Understanding ISO
To understand anything I think it is important to know what something is and what it stands for.
ISO stands for the International Organization for Standardization. ISO is an organization that sets international standards of measurements. So it doesn't matter where in the world you are, a film's ISO will have the same meaning wherever you are.
In photography ISO refers to a film's sensitivity to light. But what does that mean?
I feel fortunate enough to have grown up in an era when analog was still king. When I started to learn about photography it was all about film, I didn't switch over to digital until 2004. So I had years of experience learning the hard way of what ISO really means.
It is imperative that you understand what ISO means if you want to get comfortable working in manual mode on a digital camera.
Generally speaking, I photograph most things outdoors without a tripod. ISO is always the first thing I set (before aperture and shutter speed) and I know what ISO to use if it is really sunny out (ISO 200), cloudy (ISO 400, 800 or higher), or indoors (1600+, if I'm not shooting with a flash I will often stretch the ISO to the MAX in that case).
One thing to note is that on a digital camera, the lower the ISO the less grainy your image is going to look.
So how can you truly grasp what ISO means? I suggest looking at how the film manufacturers market their products. Fujifilm and Kodak is a great place to start since they are two of the world’s largest manufacturers of film. Let’s look at some examples:
Kodak Professional Ektar 100 is a daylight-balanced color negative film characterized by an ultra-vivid color palette, high saturation, and an extremely fine grain structure. The combination of rich colors, fine grain, and optimized sharpness benefit this film's use for nature, travel, and outdoor photography, as well as fashion, product, and other commercial applications.
Kodak GOLD 200 is a medium-speed daylight-balanced color negative film offering a versatile combination of vivid color saturation, fine grain, and high image sharpness. It has a nominal sensitivity of ISO 200/24° along with a wide exposure latitude for exposing up to two stops under or three stops over to enable working in a wide variety of lighting conditions.
The keywords in the ISO 100 and 200 film speeds are that these are balanced for daylight situations. Meaning they should be used outdoors on sunny days, unless of course you are working indoors in a studio or with a flash.
Ilford's HP5 Plus Black and White Negative Film is a traditional and versatile panchromatic film designed for general use in a wide variety of shooting conditions. Exhibiting notably wide exposure latitude, this film responds well to use in mixed and difficult lighting and provides medium contrast for greater overall control.
HP5 Plus is a flexible film type that is ideally suited for use in general photographic applications in an array of different lighting conditions.
Kodak's Professional Portra 800 is a high-speed daylight-balanced color negative film optimized for use in difficult lighting conditions. Portra 800 is very well-suited to photographing moving subjects and working in low-light conditions while maintaining true color reproduction.
To reiterate, the lower the ISO you use the less grainy your images will be. I wanted to show these examples to you so that you can start to see the differences in marketing here.
ISO 100 - 200 - is balanced for daylight. That means outdoor sunny day use. Or indoors a flash would be needed.
ISO 400 - mentions mixed lighting situations: indoor or outdoors.
ISO 800 - remains true to form in low-light situations and is well suited for moving subjects (yes sports photography and disc golf action come into play here).
If you take photographs with a digital camera the same rules apply, ISO is film speed.