Micro Four Thirds: Evolution of the dSLR?


For decades the single lens reflex camera (SLR) has referred to cameras that, with the proper mirrors and/or prisms would allow the photographer to see through the lens to frame the subject and judge focus.  When people talk about SLR cameras they generally think of a big camera where you look through a viewfinder, see through the lens, and when you take the shot, the mirror flips up to expose the image sensor.  Now the new Micro Four Thirds technology used by Olympus and Panasonic is starting to blur the lines of what we call a dSLR (digital single lens reflex) by removing the mirror(s) and prism that add complexity to the dSLR and replacing them with digital components.  Are these new "solid state" cameras still SLR cameras and what benefits do they offer? 

Single lens reflex

The primary distinction between pocket cameras and dSLR cameras is that the dSLR allows you to physically see through the lens without any conversion of the image to digital format.  That is, the light goes straight through the lens and (after some bouncing around) directly to your eyes, hence the term optical viewfinder.  Most pocket cameras see through the lens too, but they convert that light and display it on an LCD screen on the rear of the camera so in a sense you are not seeing through the lens: you are seeing what the camera sees through the lens.

The Olympus/Panasonic Micro Four Thirds system used in cameras like the Panasonic G1 and upcoming E-series cameras from Olympus work less like a dSLR and more like a compact camera in that there is no mirror and no optical viewfinder where you can see through the lens.  Instead, there is an LCD on the back of the camera and a digital viewfinder that works like an optical viewfinder on a traditional dSLR except that what you see is a digitized image of what the camera sees through the lens.  In a sense, the Micro Four Thirds system is like a pocket camera with a digital viewfinder that you can put your eye close to in order to see the viewfinder better in bright light.  Having all the features of a dSLR plus the digital viewfinder, it's a compact camera that looks and feels like a dSLR to a large extent.  In the end, whether you call it a dSLR or something else is up to you.  You could argue that point either way, but to me, the most important thing is that the Micro Four Thirds system does make cameras that look, operate, and feel like a dSLR with some useful advantages!

Contrast autofocus

Traditional dSLR's use phase detection autofocus.  This focusing method uses a beam splitter and sensors separate from the main image sensor to detect front/back focus and "predict" where the focus ring on the lens should be moved for perfect focus.  While this method is very fast (basically as fast as the attached lens can move focus), it has some drawbacks.  The main drawback to this system is that it doesn't use the main image sensor where your image is being captured.  As a result, the focus is predictive and depends on a perfect match/calibration of the camera body to the lens so that the lens actually mechanically moves to the exact position where the camera commanded.  Unfortunately, in nearly every application, there is some focus error in at least part of the range when focusing a zoom lens.  For example, your camera may focus slightly behind where you were aiming in macros up close but focus perfectly at a distance.  Lens micro adjustments offered in the menus of some later dSLR models do little to help this because they often only offer a single adjustment for the whole range.  This focus error is caused by the lens having a slightly different idea of where to go when the camera commands focus than the camera did in its AF calculation.

Because the Micro Four Thirds format doesn't use a mirror that diverts light away from the main sensor to a separate autofocus system, it is free to use the image sensor itself to adjust focus.  If you want focus accuracy, nothing beats using the main image sensor to detect proper focus since that is the sensor that will ultimately capture the final image.  With the Micro Four Thirds format, contrast detection autofocus is used where the image sensor watches what is coming through the lens and adjusts focus until maximum contrast is achieved (loosely read as maximum sharpness).  Gone are the days of fiddling with lens micro adjustments or sending your lens and body off to the manufacturer to have the lens calibrated to your camera body in order to get proper focus.  I've always been a fan of contrast detection AF but until recently, it has been too slow to compete with the phase detection AF used in traditional dSLR cameras.  Not so with the latest Micro Four Thirds cameras as their contrast detection AF is said to be as fast or faster than most dSLR cameras.

dSLR Video

Likely not by accident, the Micro Four Thirds format enabled cameras like the upcoming Panasonic GH1 to have continuous autofocus while shooting video.  Current dSLR cameras such as the Nikon D90 and Canon 5D Mark II cannot (effectively) autofocus during video shooting because their mirrors are flipped up to expose the image sensor and that disables their autofocus systems.  With the Micro Four Thirds cameras like the GH1, the sensor is free to use its contrast detection autofocus in real time just like a consumer camcorder.  Obviously this has its benefits over manual focus in certain conditions.

Other considerations

It may sound like I'm a proponent of the Micro Four Thirds format for use in "new age dSLR" cameras, but it's early in the game and right now I simply find it intriguing.  The multiple mirrors/prisms in traditional dSLR cameras certainly come at a cost and I've always believed they could be removed and benefits realized from a (mechanically) simpler design.  I do get tired of investing in cameras and lenses for traditional dSLR cameras only to find that at least 80% of them have some front/back focus issues in at least part of the zoom range.  In many cases I'm certain about where I focused yet the full size image shows the true focus just in front of or behind where I focused.  I'm looking forward to trying the Micro Four Thirds cameras to see if they put an end to that problem, yet I wonder how well those cameras will handle other aspects like being able to use the digital viewfinder in dim lighting or for fast moving objects, or whether their somewhat smaller sensors can keep up with the "big boys" when it comes to high ISO shooting.  So while I'm optimistic, I think the jury is out but suffice it to say I'll be keeping an eye on this exciting new design for what most of us will still call dSLR cameras. 

Though the Micro Four Thirds format is not limited to dSLR type cameras (it should work well for rangefinder type cameras too), due to the reduced mechanical complexity, Micro Four Thirds dSLR cameras can be as their name suggests: micro!  The camera bodies and lenses for Micro Four Thirds cameras can be made significantly smaller while using image sensors that are only slightly smaller than APS-C sensors used in most traditional dSLR cameras.  The future looks bright for this format so if you are in the market for a compact dSLR camera, keep an eye on the Micro Four Thirds offerings from Panasonic and Olympus.  There are already plenty of selections in the Four Thirds format: it's the "Micro" part that's new!  As of this writing, the only Micro Four Thirds dSLR available in the U.S. is the Panasonic G1 and that doesn't do HD video: you'll have to wait a month or so for the GH1 if you want the first Micro Four Thirds dSLR that can do HD video.


While I'm hesitant to say that the Olympus/Panasonic Micro Four Thirds format is the next phase in the evolution of the dSLR, the format has some promising advantages that seem to lend the technology to the latest crazes in photography.  Well suited for dSLR video and lending itself to more compact camera/lens sizes, the appeal is certainly there.  Keep an eye on the Micro Four Thirds format cameras and let's see if they can deliver as much in the performance department as they do in functionality.  Only the future and our trusty online reviewers will tell!


-- Mike Chaney