Recently I developed and published my new (and first) iOS application Lightmatic. Lightmatic is a digital light meter which goal is to compose every feature I need in one app.
But what is a light meter anyway? Light meter measures illuminance. It tells you how much photons bounces off some real-world object and falls then on its sensor in a fixed period (one second, for example). The unit of illuminance is the lux. But photographers don’t use luxes. Instead, we use triplets of ISO, f-number, and exposure time.
ISO is the abstract measure of photosensitivity. Photosensitivity defines the number of photons which photosensitive material (film or matrix) is ready to absorb in a fixed period. Film ISO typically varies from 8 to 6400. ISO of digital camera’s matrices can reach up to 256000.
F-number (aperture) is a lens characteristic. It equals to the ratio of the focal length to the diameter of the entrance pupil. It is commonly indicated in the format f/x where x is the f-number. A lens with f-number equal to f/1.0 transmits precisely twice as much light (photons) as a lens with f-number corresponding to f/1.4. The f-number can be changed by changing the diameter of the entrance pupil on almost all lenses.
Exposure time indicates a time interval in which shutter will be open for the light to fall onto photosensitive material (film or matrix). In the context of exposure time, integers without suffix indicate seconds. For example, an exposure time of 250 means that the shutter will be open for 1⁄250 (quarter) of a second. Exposure time 2 corresponds to 1⁄2 (half) a second. If suffix ‘ or “ follows the value, then this value indicates the number of whole seconds. For example, shutter speed 6” corresponds to six seconds.
The most straightforward light meter is a printed table in which pictograms of surrounding conditions (sunny, mild, clouds, fog) correlate to exposure triplets of the ISO, the shutter time, and the f-number. Naturally, the accuracy of such n “light meter” is significantly far from ideal, and it is impossible to obtain technically correct images using it.
The next step of evolution is a selenium meter introduced in the second half of the XX century. Selenium is known for its photoelectric properties – dependence of resistance of the ambient light level. Measuring the resistance, we can then conclude illuminance. And knowing illuminance, we can precisely calculate the exposure triplet.
Selenium meters were good, but they weren’t great. They degraded over time, especially in hot and humid conditions. The development of technology made it possible to go first to cadmium sulfate, and then to semiconductor silicon sensors. In the 1970s, exposure meters became so compact that it became possible to integrate them into cameras, thereby rapidly decreasing time from the idea to the image captured on the film.
The 21st century made it possible to use exposure meters right on smartphones. And they are not inferior in accuracy to any analog and digital brothers.
Who in 2020 may need an external light meter?
Film photographers whose cameras do not have a built-in exposure meter – a vast number of magnificent mechanical cameras.
Film photographers whose cameras exposure meters do not work at all or work incorrectly. Only about 5% of selenium exposure meters were able to survive until 2020. Cadmium exposure meters, although more tenacious, are still subject to the influence of temperature and humidity and can begin to lie after long improper storage. It is always useful to check the readings of the built-in exposure meter before principal shooting.
Pinhole photographers. Pinhole cameras do not have a lens and have an extremely small f-number varying from f/64 to f/500. Exposure times in such cameras reach minutes, hours, days, and even weeks. One also needs to take into account film reciprocity failure.
Lightmatic is one of several light meters available on iOS. Why is it worth attention?
It combines all the features that cannot be found together in any of the other applications:
Accurate basic light meter with many settings;
Film profiles with reciprocity failure curves;
Manual flash calculator.
If you shoot film, consider trying Lightmatic. Email me at firstname.lastname@example.org and tell me what features you are missing.
Lightmatic – made with ❤️ by film photographer for film photographers!