There might be myriad photo-editing apps to choose from, but new universal app Luminance is worth a look, simply for its low price of 99 cents. Luminance functions equally well on both iPad (though I’d imagine iPad 2’s built-in camera makes its use more streamlined) and iPhone.
iPad’s large screen is suited to photo editing, and Luminance uses its available space well, for the most part. You'll have to import each image into the app, which will take users without a native camera some extra time. There's no support for a batch import from the camera roll, so even if you’re on a camera-equipped device, you’ll need to cherry-pick the images you want. Once the images are loaded in Luminance, users can opt to apply one of 19 effects, including pop, black & white, twilight and weathered.
After you’ve applied the effect, you can open the adjustment panel that will display which filters were used for the effect, so you can adjust them further or remove entirely. These options include white balance, exposure, hue and saturation, tone curve, vignette and split-toning, among others. The app also lets you add these filters on top of each other. Each effect is stored in its own layer, so it’s one of the more user-friendly photo editors I’ve seen. Adjustments are made immediately, so you can see what they look like, but I experienced some lag time upon manipulating each effect. Effect combinations can be copied and applied to another image, which is ideal for favorite looks. Photos can be exported at full resolution, too.
On iPad, Luminance only works in landscape mode, and on iPhone only in portrait. Luminance is missing basic editing features, such as rotating and cropping (I don’t understand why so many apps omit these), but developers say the latter feature is on its way.