What is ISO? What does ISO mean when it comes to photography?
First of all, let’s get the pronunciation part out of the way. You will hear people pronounce ISO as “eye-so” or “eye-ess-oh.” The correct pronunciation is “eye-so.” Some may argue that ISO stands for International Standards Organization and therefore as an acronym it must be “eye-ess-oh.” This is incorrect.
The actual corporation is the International Organization for Standardization and the founders gave themselves the short name ISO from the Greek “isos” which means “equal.” Since ISO is short for “isos” it would obviously be pronounced like a word and not as an acronym. You don’t pronounce all the letters of a word unless you are spelling it. Therefore ISO is pronounced “eye-so.”
Now why is the photography term ISO (which is part of the exposure triangle) related to this corporation anyway?
The International Organization for Standardization sets standards for all sorts of manufactured products, including film and photography products. Their purpose is to make sure everyone around the world uses the same measurements and regulations and thus whatever product you get in one nation is “equal” to what you would get in another nation on the other side of the world. For example, the steps on ladders and stairs in factories are standardized. Imagine what would happen if every staircase used different distances between steps. Lots of accidents! We take for granted that printer paper size is always 8 ½ x 11 (in the US) or that a 3-hole puncher will punch 3 holes in the same places. These “standards” are the work of ISO.
Many decades ago, there was a need to standardize film speeds. Enter ISO. They created a system to numerically measure film’s sensitivity to light. There were also a competing standards known as ASA and DIN which were combined into ISO standards in 1974. So since the 1970s no matter where you bought film and no matter what brand, you were assured that it would work in your camera the exact same way. Boxes of film were sold in 4 common speeds (and some faster and slower speeds that were not as popular). These speeds were 100 (slow), 200 (slow), 400 (fast), and 800 (high speed).
Now don’t be confused by the word speed – this rating system has nothing to do with shutter speed! So what is film speed then?
The film speed indicated how sensitive the film was to light. The slower the film, the less sensitive it was to light. You would use a slow film speed when you already had good lighting, like a bright sunny day. You didn’t need the film to be sensitive to light because you had plenty of it. The faster the film, the more sensitive it was to light. You would need a faster film when you had darker or low light conditions (or for fast moving subjects). You wanted the film to react more intensely and more quickly to the limited light you had.
How do the “speeds” of film work? How did they get the film to react to light differently on those different speeds of film?
Film was created with millions of light-sensitive silver halide crystals called “grains” on the surface. A film that was slower/less sensitive had less “grains” on it. A film that was faster/more sensitive had more “grains” on it. A faster film picked up more light, but it was also more visible in the final image. Once the film was developed, images that were shot using 800 speed film appeared grainy compared to 100 speed film. The grain in a developed film image is from the crystals of silver on the film.
How do the “speeds” of film relate to digital photography?
Today’s DSLR cameras use a sensor instead of film with silver halide crystals. Digital cameras convert the light that enters the camera and hits the sensor into a digital signal. The digital (or electronic) signal is then converted or processed into a visual image. On a digital camera, increasing the ISO setting “turns up” the signal. As the signal is amplified, the increased electrical charge makes the sensor more sensitive to light, but as a consequence digital noise is created. Digital noise has a noticeable “grain” to it that is similar to what is seen in higher speed films. The digital noise may be luminance noise which affects the brightness of pixels (but not the color) or it may be chroma noise which is highly undesirable. Chroma noise consists of colored grain or specks. While both of these can be mitigated to some extent in post editing programs like Photoshop or Lightroom, at a certain point the image quality becomes significantly degraded. The threshold of where this degradation takes place will vary depending on the camera’s processor, sensor size, and megapixel count. Additionally, noise reduction processing is now built in to some digital cameras.
How does the size of your camera sensor and the amount of pixels affect how much digital noise you see?
On a point-and-shoot camera the sensor is very small. It is larger on a compact camera and largest on a full frame camera. When you pack a bunch of pixels into a smaller sensor area you will produce more grain. As you increase the ISO on a point-and-shoot camera you will get noise at a lower ISO than you would with a compact camera. And as you increase ISO on a compact camera you will get noise at a lower ISO than you would with a full frame camera. The larger sensor of a full frame camera allows for lots of pixels and larger pixel size. These larger pixels are more capable of gathering light so the reduced electrical charge required in low light does not create as much digital noise.
So what does all of this mean for your photography?
ISO is one of the three aspects of the Exposure Triangle. Knowing what you now know, that higher ISO creates grain in your images, you will want to try to keep the ISO as low as possible. This means you may have to balance the aperture and shutter speed (the other parts of the exposure triangle) to get proper exposure.
Just like in film, ISO 100 is only good when you already have lots of light. If you don’t have lots of light you will have to open up the aperture (small f/#) or increase shutter speed, or both. Depending on your subject, this is not always possible and adjusting ISO may be the only solution.
Here are a few scenarios. You want to keep ISO low, but it isn’t very sunny. You can use a longer shutter speed as long as your subject is stationary like a landscape or a flower. But if you subject is moving, like at a sporting event, you may have to adjust the aperture, or increase the ISO. You can open up the aperture to f/5.6 or maybe even f/4, but if you widen it to f/2 you may end up with a depth of field that is too shallow to get your entire subject in focus. The Exposure Triangle is a balancing act, like a 3-way see-saw, and sometimes increasing ISO will be the only way to get the image. (See our website for a free Exposure Triangle "Sliders" Cheat Sheet)
Doubling your ISO from 100 to 200, for example, doubles the amount of light sensitivity. Therefore you only need half as much light from your other settings, so you could change the aperture from f/4 to f/5.6 to get greater depth of field, or you could change to a faster shutter speed from 1/125 to 1/250th. Sports photographers will consistently use higher ISO settings than landscape photographers. Photographers that photograph in low light situations will consistently use higher ISO settings than photographers that always take photos of flowers on sunny days.
What is the takeaway?
Lower ISO is always better for your image quality. Increasing ISO will add noise to your images (how much noise will depend on the ISO level and the camera specs). Just remember, when faced with a choice of getting the shot versus only using low ISO, always get the shot even if you have to turn the ISO way up!
If you’d like a hands-on lesson with some practice using ISO settings check out the Free Lessons section of our website. There you will find lessons on aperture, shutter speed, ISO, and more.
All this talk about film bringing on some nostalgia? Want to try some old fashioned film photography?
Pull out that old vintage film camera or purchase one online! You can easily find vintage film cameras to purchase online from many sellers like Tommy's Camera Shop on Etsy, used sellers like KEH (affiliate link), or on Ebay. Film is available at B&H Video and Adorama. You need fast film for overcast days, fast moving subjects, or low light. For fast speed films, try Ilford HP5 Plus 400 or Kodak Portra 400. For very low light, try Kodak T-Max P3200. You can use low speed film for situations with lots of light, such as very bright days. Try Ilford Pan F50 or Ilford FP4 Plus 125. You can use these handheld, but a tripod can help reduce camera shake. Good luck!