Olympus OM-D E-M5 Digital Camera Review
While Olympus' PEN series of mirrorless cameras has always offered superb color rendition, they've generally struggled to keep noise down at the higher ISOs. The OM-D E-M5 seems to have similar problems, but the higher ISO speeds don't suffer from the same runaway noise problems that plagued earlier Olympus sensors. If you plan to go above ISO 1600, though, we recommend turning on noise reduction at some level. More on how we test noise.
With noise reduction kicked all the way to its maximum, you begin to see a loss of some fine details. As in most JPEG images, these are going to be noticeable first and foremost in areas with repeating patterns, such as hair in a portrait or intersecting branches on a tree. From there, areas like foliage are going to get smudged out, along with any small text that might be in your image. If you print the images at a size of 8x10'' or larger, then this will be noticeable. In small prints or if you downsize an image for the web without cropping, you're unlikely to be able to see most of this detail loss.
The Olympus OM-D E-M5 features an ISO range that extends from 200-25600 when extended. The sensor's native ISO range is 200-1600, which is where the noise performance is acceptable without having to resort to noise reduction. You can set ISO through the main menu, through the control panel, or via the on-screen guide that pops up when you press the OK button.