Size and weight are big decision factors when you are trying to find the ideal camera for your needs.
In this section, we are going to illustrate the Olympus E-PM2 and Nikon P310 side-by-side from the front, back and top in their relative dimensions. Olympus E-PM2 has external dimensions of 110 x 64 x 34 mm (4.33 x 2.52 x 1.34″) and weighs 269 g (0.59 lb / 9.49 oz) (including batteries). Nikon P310 has external dimensions of 103 x 58 x 32 mm (4.06 x 2.28 x 1.26″) and weighs 194 g (0.43 lb / 6.84 oz) (including batteries).
Below you can see the front-view size comparison of the Olympus E-PM2 and the Nikon P310. Nikon P310 is clearly the smaller of the two cameras. Its body is 7mm narrower, 6mm shorter and 2mm thinner than Olympus E-PM2. As you can tell, actually this is not a fair size comparison since Nikon P310 has a built-in lens but Olympus E-PM2 doesn't. Once you put a lens on E-PM2, its thickness will increase significantly.
Now lets look at the top view comparison of Olympus E-PM2 and Nikon P310.
Weight is another important factor, especially when deciding on a camera that you want to carry with you all day.
Nikon P310 is significantly lighter (75g ) than the Olympus E-PM2 which may become a big advantage especially on long walking trips.
Again, this comparison may not be very useful in real life as we are comparing the weight of a camera with a built-in lens to the body-only weight of an interchangeable lens camera. Once you put a lens on E-PM2, the total kit will weigh much higher.
Weather Sealing
Unfortunately neither the Olympus PEN E-PM2 nor Nikon Coolpix P310 provides any type of weather sealing in their bodies, so you have to give extra care especially when you are shooting outdoors. If weather sealing is a must-have feature for you, check these sections: Top Mirrorless Cameras with Weather Sealing Top Compact Cameras with Weather Sealing
LCD Screen Size and Features
Olympus E-PM2 and Nikon P310's LCD screens have the same diagonal size of 3.00 inches.
Unfortunately, both cameras has fixed screens so they don't tilt or flip in directions.