StopKit

StopKit is a small iOS and Mac framework for working with units of light in photography. StopKit is fully compatible with both Objective-C and Swift.

Here at Cascable, we use StopKit extensively — In our camera SDK CascableCore, camera exposure properties such as shutter speed, aperture, exposure compensation, ISO, etc are all StopKit values, and our consumer app Cascable uses StopKit to power its Shutter Robot automation tool.

Examples

Comparing Values

When comparing values, StopKit provides compare(_:) and stopsDifference(from:). Since you can't compare indeterminate values with determinate values, these methods can fail, returning ExposureComparisonInvalid or nil respectively. You can use the isDeterminate property to exclude such values from calculations.

In general, a postive stops value means "more light" or "a brighter image", and a negative value means "less light" or "a darker image". However, compare(_:) will return a result that sorts the values into what the user would typically consider "correct" (for example, aperture values have the smallest number/largest stops value at the beginning of the list).

print(ApertureValue.f8.stopsDifference(from: ApertureValue.f4))
// -2 stops (since f8 lets less light through than f4)

print(ApertureValue.f8.compare(ApertureValue.f4).rawValue)
// -1  (ComparisonResult.orderedAscending)

print(ApertureValue.f8.stopsDifference(from: ShutterSpeedValue.oneSecondShutterSpeed))
// nil, since this isn't a valid comparison

print(ApertureValue.f8.compare(ShutterSpeedValue.oneSecondShutterSpeed).rawValue)
// -100 (ExposureComparisonInvalid), since this isn't a valid comparison

Transforming Values

When transforming values, use the -adding(_:) method. Since you can't transform indeterminate values such as bulb shutter speeds or automatic values, this method can return nil. You can use the isDeterminate property to exclude such values from calculations.

In general, a postive stops value means "more light" or "a brighter image", and a negative value means "less light" or "a darker image".

let stops = ExposureStops(fromDecimalValue: 1.0)

let f8 = ApertureValue.f8
print(f8.adding(stops)) // f/5.6

let one250th = ShutterSpeedValue.oneTwoHundredFiftiethShutterSpeed
print(one250th.adding(stops)) // 1/125

let iso100 = ISOValue.iso100
print(iso100.adding(stops)) // ISO 200

let zeroEV = ExposureCompensationValue.zeroEV
print(zeroEV.adding(stops)) // 1 EV

License

StopKit is licensed under the MIT license. For full details, see the LICENSE file.