The most striking aspect of the 25mm is inevitably its diminutive size; this is one of the smallest lenses currently available for any mount. Despite this it still feels pretty well-made, and with its metal mount and use of quality plastics for the lens barrel it's certainly a notch above certain manufacturers low-end, plastic mount 50mm primes. So whilst it's certainly ultra-lightweight, it doesn't feel at all insubstantial.
'Focus-by-wire' manual focus
The most unusual feature of this lens's operation is the focus-by-wire manual focus system, which drives the focusing group indirectly via the lens's autofocus motor (as opposed to the direct mechanical connection found in most lenses). As a consequence, the feel of the manual focus ring never changes, regardless of whether the camera is set to auto or manual focus, or the focus has reached the limits of its travel (either close or infinity), and this lack of tactile feedback can be a little disconcerting in some situations.
However the advantage of this system is that it does allow an extremely long focus travel, and a correspondingly high level of focus accuracy, in such a small lens. Now this isn't really going to help manual focus using the viewfinder of any Four Thirds DSLR bar the E-3 (probably the last camera this lens will be used on), but it does allow very accurate manual focusing in live view mode.
On the camera
The lens is a perfect match for Olympus's ultra-compact E-4x0 SLR bodies, on which it gives the most pocketable SLR/lens combination currently available; it's also equally at home on slightly larger cameras such as the E-5x0 series. The only external control is the manual focus ring, which falls naturally to hand when required.
Autofocus is driven by a micro motor in the lens body, which works just fine. It's pretty quiet in operation, although not quite as refined as ultrasonic-type motors. Focus speed and accuracy is dependent upon a number of variables, including the camera body used, subject contrast, and light levels; we found focusing to be generally fast and accurate under most conditions, and certainly adequate for the likely uses of the lens.
Lens body elements
Reported aperture vs focal length
The lens allows apertures from F2.8 to F22 to be selected.
Olympus ZUIKO DIGITAL 25mm 1:2.8 Pancake Lens
Olympus M.ZUIKO DIGITAL ES-M 17mm 1.2.8 Silver Pancake Lens