How to Use a Light Meter With a Film Camera

14 min read by Dmitri.
Published on January 24, 2023. Updated on June 5, 2024.

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Setting up exposures may be more important than nailing your focus. Thankfully, there are tools that simplify this photographic process that’s essential even in the digital age.

In this guide, I’ll show you how to use seven ways to measure light, starting with my favourite: using free mobile apps.

Learn how to use: Light meter apps (reflective). External reflective light meters. External incident meters. Built-in uncoupled light meters. Built-in coupled light meters: match-needle type. Built-in coupled light meters with automatic priority and full-auto metering modes. The Sunny 16 rule (estimating using sight, without a meter).

☝︎ Further reading: “A Beginner’s Guide to Shooting Manual Film Cameras.”

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Lumu — a free light meter app that uses your phone’s camera to calculate the shutter speed and aperture combinations with live preview.

Light meter apps (reflective).

I’ve been using Lumu, a light meter app for iPhone, for over five years. It has a clean, simple interface, and a spot-metering mode. My favourite thing about apps like that is the live preview.

Live preview in mobile meter apps greatly simplifies the transition from a digital camera to film. You can see on your screen how bright/dark your image will look once developed. With a spot-metering mode, you can ensure that a particular part of your scene is treated as 18% middle grey (more on that below). Or, you can choose settings that purposely under- or over-expose your image while seeing what they may look like on film instantly.

To use a light meter app, you’ll need to:

1) Input your film ISO. On Lumu, it’s the bottom number on the screen.

2) Select a shutter speed — typically faster than 1/60th of a second hand-held to avoid motion blur and always below your camera’s fastest shutter speed. On Lumu, this is the middle number.

3) Point your mobile device towards the subject/scene and take note of the aperture value and shutter speed that your light meter shows.

4) Set your camera’s shutter speed and aperture to match the light meter’s.

5) You can now take your picture.

For step 2, you can also select an aperture instead, which will stay constant along with film ISO, while the light meter will update the correct shutter speeds when you point it toward your subject. Setting an aperture as a constant means metering in aperture-priority mode; when the shutter speed is constant — we’re in shutter-priority mode.

✪ Note: There’s no ISO-priority mode in film photography as it’s always assumed to stay constant (because your entire roll must be developed at once and thus must retain identical sensitivity across all frames).

☞ Switching metering priorities is a little awkward on Lumu — you’ll need to tap the numbers until they show arrows (this sets them as a constant).

Left: no spot metering. Right: spot-metering to correct backlit exposure.

Most light meter apps will let you use a spot-metering mode, during which the exposure is metered within a small circle, ignoring the rest of the scene. This is useful when metering for backlit scenes (when the sun is shining into the camera lens). In a situation like that, you can point your spot meter directly at your subject/object of interest, which will then correct the exposure for that part of your photograph. This action will tell the meter to ignore the flood of sun rays that otherwise confuse into thinking that the entire picture needs to be as dark as possible.

Whether you use an app or a physical spot meter, keep in mind that pointing it toward dark spots will force it to brighten your scene (via the settings it suggests for your camera) until that piece of the image appears to have a neutral brightness or averages to 18% middle grey. Pointing it at the snow, for example, may yield readings for your camera that’ll render it dark grey on film (you’ll also observe that result via live preview). On the other hand, if you’re photographing something on a dark background (like a black curtain), your photo may end up unnaturally bright.

You can test the above with an app with a spot-metering mode and live preview (like Lumu) to understand what I mean better.

Thankfully, most scenes have a large variety of brightnesses which makes reflective light meters work. If you switch the spot-metering off, the light meter will often gather more data from the scene for a better average. Alternatively, you can point your spot meter at objects of various brightness until the live preview shows the desired luminance (bearing in mind that it may get rendered differently on film).

✪ Note: Do you have a favourite light meter app? Let me know in the comments below.

Light metering apps can be considered accurate enough for all types of film photography. However, dedicated and in-camera light meters have their advantages.

External reflective light meters.

A reflective light meter will measure the light that reflects off your scene. Light meter apps belong to that category, as do some real, physical devices — including spot meters.

Photo by Annie Spratt.

✪ Note: A spot meter is only different from a general reflective light meter in that it measures reflected light from a narrow angle-of-view, like a telephoto zoom lens that can focus on one small detail.

Some photographers prefer physical light meters as they are often faster to use: there’s no app to load or phone to unlock. Strategically placed tactile, dedicated controls are also advantageous to slippery, monolithic smartphone surfaces.

Selenium reflective light meters don’t use batteries — another something an app can’t do. You can recognize them by the translucent plastic bubbles that you point towards the scene; selenium meters may also be built-in to film cameras. The downside of this technology is poor low-light sensitivity and overall accuracy. These types of meters may be good enough for many or most applications; still, a battery-powered device is likely to give better results.

Not all external light meters will look like the apps. Their design evolved over a hundred years, changing every so often. But the general principle is still the same: set the ISO, choose a shutter speed or aperture, point the device toward the scene, and transfer the values it outputs onto the camera.

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Photo by Benjamin Balázs.

External incident meters.

Incident light meters calculate the exposure by measuring the light that collects onto their white semi-spheres directly from the sun, moon, or any light that reaches it. Incident light meters tell you how bright it is where you place them.

Incident light meters do not get confused by overwhelmingly bright or dark subjects/objects. There’s no need to worry about 18% middle grey. Because of that, it’s easier to get an accurate reading with an incident meter than with a reflective meter.

However, you will need to walk up to your subject and place the meter next to them to take a reading. This may not be suitable when photographing far-away or fast-moving stuff. These types of light meters also tend to be more expensive.

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Voigtländer Vitessa L2’s uncoupled reflective light meter — top view.

Built-in uncoupled light meters.

Most modern cameras have built-in reflective light meters that can automatically set or guide your shutter speed and aperture. Film cameras have had those devices as part of their design for nearly a hundred years — but they weren’t always as tightly integrated as they are now.

Selenium light meters (the kind that needs no battery power) were the most popular way to measure light in the mid-century. Unfortunately, integrating them into a mechanical camera is a complicated task. And so, manufacturers embedded the meters without connecting them to the camera controls.

The resulting uncoupled light meter design required the photographer to measure light with their camera (as you would with an external light meter), then set the aperture and shutter speed manually by reading the meter’s output.

Voigtländer Vitessa L3 uses exposure values for shutter speed and apertures. These numbers are meant to be transferred from the light meter.

Using an uncoupled meter isn’t much different than having a hand-held light meter. The disadvantage of such a design (as compared to coupled designs, discussed below) is the extra step of dialling the metered values into the camera. The upside is not having to carry another device and faster operation — since all the controls are in one place.

Another advantage of an uncoupled meter design over many coupled designs is full control over camera settings. Fully automatic cameras will give their meters full control over your camera’s exposure, making correcting meter’s mistakes (like incorrect 18% grey balance and backlit scenes) difficult or impossible. An uncoupled light meter will only give “suggestions” that the photographer may use on the camera or correct with a better reading.

✪ Note: It’s not uncommon to use a good external meter for more accurate readings, together with a camera that has a built-in light meter.

Built-in uncoupled light meters often use an EV (Exposure Value) system where they point to a single number (once you set them to a correct ISO) which you can transfer onto your camera’s lens.

☝︎ Further reading: “What Is Exposure and How to Measure It.”

Built-in coupled light meters: match-needle type.

External reflective meters were the first tools we used to measure light accurately; uncoupled built-in meters followed. The final step in camera exposure automation is built-in coupled light meters.

Voigtländer Vitomatic I with a coupled match-needle reflective light meter (a half-moon-shaped window on the right-hand side of the top plate).

But not all coupled light meters are built the same. Their first generation simply connected the meter to the shutter speed and aperture mechanically to make them “aware” of the camera settings. This resulted in a match-needle system that cleverly guided the photographer toward selecting the correct shutter/aperture combination.

To use a match-needle built-in coupled light meter, like the one on the Voigtländer Vitomatic I camera, simply ensure that the needle matches the little arm with a circle whenever the camera is pointed towards your scene.

This type of coupled light meter retains all control over shutter/aperture in photographers’ hands. You may choose to match the needle or ignore it in favour of another meter or your Sunny 16 estimate.

Some coupled meters are the result of multiple or strategically placed light sensors inside a camera. Center-weighted built-in meters will prioritize the middle of the frame and partially ignore the fringes. Whereas bottom-weighted meters will ensure that parts of your scene below the horizon are well-exposed and partially ignore the bright sky.

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Minolta TC-1 uses a built-in coupled automatic light meter in aperture priority mode.

Built-in coupled light meters with automatic priority and full-auto metering modes.

Built-in coupled light meters got a major upgrade around the 1970s. Camera manufacturers have finally figured out how to change camera settings based on light meter measurements automatically. Suddenly, there was no longer a need to adjust the aperture and shutter speed by hand based on the numbers given by a meter or to match a needle — everything happened in-camera on its own (almost).

The first coupled automatic meters still relied on the photographer to (sometimes) adjust one of the settings: aperture or shutter speed.

Such meters are either shutter- or aperture-priority by name. As you may remember from the above, an aperture-priority mode gives the photographer full control over the aperture while the rest remains up to the meter. With a shutter-priority mode, you’ll control the shutter speed as the meter adjusts the rest automatically.

Canon Canonet QL-17 GIII uses shutter-priority metering with an option for fully-manual exposure settings.

Those types of meters would often display the camera settings they’ve calculated in the viewfinder or elsewhere on the camera. For example, with the shutter speed set to 1/30s, a built-in shutter-priority meter will point its needle to 𝒇5.6, indicating its measurements inside the viewfinder of Canon Canonet QL-17 GIII.

Other cameras, like Yashica Electro 35, would only warn you when the metered shutter speed is below 1/30s during insufficient light (and thus runs a risk of introducing motion blur) or require shutter speeds above the maximum the camera can fire at (typically above 1/500s). As a photographer, you can use this feedback to increase the aperture size (lower 𝒇-number) to allow more light in for faster shutter speeds or decrease the aperture size to slow down the shutter speed.

Yashica Electro 35 GT. Its yellow “slow” indicator lights up when the aperture-priority meter estimates a matching shutter speed would be slower than 1/30s. The red “over” indicator means that the camera’s shutter speed is insufficient according to the meter readings.

The trade-off to shutter/aperture priority coupled meters’ convenience is the need to rely completely on their readings, which can cause problems in some situations. Being reflective-light measuring devices, those meters are prone to errors in backlight situations; you also won’t be able to deliberately make your scene brighter or darker than it appears to the meter.

Some cameras, like Minox 35 GT, had a dedicated backlight compensation switch, which changed the camera metering to add a couple of stops of light to the scene. This helped with the common backlight situation. Other cameras shipped their cameras with an option to disable their coupled meters and shoot manually by relying on an external meter reading.

Minolta TC-1 lets you compensate for its automatic metering by adding/removing stops of exposure (making the result brighter/darker than internally metered). Using that tool that lets you add +2 stops (or any amount) of brightness and avoid black silhouettes in your image. This camera also comes with a spot-metering mode which I often use instead to set my exposures.

Older cameras that use coupled automatic light meters but rely on you to input your film’s ISO (instead of automatically reading that from the film canister’s DX code) could be hacked. For example, Electro 35 with an ISO 400 film can be “fooled” to take an image that’s two stops brighter by setting an ISO dial on the camera to 100.

✪ Note: There’s little practical difference between shutter- and aperture-priority modes. Still, many photographers have a preference and it’s often towards aperture-priority modes as it makes easy shooting “wide-open.”

Olympus Infinity Stylus Epic (Mju II) uses full-auto coupled metering.

Many newer film cameras implement full automatic metering that needs no input from the photographer whatsoever. It completes the job left by the shutter/aperture priority meters by selecting settings least likely to introduce motion blur with an optimal aperture.

Full-auto meters make choosing wider apertures to generate bokeh or slower shutter speeds for deliberate motion blur difficult or impossible — especially if they read film ISO via DX code. Some examples of such cameras are Olympus Mju I & II, Konica Big Mini 201 & 302, as well as many others in the point-and-shoot category. But the speed and convenience of only having to frame and press the shutter to get a well-exposed photograph most of the time has its advantages.

The Sunny 16 rule (estimating using sight, without a meter).

While there’s a certain convenience in trusting technology with measuring light for our photographs, there are some disadvantages. Built-in meters can make mistakes (even if they’re perfectly calibrated) and it can take some fidgeting to adjust them for working in complicated scenes. An external light meter is another gadget that weighs you down and requires maintenance/time on the scene.

This is why learning how to measure light using our eyes can be very useful. There’s nothing to carry, point, and program — one look and a little bit of mental math is enough to set the right shutter speed and aperture.

The Sunny 16 rule guarantees that in full sun, an aperture of 𝒇16 combined with a shutter speed that inversely, approximately matches your film’s ISO will give you a correct exposure. Photographers have been using this method for decades; however, it becomes complicated to use in the shade or artificially-lit places.

“Master the Sunny 16 Rule!” is a course that explains how the Sunny 16 rule works in greater detail and shares a few methods for effortlessly applying it in the shadows, various weather, and artificially lit areas.

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Thanks. That was easy to follow and I found it useful. Currently, I’m using a Sekonic L398 (self powered; possibly with a silicon cell instead of selenium), and occasionally a Weston Master III (the same as the one shown in your third-from-topmost photo). I prefer to use the Weston with a grey card, therefore it’s not as convenient as the Sekonic in ambient-reading mode. Good luck eh! A friend recommended your “Moscow Dayze” — I bought it and really found it fascinating. Your photos there are an unexpected insight into the ebb & flow of Moscow. Thx again.

Oh yes! I’m hand-metering for my Rolleicord IV; new to me from a big eBay seller in Prague. Currently using Foma 100 (Rodinal; soon D96). Also, I’m planning to do quite a bit of work with Double-X (to be tested in D96; Df96 wasn’t my cuppa tea) and Potsdam 100 — working well in D96. C ya. Good luck. I’m in Victoria, BC.

Light meter apps (reflective).

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Dmitri