Shrunken Micro Four Thirds optics explained
In the theoretical world of optics, the focal length of a lens is the distance from the optical centre of the lens to the focal plane. Think of two cones, one projecting out from the rear of lens onto the focal plane and one projecting out towards the subject in front of the lens. Where the two cones meet is the optical centre of the lens. With a simple lens or pin hole camera, the cones would have the same angle of projection on either side of the optical centre.
For example, in traditional terms, with 35mm film cameras, a 50mm ‘standard' lens provides a 47 degree angle of view. This is the angle of view calculated from those projected cones I mentioned earlier. It can be visualised in the diagram below:
Above is the 'pinhole camera' lens idealised view of how a lens works.
Here 'A' is the distance from the optical centre to the focal plane, so A is the focal length of the lens.
Here, the angle of view of the lens has not changed, but the distance of the centre point of the lens
from the focal plane on the right has increased, as is the case in many cameras.
The 'effective' focal length has not changed because the
angle of view 'B' has not changed.
However, 'A' is now longer than the effective focal length.
Optics in the lens design compensate
for the change in lens to focal plane
the greater that distance change the
more powerful the optics need to be and the bigger
and bulkier the lens becomes.
In reality, as the light is refracted by several lens elements that form a composite lens, for lenses of short focal length, which means wide angle lenses, the actual distance between the focal plane and the optical centre of the lens can be several times the theoretical focal length. For example, the widest wide angle lens in the Four Thirds DSLR lens range is the Digital Zuiko 7-14mm f/4 zoom. At 7mm, its angle of view is about 116 degrees, but in reality the optical centre of the lens is maybe ten times longer than its effective focal length.
To project 116 degrees of view from that distance onto the sensor area means a lot of optical power at work in the lens design. And this is why there is so much glass, punctuated by a massive dome of a front element on the 7-14 lens. Olympus' optical design is of the highest quality and the 7-14 Digital Zuiko is a superb lens, but only because it's as large and heavy as it is.
I've seen a mock up of the forthcoming 7-14 f/4 mFT lens that will be coming from Panasonic Lumix. It will provide exactly the same optical view and brightness as the original 7-14 lens, but it's dramatically smaller. It will probably be at least half has heavy, nearly half as slim, and shorter too. Think of one of those mini soft drink cans you often get on planes compared to a normal size pop can and you kind of get the idea, even if that is perhaps an exaggeration of the relative difference in size.
So there you are; you can have a camera with a much larger sensor than a compact camera, with much of the usability of a DSLR, but with much smaller bodies and lenses. Micro Four Thirds is the first of a new breed of non-reflex interchangeable lens system cameras, and it's expected that other famous name manufacturers will eventually launch similar systems of their own.