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.

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How Micro Four Thirds has managed to make DSLRs look big and bulky

DPNow How Micro Four Thirds has managed to make DSLRs look big and bulky
Here is a summary or excerpt from an article that has just been published on DPNow: Panasonic Lumix... (more)

Bob Ross Re: How Micro Four Thirds has managed to make DSLRs look big and bulky
Hi Ian, Nicely explained and illustrated. Another size feature is that while they have shrunk the c... (more)

Ian Re: How Micro Four Thirds has managed to make DSLRs look big and bulky
A G1 is scheduled to be with me tomorrow (Wednesday). Ian... (more)

DennisP Re: How Micro Four Thirds has managed to make DSLRs look big and bulky
Ian - As one who has expressed previous interest in this camera since it was announced, I am looking... (more)

Ian Re: How Micro Four Thirds has managed to make DSLRs look big and bulky
Hi Dennis, The review camera arrives tomorrow. I don't think the weather is going to be very ... (more)

Bob Ross Re: How Micro Four Thirds has managed to make DSLRs look big and bulky
Sounds good....you get a new camera to test and I get a new president to....:eek: Bob... (more)

Stephen Re: How Micro Four Thirds has managed to make DSLRs look big and bulky
The December issue of Digital Photo magazine has a review of the new camera............................ (more)

Ian Re: How Micro Four Thirds has managed to make DSLRs look big and bulky
Is that, um - a badly written/produced review, or - a poor verdict on the camera? :) Ian... (more)

Stephen Re: How Micro Four Thirds has managed to make DSLRs look big and bulky
It all seemed somewhat subjective. A little about its features and build, a similar amount descr... (more)

Ian Re: How Micro Four Thirds has managed to make DSLRs look big and bulky
Production samples have only been available for a few days so I doubt that this magazine properly ... (more)

Stephen Re: How Micro Four Thirds has managed to make DSLRs look big and bulky
Bearing in mind how much in advance these mags work, it seems unlikely then that they used a produc... (more)

DennisP Re: How Micro Four Thirds has managed to make DSLRs look big and bulky
Hi Ian - I don't know if your review G1 arrived today, but I have just been into town to see if our ... (more)

Ian Re: How Micro Four Thirds has managed to make DSLRs look big and bulky
Dennis, you have beaten me to it! :) No, my review camera has been delayed by one day. I'm hoping ... (more)