The iPhone 4S' camera does an admirable job in a variety of lighting conditions, particularly when you remember that the device is a phone first, and a camera second. Still, if you find yourself shooting in low-light situations, you probably have noticed that your photos look blurry or grainy. Cortex Camera is a $2.99 app that can reduce this graininess or noise when shooting in low light. Here's how it works:
Instead of taking a single, still photo, Cortex Camera actually captures a short video when you hit the shutter button. Thus, you will need to hold your iPhone still during the 3 seconds or so when Cortex Camera is taking this video, or use a tripod. After another few seconds of processing, Cortex Camera merges 100 frames of video into a single image, saving it to your iPhone's Camera Roll. You can tap to focus and pinch to zoom.
In my experience, I found that Cortex Camera greatly reduced noise in low-light conditions. And I was able to get crisp shots without the need for a stand or tripod. Photos taken with Cortex Camera are a bit narrower than what you get from the iPhone 4S' camera. For more comparisons, check out this page from Cortex Camera.
Cortex Camera requires no more than launching and hitting the shutter button in the middle of the bottom edge of the display to snap a shot. You can view your Camera Roll without closing the app by tapping the button in the lower-left corner, and you can access the settings by tapping the small Info button in the lower-right corner. There are only four settings to worry about. You can choose to capture images as JPEGs and or PNGs, as 2-megapixel or 8-megapixel images, and you can turn the flash (aka, "torch") on to illuminate your subject. The last is an alignment option, which you can turn off in order to show movement in a shot.