Recently I developed and published my new (and first) iOS application Lightmatic. Lightmatic is a digital light meter whose 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 many photons bounce off some real-world object and fall 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.
Photographer with the light meter on the set, image from Wikipedia
ISO is the abstract measure of photosensitivity. Photosensitivity defines the number of photons that 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 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 an f-number equal to f/1.0 transmits precisely twice as much light (photons) as a lens with an 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 the 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, this value indicates the number of whole seconds. For example, shutter speed 6” corresponds to six seconds.
Image from Wikipedia
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 exposure time, and the f-number. Naturally, the accuracy of such a “light meter” is significantly far from ideal, and it is impossible to obtain technically correct images using it.
Image by Steven Byrnes
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 on resistance of the ambient light level. Measuring the electrical resistance, we can then conclude illuminance. And knowing illuminance, we can precisely calculate the exposure triplet.
Analog Sekonic, image from Wikipedia
Selenium meters were good, but they weren’t great. They degraded over time, especially in hot and humid conditions. Technology development made it possible to switch 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, decreasing time from the idea in the photographer’s head to the image captured on the film.
Digital Sekonic, image from Wikipedia
Now we can use precise digital light meters right on our phones. And they are not inferior in any way to their analogue and digital brothers.
I’ve tried almost every light meter available for iOS but haven’t found one with the combination of features I need:
- Easy to use UI;
- Pinhole support;
- Reciprocity curves for pinhole mode;
- Manual flash calculator.
So, I developed Lightmatic. If you shoot film, consider trying it. Email me at email@example.com and tell me what features you are missing.
Lightmatic – made with ❤️ by film photographers for film photographers!