The Micro Four Thirds Magic Size Trick

The first Micro Four Thirds camera, the Panasonic Lumix DMC-G1, goes on sale at the end of this month. The G1 looks like a DSLR in miniature, even compared to today's smallest DSLRs. You can change lenses with the G1, but it's not actually a DSLR as there is no reflex mirror and prism for through the lens (TTL) viewing. Instead, there is a state of the art 1.4 megapixel electronic viewfinder. But this is no fancy bridge camera. The G1 has a DSLR-sized sensor, so just how can the G1 and, especially, its lenses be so small?

With a single lens reflex camera, the lens must be situated well forward of the film or, nowadays, sensor plane, which is also known as the focal plane. This is to accommodate the swinging reflex mirror and, behind that, the focal plane shutter, which occupy space in-between the back of the lens and the front of the focal plane.

Why is this a disadvantage? It makes the design of wide angle lenses, in particular, very complex and the result is large and heavy optics. You may have noticed that rangefinder and similar cameras, for example, have much smaller lenses. This is because the back of the lens can be positioned much closer to the focal plane – because there is no mirror in the way.

The Micro Four Thirds difference

The Panasonic Lumix DMC-G1 (left) has the same size sensor and a lens with virtually the
same zoom range as its DSLR sibling on the right, the DMC-L10, but as you can see the size
difference is remarkable. And the L10 is not a large DSLR by any means.

The new Micro Four Thirds system platform from Panasonic and Olympus has evolved from the Four Thirds DSLR system platform. Micro Four Thirds shares the same DSLR-sized sensor as the original Four Thirds DSLRs, but Micro Four Thirds cameras and lenses can be made much smaller and lighter, enabling the cameras to be shrunk too.

The key difference between Micro Four Thirds (mFT) and Four Thirds is that the distance from the lens mount flange to the focal plane has been roughly halved from 4cm to 2cm by eliminating the need for a mirror box. In fact, the rear of the lens can extend past the flange up to a certain point inside the camera body, bringing the lens and focal plane even closer. Naturally, this makes the depth of the camera body significantly less.

By shortening the lens to focal plane distance like this, lens optics at short and medium focal lengths need to apply much less refraction power to the incoming light. As the optics need to be less powerful, the optical design can be simpler and much more compact. This enables lenses to be shorter, slimmer and lighter, without sacrificing brightness, nor requiring the sensor to be reduced in size.

Smaller lenses but same size sensor

Micro Four Thirds uses the same 18x13.5mm sensor size sensor of Four Thirds DSLRs, making it relatively huge compared to compact and bridge camera sensors, even though it's one of the smallest sensor sizes used in DSLRs. It's about five times the area of a the sensor used in a Canon Powershot G9 or Panasonic DMC-LX3 compact camera. As the mFT sensor size is the same as that used in Four Thirds DSLRs, mFT cameras can be used with original Four Thirds lenses through the use of a spacer tube.  However, mFT lenses are not compatible with Four Thirds DSLRs and a different lens mount bayonet has been implemented to prevent the fitting of mFT lenses on original FT bodies.

Spaced out backwards compatibility

The spacer maintains full electronic communication between a mFT body and a Four Thirds lens, so lens identification, aperture setting and triggering, fly by wire manual focus, and autofocus signalling are retained.

AF compatibility limitations

There are some AF limitations to consider, however. Without a reflex viewfinder system, mFT cameras can't autofocus using phase detection. Instead, contrast detection, using the main camera sensor, has to be used. The first mFT camera to be launched, the Panasonic Lumix DMC-G1, can't autofocus Four Thirds lenses that have not been optimised for live view contrast detect autofocus. Olympus has managed to provide workaround mode that enables all Four Thirds AF lenses to autofocus* in live view mode and it will be interesting to see if Olympus implements this solution again in its own mFT models.

There are some other technical factors in the compatibilIf you are interested in the specifics of mFT compatibility with older FT lenses, an article on one of our other sites, Four Thirds User, could be useful.

Now see page 2 for a slightly more technical explanation as to why Micro Four Thirds lenses can be made so small.

Reader feedback:

Discuss this story:

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 distance and 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.