Tadaa SLR is a free app that lets you create depth-of-field effects after you take your photo.
Open an image on your iPhone, then mask off the area where you want to preserve your focus. This opens the door for bokeh effects, which traditionally are often done with special lenses.
Using the app is easy. Open a photo and use your finger to define a mask. Thanks to edge detection, you don't have to be exact. Anything you mask will remain in focus, while anything outside the mask will blur. The next step gives you the ability to adjust the out-of-focus parts, crop the image and adjust things such as saturation and contrast. Filters can adjust the colors and render attractive black-and-white images.
Tadaa SLR adds a few tricks you might not get from other apps in your collection. For free, it's worth downloading and learning, because if used well, it can enhance anything from landscapes to tabletop photography to portraits. The app also gives you access to your camera if you want to use the app while shooting "live."
Tadaa SLR does a pretty good job of mimicking depth-of-field adjustments in an SLR camera, so give it a try and see if it works for you. Like any effect, selective focus and blurring can be overdone.
Tadaa SLR is not a universal app, so it's best suited to the iPhone or iPod touch. It requires iOS 6 or greater. Tadaa also produces a nice pseudo-3D app I reviewed earlier this year.