04 Apr understanding iso
Welcome to the ISO masterclass! Everything you possibly wanted to know, or maybe didn’t know about ISO in relation to photography. I say in relation to, because ISO stands for International Organization for Standardization . Their founders decided to give it the short form ISO instead of IOS. ISO is derived from the Greek ‘isos’, meaning equal. They do a lot more than just standardization of photography measurements. Specifically for us though, they define the process of measuring the sensitivity of light. If you want to see exactly how to do it, you can buy the details from the ISO here. I think its about $110. Sticking with a more practical lesson here, lets move on.
It all started with film
Back in the day, or heck even present day if your’e a hipster like me, film was rated in these ISO numbers. For example the film in the image above, is ISO1600. What does that mean though? Its corresponding to the speed of the film. Faster film, means it will expose an image with less light. Which in turn will either yield a faster shutter speed or availability of a smaller aperture. Film is made of a long strip of plastic and then coated with silver halide crystals. The fineness of these crystals is what determines the speed. The larger the crystal, the grainier the image, the faster the film. Digital is another story.
move over film!
With the onset of digital came a new method of increasing light sensitivity. Digital camera sensors are made up of photo receptors. They measure photons. Sensitivity is either increased or decreased by changing the photo receptors amplification. In laymen terms, more sensitivity, more juice, more interference (noise). This really only happens to an extent, each camera will vary but past a certain point amplification will reach a maximum (say ISO1600) and then its software based after that. You’re post processing the image in camera in a nutshell to boost the shadows. Lets take this to the practical side and see some examples.
Here is an example image shot at ISO 160. Nice and clear right? Lets bump the ISO up.
Ive added 2 stops of adjustment to the exposure in Photoshop to illustrate the detail loss. As you can see the loss of detail is mostly in the shadows and more noticeable past ISO 1600. This is because you run out of sensor amplification and it goes to digital. A good tip here is if your shadows are the ones losing the detail and not the highlights… over exposing might yield some benefit in image quality when shooting higher at a higher ISO. This is called ETTR (exposing to the right).
In the above example I’ve pushed the image even further. Notice how the highlights still look nearly the same while the shadows look pretty terrible.
Lets talk about stops. ISO is the easiest one to remember. If you go from ISO50 to ISO100 that is one stop. Every time you double the number it equals a stop. ISO200 to ISO400 and so on. So what does this mean for us? Given a fixed aperture, the only other adjustment left is shutter speed. If you increase your ISO, you also increase your shutter speed. One stop ISO is equal to one stop shutter speed. Remember this, its on the test later.