What “exposure” means on a film camera
On a film camera the word exposure is used in two ways. The first is the amount of light that reaches the film for a single frame. The second is a single frame on the roll, which is why a roll is described as having 24 or 36 exposures. Most of this guide is about the first meaning, because getting the amount of light right is what separates a usable negative from a thin or muddy one.
Unlike a digital camera, you cannot check the result on a screen or change the ISO between shots. You commit to a film speed when you load the roll, and you only see the outcome after development. That makes understanding exposure more important, not less.
The three controls: ISO, aperture, and shutter speed
Three things decide how much light the film receives.
- Film speed (ISO). This is a property of the film you bought, such as ISO 100, 200, or 400. Higher numbers are more sensitive to light and usually show more grain. The speed is fixed for the whole roll. For a closer look at one common speed, see ISO 200 film settings.
- Aperture. The size of the lens opening, written as an f-number. A smaller number such as f/2 lets in more light and gives a shallower depth of field. A larger number such as f/16 lets in less light and keeps more of the scene sharp.
- Shutter speed. How long the shutter stays open. Fast speeds such as 1/500 s help freeze movement. Slow speeds such as 1/30 s let in more light but risk blur from subject movement or camera shake.
Each full step on the aperture or shutter scale doubles or halves the light, and this unit is called a stop. Opening the aperture by one stop and halving the exposure time gives about the same exposure, which is why many different combinations can be correct for one scene.
How to measure the light
You have a few options for deciding on settings.
- The camera’s built-in meter. Many 35mm cameras from the 1970s onward have one. It is usually a good starting point, but older meters may be inaccurate or depend on batteries that are no longer made in the same form, so it is worth checking them against another source.
- A handheld light meter. A dedicated meter, either reflected or incident, is a reliable way to measure the scene independently of the camera.
- A phone meter app. Convenient as a backup, though accuracy depends on the phone’s sensor and the app.
- The Sunny 16 rule. In bright, direct sun, set the aperture to f/16 and the shutter speed to the reciprocal of the film speed. With ISO 200 film that is roughly 1/200 s, or 1/250 s on most cameras. Open up by about a stop for each step down in light, for example to f/11 for slightly hazy sun and f/8 for light overcast, as a rough guide.
A meter reads the scene as a middle gray, so very bright scenes such as snow can come out underexposed and very dark scenes can come out overexposed unless you adjust. The metering modes guide explains why.
Negative film, slide film, and latitude
Latitude is how far from perfect exposure a film can drift and still give an acceptable result.
- Colour negative film generally has a generous latitude, particularly toward overexposure. Many photographers expose it slightly generously, for example by rating ISO 400 film at 200, because shadows hold detail well and highlights tend to be forgiving.
- Black and white negative film behaves similarly in many cases, although results depend on the film and developer.
- Slide (reversal) film has much less room for error. Highlights clip quickly, so metering for the highlights and keeping exposure accurate matters more.
A thin, underexposed negative loses shadow detail that cannot be recovered. A dense, overexposed one can often still be scanned or printed, which is why the common advice is that when in doubt with negative film, give it a little more light.
Long exposures and reciprocity
At very long exposure times, film often becomes less sensitive than its box speed suggests, an effect called reciprocity failure. The point at which it matters, and the compensation needed, varies by film, so check the manufacturer’s datasheet. It becomes especially relevant for night work and for pinhole photography.
Exposures on a roll: 24 or 36?
A “36 exposure” roll gives you up to 36 frames, and a 24 exposure roll gives up to 24. Neither is better in terms of quality, since the film is the same. The choice is about cost and how fast you shoot.
- A 36 exposure roll usually costs less per frame.
- A 24 exposure roll is quicker to finish and develop, which is useful when testing a new camera or film.
You can sometimes squeeze an extra frame or two from a roll, but it depends on the camera. Half-frame cameras fit roughly twice as many pictures on a roll, as explained in half frame vs full frame film cameras.
Is film ruined if it is exposed to light?
It depends on how much light and whether the film was already developed.
- Opening the back of a camera with an unprocessed roll inside will fog the film, at least the portions that see the light. Frames near the outer turns of the roll are the most affected.
- A small amount of light, such as a loading mishap in shade, may only affect the first frame or two.
- Airport X-ray scanners are a separate concern. Sensitivity depends on film speed and the number of scans, so keep high-speed film in a hand-inspection bag if you can.
Developed negatives are no longer light sensitive in the same way, so they can be handled in normal light.
Should exposure be high or low?
Neither extreme. The goal is enough light to capture shadow detail without losing the highlights. For negative film, slight overexposure is usually safer than underexposure. For slide film, err on the side of protecting highlights.
A simple workflow for your next roll
- Choose a film speed that suits the light you expect, and set it on the camera if it has an ISO dial.
- Meter the scene, or use Sunny 16 as a check.
- Pick the aperture for the depth of field you want, then set the shutter speed to balance.
- Make sure the shutter speed is fast enough to avoid blur, or use a tripod.
- If the scene is unusually bright or dark, adjust by one or two stops.
- Keep notes on a few frames so you can compare them with the results later.
Over a few rolls you will learn how your camera and meter behave, and exposure becomes much more intuitive.