What does ISO mean in film photography?

What is film speed?

Film speed is a standard measure of how strongly a film reacts to light. The ISO system, set out in 1974, replaced older ASA and DIN ratings, as Wikipedia explains under film speed. The number printed on the box, such as 400, is the one you see on cameras and in app names.

A faster film needs less light to make a good exposure. A slower film needs more.

What is a stop?

A stop is a doubling or halving of light. ISO 200 is one stop faster than ISO 100, and ISO 400 is one stop faster again. That means ISO 400 needs half the light that ISO 200 does for the same exposure.

This is why the numbers climb in doubles: 100, 200, 400, 800, 1600, 3200. Each step buys one stop of dim-light ability.

Why is faster film grainier?

Faster films use larger light-sensitive crystals to catch more light, and larger crystals show up as coarser grain. The trade-off is built into the chemistry. Wikipedia describes the texture under film grain. For a comparison with phone sensors, read film grain vs digital noise.

Which ISO should I choose?

1. Think about the light you will usually shoot in.
2. For sunny days outdoors, ISO 100 or 200 gives the finest detail.
3. For a mix of indoors and outdoors, ISO 400 is the common all-rounder.
4. For evenings, parties without strong flash and city streets at night, ISO 800 or higher helps.
5. If you are unsure, go faster. Film copes better with extra light than with too little.

What does push and pull mean?

Pushing means rating the film at a higher ISO than its box number and then developing it longer to compensate. Pulling is the opposite. Pushing raises contrast and grain, and it lets you shoot in darker light than the film is designed for. Wikipedia has an article on push processing.

Pro adds push and pull options to Proofsheet. The effect is digital and is applied when the roll develops, so it changes how the look is rendered in stops.

What do the numbers in Proofsheet look names mean?

They are labels, like a number on a box of film. Harbor 400 is a warm, everyday color look. Nightbus 800 is made for streetlights with a red glow around highlights. Frost 100 is cool, crisp and fine grained. Static Mono 3200 is black and white with heavy grain. See the full list on the film looks page.

They are not claims about real film stocks. Proofsheet's looks are its own digital styles and do not imitate any film or camera brand.

Does ISO mean the same on a phone?

The scale is the same, and on a digital camera, raising ISO raises the gain on the sensor and with it the noise. On film, the number is fixed by the emulsion. A phone camera often picks its own ISO automatically, which is one reason it can feel hard to control. Proofsheet lets the iPhone camera work as usual and applies the film look later, which is why the viewfinder shows the plain scene.

How do I pick a look when the numbers are only labels?

Read the one-line description in the picker, not the number. Harbor 400 is warm everyday color. Paper Mono is clean black and white. Sundown 160 is golden hour, all day. Snowfield 50 is bright, low contrast and airy. The number gives a hint of mood, such as fine or heavy grain, and the description tells you the rest.

The picker shows sample frames through whichever look you focus, so preview them. The twelve looks are listed on the film looks page.

Do I need to think about ISO at all in an app?

Not as a setting. The iPhone camera exposes the scene on its own, and the look is applied afterward. The ISO label simply helps you pick a mood. Think of it the way you would think of the speed number on a box of film at a store: a hint about grain and light, not a control.

What is reciprocity and should I care?

At very long exposures, film stops responding in a simple way, and you need more light than the meter suggests. That is called reciprocity failure. It matters for night scenes with slow shutter speeds and rarely for everyday shooting. A single-use camera never gets there, and neither does a flash picture.

If you try long exposures, check the data sheet for your film, since different stocks behave differently.

Which numbers should a beginner remember?

• 100 for bright days.
• 400 for almost everything else.
• 800 for dusk and dim rooms.
• 3200 only when you want heavy grain and the scene is very dark.