"You have a 100mm f2.8 lens on a 4/3rds? So what, it's only = to a 200mm f5.6 on a FF!" We hear stuff like this a lot. It's only partially true. This happens every time a new prime is introduced for m4/3rds. The equivalency thing only applies to angle of view, nothing else. The lens on the 35mm sensor isn't "faster" but the sensor may be more sensitive. A 12mm lens on a 4/3rds sensor gives the same angle of view (imaged area) as a 24mm lens on a 35mm (notice I didn't say full frame, because the 35mm format is a hoary old format from early film days?) format camera. The physical sizes of the lenses will be different, but because of the way wide angle lenses are designed (some have huge front elements, etc), it's best to use example lens focal ratios and focal lengths on longer lenses, to get an idea of how each aspect of gear impacts physical size. We take a 100mm f2.8 lens on a 4/3rds and a 200mm f2.8 lens on a 35mm format, you get the same angle of view if the cameras are at the same distance from the subject. The lens speed is the same and you'll see the same image density (more or less, at a set ISO) from both cameras if the aperture and shutter speed are the same. What will be different physically will be lens size. Roughly speaking, lenses of double the focal length and the same speed as another lens will be twice as large, the diameter of the optical elements or the "clear aperture" must be twice as wide because focal ratio is focal length divided by optical element diameter at its widest or "clear aperture." So, a 100mm lens of f2.8 is going to need a clear aperture of at least 36mm while a 200mm lens of f2.8 focal ratio will need a lens aperture of 2x that or 72mm. The qualifier here is that sometimes manufacturers use a larger front element than needed to avoid certain image aberrations and physical flaws the lens might have. Thus, a lens that provides the same area coverage and speed on 35mm as a lens that does on 4/3rds is going to be roughly twice as large and probably 2-3x as heavy because the lens elements are (by area) much larger. This will vary given glass types used, element number, housing, etc, but again, wide angle lenses have vastly different designs so may not adhere completely to this rule. Does the lens with the larger clear aperture gather more light? Yes, light collection is solely determined by lens surface area, but since you are dividing that light over an equivalently larger surface area (the light cone encompassing the 35mm sensor) it provides no additional light at the sensor IF the pixel density is the same as the 4/3rds sensor, and if the aperture and shutter speed is identical. But, IF the pixel density (the pixels are larger) is not as high in the 35mm sensor as the 4/3rds, then each pixel IS getting more light in the 35mm sensor. Right now, a 4/3rds sensor with 16 megapixels has a much higher pixel density (pixels are smaller) than a 35mm sensor with 24 megapixels, so each pixel in the 35mm camera gets more photons. In fact, there are no 35mm sensored cameras with pixel densities as high as a 16 megapixel 4/3rds sensor at this point. Maybe Canon will release a 48mp 35mm camera? The final image produced by either of those two sensors is dependent on a lot of other things, once the light hits the sensor. Arguably, the pixels in the less dense 35mm sensor are more "sensitive" (because they collect more photons by virtue of their larger size) which results in the better high ISO performance they show. The larger the pixels, the greater their "photon well capacity." But the end result depends on so many other things in the camera that the advantage isn't as dramatic as it might seem, except in the case of something like the Nikon D3s/4 which have huge pixels and very low pixel density and that sensor has fantastic high ISO performance.