If you're a photo beginner who's recently decided to switch over from Auto-everything, this is a great place to start. Taking greater control over your images can give a great sense of satisfaction and will allow you to compensate accordingly when you inevitably miss a shot because yes, it still happens to the best of us.

In photography and videography, there is a concept called the exposure triangle. Typically, it's one of the two first lessons that most photography students will start with, the other being image composition. Essentially, there are three primary aspects that must be kept balanced in order to get a correctly exposed image.

If an image is underexposed, it could be too dark to make out details, and if it's overexposed, you could end up with too bright a photo. To every photography rule there is an exception, but generally, you'll want to keep the majority of the details in your image visible. This is where the exposure triangle comes in.

There are three basic aspects that contribute to the exposure of an image. Your Film Speed or ISO, Shutter Speed & Aperture all play into how much light hits your sensor or film, and a photographer must keep these three balanced properly to avoid a completely black or completely white image. Sounds complicated, doesn't it?

Well, it can be difficult at times, but this is also where the fun part of photography comes in. By manipulating one or two of the aspects, you can create wickedly cool effects that you may not be able to achieve on Auto-everything-mode. Want a cinematic blurred background? Open up your aperture to let in more light but then you've got to compensate that extra light with a lower ISO setting or a higher shutter speed. Today, we're going to learn about ISO, or film & sensor sensitivity.

Disclaimer about the Exposure Triangle in the Digital Age

The majority of imaging concepts were developed at a time when analog film was the primary medium. Today's modern-day sensors can do things that early photographers could never dream of, like sharpening, selective blurring, or shooting a digital RAW file and pulling out details you didn't even know were there.

That being said, the basic ideas still apply. Photoshop, Lightroom, Capture One, or any other sophisticated editing software might make some of this seem obsolete when you can easily change so many imaging aspects after the fact. However, those programs can't pull details that aren't in the digital file. Not to mention, getting it right in-camera cuts down on your editing time, and it's a lot more fun. So quit being contrarians, you fancy editing-suite wizards.

In order to save you a bit of extra reading and myself a lot of extra typing, I'm going to talk mostly about film going forward, but the rules generally apply to both.

Intro to ISO

Two of the three points of the Exposure Triangle affect how much light goes through your lens and hits your film. Shutter speed and aperture values can generally be set by the camera and lens, but your Film Speed is generally set and can't be changed. ISO is sometimes called 'film speed' because of how quickly the film reacts to a provided light source.

Your film's speed is generally referred to as an 'ISO Rating' these days. Previously, other standards were used like ASA or DIN, but in 1974, the International Organization for Standardization became the, uh, standard. In fact, many vintage film cameras have ASA/ISO conversion charts printed directly on them for easy reference.

Essentially, this numbered rating system would tell the user just how light-sensitive their film was. The higher the number, the more sensitive the film. For example, an ISO 400 film could commonly be used for both indoor & outdoor settings, where an ISO 100 film could handle the brighter outdoor sun without being too underexposed. An ISO 800 or even 1600 film would definitely be better suited for indoor or nighttime photography due to its ability to absorb so much light from a scene.

It does get a bit more complicated than this, though. Analog film is often used at different ISO ratings than what's on the box, allowing the shooter to get clearer low-light performance or alternate colors. This is an exceptionally deep rabbit hole that many experienced film photographers find themselves delving into over time. Each individual brand of film has its own color spectrum and performs differently when over- or under-exposed and can be 'pushed' or 'pulled' to the correct exposure in the development process. The Darkroom has an excellent Film Index to get you started with these kinds of experiments.

What makes a film more light-sensitive?

In order to explain why a film stock might receive a higher ISO rating than another, we've got to take a closer look at how analog film works. Embedded within photo film is a layer of light-sensitive silver halide crystals that show an image when going through the development process. Think of these as a kind of organic pixel. The different sizes and shapes of these crystals determine how sensitive a film will be. The larger the crystals, the better they can quickly absorb light.

This also means that these crystals will start to become more prominent in your image, however. An image shot on ISO 100 may have smooth gradients and transitions between light- and dark- areas, whereas Ilford Delta 3200 will have more noticeable texture from these crystals. This 'bumpy' texture is called film grain, and it refers to the natural structure of the halide crystals embedded within the film.

On a digital sensor, things are quite different. Instead of having light-sensitive natural crystals, there is an array of light-sensitive photosites or pixels in rigid squares or rectangles. This is one particular reason why high-ISO digital noise in a dark photo isn't as pleasing to the eye as the natural grain structure of high-ISO film. The digital patterns tend to come through as inorganic blocks instead of natural-looking crystal blobs.

Generally, with 35mm film, a photographer would be locked into using one particular ISO-rated film until the end of the roll. Many medium format cameras offered swappable film backs so you could load two different types of film, take a shot with one module and then switch to the other roll of film quickly without having to reload an entire spool.

Digital sensors allow you to change sensitivity on the fly as well. Dialing up your ISO level on a digital camera will brighten your image, but it's generally at the cost of having a smooth, noise-free picture. Changing this value on a digital camera actually amplifies the pre-existing value instead of truly changing the sensor's sensitivity–another reason why excessive digital noise can look more like static than film grain.

ISO 100, Sony a7s III, 85mm f/4 @ 1/4 shutter
ISO 102400, Sony a7s III, 85mm f/4 @ 1/5000 shutter

What does all of that actually mean?

Across the board, a higher ISO setting will get you a brighter, more exposed image. This nearly always comes with more grain or noise in the details of the image. Many modern-day digital cameras will do the honors of applying an automatic noise-reduction filter to the image so that you see more subject and less distortion.

Taking a close look at the following examples, it's easy to see how fine details turn into a kind of digital static at extremely high ISO levels.

100 ISO Detail
102400 ISO Detail
100 ISO Detail
102400 ISO Detail

By keeping your ISO as low as possible, you'll be getting a smoother, finer grain structure in film photos and less noise in your digital photos. One could argue that you may always want to shoot at the camera's lowest ISO setting available. However, by being more flexible with your ISO setting and understanding the level of grain you're comfortable with, you can feel free to experiment with the more fun settings of your camera like shutter speed and lens aperture, which we'll go into next time.

Read Part II here, where we'll talk about Shutter Speed

Read Part III to learn more about Apertures & f-stops