Everyone's favorite photo filtering and sharing app for iOS got a significant update on Friday afternoon. Version 2.1 of Instagram adds a new filter, a tool for easily enhancing low-lit photos and a redesigned navigation.

Sierra, the latest filter to join the Instagram family, is a white-bordered filter that adds a lightened, low-contrast vintage look to photos. As far as Instagram filters go, it's pretty standard stuff, but it's always nice to have new options. The more substantial addition to the app is a feature called Lux, which lets users automatically increase the brightness of photos and boost the contrast. The option is meant to offer a way to improve underexposed photos and make them more Instagrammable.

The visual overhaul of the navigation UI comes five months after the app's camera was redesigned in version 2.0. This iteration appears to complete a larger redesign process was undertaken last year. The new version uses new icons and UI elements that feel like iOS-centric, which suggests an Android version may be up next.

Instagram CEO Kevin Systrom confirmed last year that building an Android version of Instagram is "a major priority" for the company, and the company is known to be working on such an app. It's really not a matter of if, but when. Last week, rumors began swirling that Instagram for Android could be imminent. We reached out to Systrom, who declined to give any specifics about a timeline.

For Instagram, Android is the most logical next step for growth. Having stirred early buzz in the tech press and later named Apple's iPhone app of the year for 2011, the service has done quite well, especially considering it only exists on iOS. It now boasts over 15 million users on Apple's mobile operating system alone.

Launching an Android app will expose it to a massive number of potential new users. Android commands more than 46% of the smartphone market, according to Nielsen. If its success on iOS is any indication, the service can expect to see its user base flourish once the Android version drops.

The other top priority at the company's headquarters is building out a Web version of the service. This one is a little less urgent, because they are so many third party Web UIs for Instagram, and probably not as much demand for an official one as there seems to be for an Android app.