mentioned film because to understand ISO, i think it helps to understand how it works with film – specifically black and white film. Besides, i still shoot film sometimes (but the suspense kills me). So imagine we’re talking about film here – the same general general terminology is used for digital sensors too. ISO is referred to as the sensitivity or speed of the film. Readily available films range in speeds from less sensitive ISO 100 (slow) to very sensitive ISO 1000 (fast).
It helps to understand how film actually makes images to better understand ISO (things may get a little technical for a bit…if that kind of thing bothers you, feel free to skip down to the photos). Film is an emulsion containing grains of light-sensitive silver halide crystals. When light (photons) hits these crystal grains they undergo a chemical reaction. Later when you develop the film, the crystals that have been exposed to light turn dark during processing. Remember in film, you capture a negative of the image. When you make a print of that negative, all of the black sections will be white showing how much light was reflected from the subect.
In low ISO films, the size of grains of silver halide crystals is really small, and there are a lot of them. As you increase the ISO, the size of the crystals increases and there are fewer of them. So if you think of the relationship between the light (photons) coming into through the lens to the crystals in the film as being 1:1, you would need fewer photons in higher ISO film (with fewer, larger crystals) to expose an image than with low ISO film (with more numerous, smaller crystals).
So slow film is considered slow, or less sensitive because you need a lot of light to match up with all of those small crystals…and one of the ways you let a lot of light into the camera is by keeping the shutter open for a longer time period – a slower shutter speed. Fast film requires a less light (because it has fewer, larger crystals), so you can crank up the shutter speed to something really fast, let in just a little light, and you’ll still get an exposed image.
The thing is, the faster your film and the larger the grains the more…um…grainy your finished photo is going to be – you can actually see the individual grains of silver halide in the finished print. People used to shoot ISO 400 film all the time, because it was a nice sweet spot for sensitivity – it was slow enough to have a small grain size, yet fast enough to capture images in most light situations. In general, you want to shoot at the lowest ISO you can in order to avoid having a grainy photo.
In digital photography, the same notion applies – even though we’re no longer dealing with grains of silver halide. Digital cameras use what’s known as “magic” to create their ISO range. Actually, the sensors in digital cameras are not magical, but rather analog devices. They’re analog devices in the strictest sense because they measure the amount of light hitting the sensor. A digital device just measures 0’s and 1’s, on or off , whether or not light hits the sensor – something that wouldn’t give you the detailed “how much light” kind of information that an analog device can. Of course, almost everything else in a digital camera is digital – especially the software which translates that analog information to create the image.
Whether you use film or a digital camera, you may be thinking, all this talk about grain and noise – what does it really mean?
Here’s the same scene taken with two different ISO settings. The one on top was shot as ISO 100, the one on the bottom was shot using ISO 1600.
You can see what i mean by noise in the photo on the bottom. You may not be able to make it out in a small image, but when you enlarge an image with a lot of noise, it gets noticeable quickly.
Today’s DSLR’s have great high ISO (above ISO 800-1000) quality and can produce images that are not nearly as grainy as their film counterparts.
The thing you need to be aware of, is that different camera manufacturers and different makes and models have different levels of ISO sensitivity (some may only go up to ISO 1600, whereas other go up to outrageous levels like 12,000). Within those ranges are differing levels of noise, so while one camera may go up to ISO 3200, it may be really noisy when compared to a different camera and sensor. ISO performance also increases with sensor size, so DSLR’s are going to have better high ISO performance than digital point-and-shoots because they have a much larger sensor.
There is, as i’ve said, a lot of variation between how well a camera can produce a non-grainy image at high ISO. Generally speaking, high ISO with low noise is expensive, and you get what you pay for. If you’re interested in photographing in low-light situations, be sure to pick a camera with low noise in the 1600 – 3200+ ISO range. Point and shoot cameras generally start to get pretty noisy around ISO 800, so if you’re looking to do a lot of shooting in low light, you’re probably going to want to go with a DSLR. There are also a lot of sites that compare different camera models and show actual shots from the different cameras at different ISO’s, so you can see for yourself. One such helpful site is dpreview.com. It has the most in-depth reviews of almost every make and model of digital camera available i’ve ever seen.