Longtime Exposure Calculator
Price: Free/ Available for iPhone, iPod touch, iPad
Long-exposure photography fanatics will know that an ND filter can be essential when it comes to extending exposure time for those all-important open shutter shots.
Longtime Exposure Calculator (opens in new tab) by HPR-Solutions is a free iPhone, iPad and iPod touch app that enables you to dial in a projected shutter speed to one column and then 'add' an ND filter as graded in both f/stops and filter names (eg 3 stop or ND8) in the other column. The app then calculates the adjusted exposure.
While it is, in part, possible to TTL meter with an ND filter attached to the front of a camera's lens, the results won't always be accurate, and there's a point where things get so dark that it's not possible - especially with in-vogue super-dense filters. Having an off-camera calculation method such as this makes it quick and easy to figure out exposures without so much as needing to put those brain cells into overdrive while scrawling workings-out on paper.
The Longtime Exposure Calculator app has exposure in-put values that range from 30 seconds to 1/8,000th of a second, which encompasses the range of even a professional DSLR.
The ND filters list is detailed too, covering from ND2 (1 stop) all the way down to ND1,000,000 (20 stops) and even puts in useful references for filter stacking, such as '2x ND8' or 'ND2 + ND3' to enhance that connection with reality.
A variety of filter types that could be dragged and dropped to create customised user stacks would improve the app, although this could complicate things for the more casual user.
On the downside, this app isn't going to be particularly helpful to film users, because it doesn't take into account film types or reciprocity failure (which varies depending on which film you're using).
The app also resets when it's exited (even with iOS 4), which means you'll have to remember the given exposure value. And, seeing as that can range from 1/8,000th of a second all the way up to a year, there could be a lot to remember if you have a lot of filters.
Hey, it's a free app, so it's hard to complain. Longtime Exposure Calculator performs a calculation task, and it does it well.