If you're serious about photography and particularly if you're using a vintage camera you're going to need a light meter but a decent digital meter will set you back at least $100 (although high quality light meters are more like $200 or more).
This simple device consists of a diffuser dome that clips over the front camera of an iPhone (versions are available for the iPhone 4 and iPhone 5). The dome integrates the incident light falling on it and the free Luxi application provides a read-out for the light level in Lux and fixing the f-stop, the exposure time, or the ISO value calculates the other two values. In tests with far more expensive dedicated light meters the Luxi gave almost identical readings.
I tried the Luxi on my iPhone 5S and while it doesn't fit the aftermarket case kit I use, it will fit with just the kit's tempered glass screen in place (I may try to modify the case by removing the top left corner so it doesn't interfere with the Luxi).
The Luxi app is good but could do with better documentation (the app itself doesn't cover calibration which is poorly covered on the company's web site). If you want to use a more polished alternative you can use any light meter app that uses the iPhone's front-facing camera such as the Pocket Light Meter (free) or the gorgeous FotometerPro ($0.99)
The Luxi was launched as a Kickstarter project in March last year and the desirability of such an effective but cheap and simple device was obvious from the fact that they were aiming for $10,000 and wound up raising $120,921.
If you're into photography you need a Luxi; totally brilliant!