FilterCalc is a free Android app for photographers who use filters on cameras with no TTL metering (e.g. vintage rangefinders, TLRs etc.). “While there are many apps out there that help you compute exposure compensation in terms of aperture / shutter speed, there are none that allow you to calculate the offset in terms of ISO,” says developer Tatyana Skymyrka. “A traditional technique in using filters is to deduct the number of stops/filter factor off actual ISO/ASA you’re using and use the newly ‘computed’ base ISO in your light meter. Readings then result in proper exposure. While computing 1-2 stops is not a big deal, attempting to calculate offset say of 6 3/4 stops in a field can be a challenge. This is precisely what the app addresses.” Compatible with devices running Android 2.3.3 and higher, filterCalc can be downloaded from the Google Play Store.