Panasonic GM1 Tech Insights

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Tech Insights: Making it smaller, and the consequences thereof

Panasonic did a number of things to shrink what's basically a GX7 into the incredibly small frame of the GM1, and some of those changes help explain the performance tradeoffs relative to its larger sibling. Let's take a look at some of these.

Shutter Assembly. One of the biggest changes in the GM1 is its all-new shutter assembly, and the changes there bring not only tradeoffs but at least one benefit as well.

The shot above shows the shutter assemblies from the GX7 and GM1 side by side. As you can see, the difference is pretty dramatic, the GM1's shutter assembly, on the right, occupies 80% less volume(!) than that of the GX7. The space savings comes from eliminating one shutter curtain entirely, and from moving to a stepper-motor actuated shutter, vs the spring-loaded unit that's been de rigeur on Micro Four Thirds cameras from the beginning.

Eliminating one of the shutter curtains means using an "electronic first-curtain" technique, in which the beginning of the exposure is determined electronically, row by row of pixels, down the sensor face. This mimics the opening of the first curtain of a conventional focal-plane shutter. The exposure is ended by moving the second curtain down the frame in a controlled fashion.

The trick here is that "in a controlled fashion" part. In a normal focal-plane shutter, the two curtains move at a relatively constant but not terribly precise speed (accuracy within a few percent is perfectly fine), and the duration of short exposures determined by the width of the slit between the two curtains. It's a pretty much an all-mechanical process, with the energy provided by a spring. Such shutter mechanisms have motors in them, but the motor's sole function is to raise the blades back to the starting position again, and rewind the spring so it'll be ready for the next shot. None of this has to happen with any precision.

With an electronic first curtain, though, the speed of the falling curtain has to be very precisely coordinated with and matched to the row-by-row electronic beginning of the exposure. If the curtain drops even a little bit faster or slower than the row-by-row clocking occurs, the result will be an obviously uneven exposure from the top of the frame to the bottom.

So with an electronic first curtain, you can't simply pull a trigger and let a spring do the work, you have to drive the second curtain electronically in a very precisely-determined manner. This was a key enabling technology in the GM1, namely a stepper motor fast enough and precise enough to match the row-by-row enabling of the electronic first curtain.

This literally amazed me. It's no problem to drive things rapidly with electromagnetic actuators, but I was stunned that a stepper motor could do the job quickly enough. The total time it takes the shutter curtain to travel from the top to the bottom of the frame is right around 10 milliseconds. To produce that motion, the stepper motor has to take a large number (hundreds? thousands?) of steps to advance it incrementally. Even if we assumed a very coarse resolution of only 100 steps from top to bottom, this means that the stepper motor is working at a rate of 10,000 steps/second - and I suspect it's actually taking a good many more steps than that, to avoid any exposure irregularity caused by the curtain jumping ahead many pixels at a time. Like I said, pretty amazing.

The tradeoff here, is that 10 milliseconds top to bottom is as fast as they can move the curtain, and they need to allow a little slack on either site for getting things going and stopping them again. Bottom line, this means that the Panasonic GM1's X-sync speed is only 1/50 second, and the fastest shutter speed with a mechanical second curtain is 1/500. (All-electronic shutter speeds go as high as 1/16,000, though.) That's pretty sluggish by modern standards, and it means you're going to have a hard time knocking down ambient light in your flash shots. Also, while we haven't had the opportunity to test a production sample of the GM1, in the past, cameras that we've tested with electronic shutters can have problems dealing with very bright light sources. I have no idea if this will prove to be the case with the GM1, but I felt it appropriate to at least voice the concern at this point.

I said that there was at least one benefit associated with the new shutter too, and it's not an insignificant one. To the best of our knowledge, all Micro Four Thirds cameras to date have used either the same Copal shutter (like the one from the GX7, shown above), or some close variant of one. We've found that these shutters produce a strong vibrational impulse when the first curtain drops and slams against the bottom of the shutter assembly. Depending on the lens in use, and how susceptible its elements are to moving about in response to vibration, the shutter vibration is often strong enough to produce blur in photos, in some cases, even with the camera locked down on a tripod.

We first observed this in the original Olympus E-P1, with its rather loose kit lens. (It drove us buggy for a full month, you can read the full story of our sleuthing and reverse-engineering analysis of the E-P1's "blur anomaly" here. Interesting reading for the uber-geeks out there.) We've seen the same phenomena to a greater or lesser extent with every Micro Four Thirds camera we've tested to date. In most cases, the manufacturers have tightened the tolerances for their lenses, so the vibration doesn't have as deleterious effect as we saw with the E-P1 and its original kit lens, but it's always there to one degree or another. (Also note that this impulse happens too quickly for a camera or lens' IS system to compensate. In many cases, the action of the IS system attempting to cope with this makes matters worse, not better.)

With the Panasonic GM1 and its electronic first curtain, though, this vibration problem should go away completely. There's no first curtain to slam into the bottom of the shutter assembly, and the very light second curtain being driven via the stepper motor should contribute little or no vibration as well. (Given this, we'll probably swap out the GX1 test body we're currently using for SLRgear lens tests in favor of a GM1. Besides, I'd love to have a little GM1 around. ;-)

Magnesium "Unibody" Casting. The shutter mechanism is a big part of the downsizing story, but another important part is what I'll call the Panasonic GM1's "unibody" frame casting. When weight is important, a lot of cameras resort to a magnesium-alloy frame to provide structural strength. Typically, though, the magnesium is used relatively sparingly, and a lot of aluminum added to fill in the spaces, providing mounting points for lens, shutter, and circuitry. There are really two reasons for this sparing use of mag-alloy. First, magnesium is a lot more expensive than aluminum, so using a lot of it will drive up your materials cost. Secondly, it's generally more cost-effective to either cast or machine the fill-in bits of metal, rather than trying to come up with a die-casting mold complex enough to provide all the needed attachment points on its own. Depending on the design, it may be outright impossible to do it all with a single part.

With the Panasonic GM1, space and weight were overriding concerns, so the engineers decided to push the mag-alloy die-casting to the max, designing a two-piece framework capable of supporting all the rest of the GM1's innards by itself, without any other aluminum bits added in. The shot above shows the two halves of the frame assembled to each other; we unfortunately neglected to grab a shot of the incredibly complex system of bosses, holes, and mounting points that cover the inner surface of the front piece.

We don't have any photos from Panasonic showing the GM1's guts actually assembled to the mag-alloy frame, but everything is basically stacked up and attached to it, circuit board, lens, sensor assembly, heat sink, etc. The LCD screen and rear-panel buttons are supported from the back half of the frame.

I mentioned the heat sink above; it turns out that that's the source of some of the performance tradeoffs in the Panasonic GM1. Both the sensor and image-processing circuitry in digital cameras generate heat, and heat is the enemy of image quality. As a sensor heats up, noise levels increase dramatically, so keeping the sensor cool is pretty important. Then, of course, there's the matter of not letting the circuitry simply cook itself. Beyond a certain internal temperature, circuits will sustain irreversible damage.

The only way to get rid of excess heat is to conduct it away to the outside of the camera's body, and thence to the surrounding atmosphere. With a body as tiny as the GM1's, though, there's not much surface area available to dissipate the heat. Thus, while the sensor and processor in the Panasonic GM1 are the same as in the GX7, some performance parameters had to be dialed back to keep the heat generated to a level that could be safely dissipated by the smaller body.

Heat-related performance limitations show up in two areas on the GM1, namely video and bulb exposure. While the GX7 can shoot 1080 video at a full 60p frame rate, the GM1 is restricted to 1080 60i or 30p. Clocking data off the sensor and processing it rapidly enough to produce 60p video just generates too much heat for the GM1's tiny envelope. Ditto for bulb exposures. Apparently, image data is being read out from the sensor and processed more or less continuously in bulb mode on the GX7, but doing so would again have produced too much heat for the GM1's tiny body.

Circuit Board. Speaking of the circuit board, that was another area where significant changes were made to help squeeze everything smaller. The main circuit board in the Panasonic GM1 has a large cutout in the middle of it, to accommodate the lens assembly. There are basically two lobes, the larger on the right side of the camera, with a thin bridge of circuit board sneaking under the lens to connect the two. (I'd hoped to have a photo to show you, but hadn't received it by press time; if we get it later, we'll drop it in here.)

Flash Unit. The last piece of the miniaturization puzzle was the flash head. Most flash heads use a pantograph-like design, with two frames, each as long as the camera is thick. In most cases, this gets the flash head a decent distance from the lens, to help reduce red-eye, and also to prevent the lens barrel from casting shadows. The Panasonic GM1's body is so thin, though, that a conventional pantograph design would have left the head barely clearing a standard Micro Four Thirds lens barrel. The solution was to attach the flash head to a little spring-loaded piston that sides upward when the flash is popped up, adding a crucial few millimeters (5-10mm?) to the open height of the head. Panasonic also told us that they used the inside of the little piston to hold the flash circuitry and large flash capacity, helping to further reduce size.

The Miniaturization Bottom Line. Panasonic has done an incredible job of packing full-sized camera features into an incredibly tiny body, with surprisingly few tradeoffs made along the way. The only performance differences between the Panasonic GM1 and the GX7 whose innards it shares are: