Size and Weight Comparison
Size of a lens is an important deciding factor when comparing two lenses.OM System 12-40mm F2.8 PRO II is the longer of the two lenses at 84mm. The Panasonic 14-42mm F3.5-5.6 OIS with a length of 64mm is 20mm shorter. Besides being longer, the OM System 12-40mm F2.8 PRO II also has a larger diameter of 70mm compared to Panasonic 14-42mm F3.5-5.6 OIS's 61mm diameter.
Weight of a lens is as important as its external dimensions, especially if you are planning to hand hold your camera and lens combination for longer periods. Panasonic 14-42mm F3.5-5.6 OIS weighs 165g, 131% (217g) lighter than the OM System 12-40mm F2.8 PRO II's weight of 382g.
Below you can find the dimension and weight table of the 'Standard Zoom Lenses in Micro Four Thirds Mount for size comparison purposes.
Standard Zoom Lenses in Micro Four Thirds Mount for Size Comparison
Lens Mounts
Both OM System 12-40mm F2.8 PRO II and Panasonic 14-42mm F3.5-5.6 OIS has the same Micro Four Thirds lens mount. Some of the latest released cameras that are compatible with the Micro Four Thirds lenses are Panasonic GH6, OM System OM-1 and Panasonic GH5 II.
Panasonic 14-42mm F3.5-5.6 OIS is also available in Micro Four Thirds mount.
Focal Range
OM System 12-40mm F2.8 PRO II has a focal range of 12-40mm and 3.3X zoom ratio which has a 35mm equivalent focal range of 24-80mm when used on a FourThirds format camera. Panasonic 14-42mm F3.5-5.6 OIS has a focal range of 14-42mm and 3.0X zoom ratio which has a 35mm equivalent focal range of 28-84mm when used on a FourThirds format camera.