External light meters

The question arises again and again as to whether external light meters are “more accurate” than those integrated into the cameras. Or whether you can use external exposure meters to check the integrated ones. Unfortunately, there are some pitfalls.

What, where and how does the film camera actually measure?

With many modern photo cameras you can set how many measuring points should be used and how they should be weighted among each other. There are also different modes (aperture or shutter speed priority, etc.). But with a film camera, especially a small format film camera, that’s mostly not the case – the choice of film speed dictates the shutter speed, and the camera then adjusts the aperture to match. That’s mostly it.

In addition, the operating instructions for film cameras usually do not mention the measurement method used. Do they use an average over the whole image? Or just one area in the middle? Do they even have multiple sensors? Cameras that don’t measure through the lens also often puzzle filmmakers because they always seem to measure the same “image area”, even if the camera has a zoom lens.

The following series of images taken with a mobile phone shows why it is not entirely unimportant where and what the camera measures. Because within the same subject, apertures can range from f/14 to f/51.

For a comparison between external and internal exposure meters, it has proven useful to measure “centre-weighted”. With ISO 100 and an exposure time of 1/40 second, most small format film cameras should use the value from the last picture, i.e. an aperture that is close to f/32.

Actual film speed – and what the camera makes of it

Let’s take, for example, the film Kodak Vision3 200T and the Super8 standard: In artificial light, the film actually has a film sensitivity of ISO 200/24°. However, the Super8 standard does not provide for this value at all. The cartridge is notched in such a way that most cameras recognize the content as “artificial light film with ISO 160/23°”. And we already have the first possible deviation or source of error. Even Single8 filmers are not fine here, since the standard provides “all integer DIN values” from ISO 16/13° to ISO 400/27°, but most cameras only ISO 25/15°, 50/18° , 100/21° and 200/24°. So is it better off with a camera that allows you to set the film speed manually? Unfortunately, the answer is “no”, since there are usually gaps in the selectable sensitivities. With the quartz DS8-3 already shown above, for example, you can only choose from the DIN values 12, 14, 15, 17, 18, 20, 21 or 23. The only advantage is that you can see what value you have set here, whereas with the Super8/Single8 “automatic cameras” you have to know exactly what the film manufacturer has notched and what value the cameras make of it.

Integrated filters

A point that is often forgotten when using external exposure meters is that most Super8 cameras and some Single8 cameras have integrated filters that allow artificial light films to be used in daylight without a color cast. These filters mostly correspond to a “Wratten 85”, which causes a light loss of 2/3 stop. Unfortunately, these filters are often automatically swiveled into the beam path. The exposure meter on the camera is aware of this, but unfortunately not an external exposure meter. Here it only helps to familiarize yourself with the system and camera.

Loss of light through viewfinder

The camera’s SLR viewfinder can also cause a “loss of light”: With a few small format film cameras, you only have a flickering image in the viewfinder. The reason for this is that the camera has a movable mirror, usually an oscillating mirror: when the film is being exposed, the mirror is swiveled away from the beam path. In this way, all of the light falls on the film (orange arrow), while the path of the light into the viewfinder (green arrow) is interrupted. Only while the film is transported one frame further and therefore cannot be exposed at the moment, is the mirror back in the position shown below – the orange arrow is interrupted while all the light is reflected into the viewfinder. If this is the case, then there is no loss of light. However, most SLR small format film allow you to see an image in the viewfinder at all times. A static, semi-transparent mirror is then used here. The light can thus permanently take the green and the orange path. Disadvantage of the whole thing is that the light for the viewfinder is “cut off” all the time and doesn’t reach the film. How high this light loss is should actually be in the respective manual. Emphasis on “should”. Because I haven’t found such a statement anywhere. Unfortunately, at first you can only guess. I would start from this value, which I found for the Sony α99-II from 2016: It also uses a static, semi-transparent mirror, which is said to result in a light loss of 1/2 stop.

If the camera doesn’t have a reflex but only a Newtonian viewfinder, then at least you don’t have to worry about a loss of light through the viewfinder.

Shutter and Co.

At first glance one might think that “exposure time = 1/film speed” applies, ie 1/18 s at 18 frames per second. Unfortunately, this reasoning is wrong , since the film has to be transported further and must not be exposed during this time. Most cameras have a “sector diaphragm” for this, which interrupts the incidence of light on the film. The “sector shutter” is sometimes also called “wing shutter” or “circular shutter”. Most small format film cameras have a fixed aperture shutter, also called the “open angle”. However, some cameras also have a “vario shutter” that allows for several opening angles and can sometimes be closed completely for manual fade-ins/downs. If you don’t have a manual, you can assume an opening angle of 180°. However, very few small format film have exactly this opening range. The vario shutter of the Bolex H-16 REX, for example, can be opened from 0° to a maximum of 145°, while the Fujica AX100, for example, should have a fixed angle of 230°. However, if you have the exact opening angle, you can also use it to calculate the exposure time: Exposure time = (1/film speed) * (opening angle/360°).

But to complicate matters further, not all film cameras have a “revolving shutter”. For example, the Pentaka 8B has a “sliding shutter” while other models have a “swing shutter”. Although this does not change anything about the “light-tight covering of the film during transport”, it means that the exposure time cannot be calculated from the opening angle. Here you can only guess without values from a manual and assume exposure time = 1/(2*film speed), ie 1/32 s at 16 frames per second.

manufacturer information

Some manufacturer information on the exposure time of the cameras is also strange. Let’s take the values of the Quartz DS8-M, for example, which has a sector aperture with a fixed opening angle:

There are also such “slips” in the manual for the Nizo 4056:

In some manuals, eg the Nizo 4056, the loss of light through the viewfinder (see above) seems to have already been included in the information on the exposure time. Because otherwise the difference between the calculation result (opening angle = film speed x duration x 360°) and the opening angle specified in the manual can hardly be explained.

exposure meter

Some handheld exposure meters have a “cine mode”. You have to be careful here, as many amateur devices from the 1950s/1960s assume a fixed exposure time of 1/32 s, i.e. a film speed of 16 fps, at which each film frame is transported half the time darkened. Devices that are aimed more at professionals sometimes also have a “cine mode”, which, however, is more likely to have a fixed film speed of 24 fps. A look at the light meter manual is therefore always recommended.

final word

A comparison of the values of a separate and an integrated exposure meter is not that easy. Because if the manufacturer doesn’t reveal how, where and what the camera measures and the camera also incorrectly recognizes the film sensitivity, uses a filter “just like that” and then doesn’t even let all the light through to the film, you really have to be careful to consider all the factors on the external light meter as well.

But the complete abandonment of the camera’s internal light meter needs to be well thought out and can only succeed right away if all the required values can also be found in the manual – hopefully still available.