Photography is governed by the laws of physics—and a ton of maths. For quick snapshots, relying on your camera’s auto-features is fine: modern cameras hide most of the calculations behind autoexposure and autofocus modes. For carefully taking a shot, however, manually setting you camera gives the most creative control.
The relationship between shutter-speed, aperture and ISO is not difficult to calculate, but some situations are made much easier with a little help. It can be especially awkward to do calculations in your head for long exposure photography. For macro, landscape and wildlife photographers calculating the depth of focus can be tricky. Using a calculator guarantees accuracy.
PhotoBuddy for iOS and Photo Tools Pro for Android provide both exposure and depth of field calculators in a single app. Other apps like Long Exposure Calculators for iOS and Exposure Calculator for Android are limited just to exposure calculations but allow you to take neutral density filters into account—perfect for long exposure photographers.