But what does the feature actually do?
Put shortly, the multi aspect sensor is an oversized sensor. It covers a larger area than the normal Four Thirds sensor, which is 17.3mm x 13.0mm. The larger sensor allows for a constant diagonal field of view at 4:3, 3:2 and 16:9 aspect ratios, with the latter normally being used for video. It also means that parts of the corners of the sensor are never used in any of the modes.
It can be illustrated with this picture. The green outline shows the normal Four Thirds sensor size, while the oversized sensor is the larger outline. Non-oversized sensor cameras, like the GH3, use the orange box for video recording, while the GH1 and GH2 use the larger red box:
Here is a short video demonstration which illustrates the advantage of the multi aspect sensor. First, it shows the Panasonic GH2, while switching between photo mode (4:3) and video mode (16:9). When switching to video mode, the image grows a bit in horizontal size, while it shrinks a bit vertically.
Later, it shows the same with the Panasonic GF3, which does not have a multi aspect sensor. When switching to video, the top and bottom row disappear, but the horizontal size remains the same. This is because the camera lacks the multi aspect feature:
Ok, so that was the technical stuff. Now again, what are the consequences of losing the multi aspect sensor?
Most zoom lenses start at 14mm. This is not too wide, but it corresponds pretty much the standard wide field of view for kit zoom lenses. For example, APS-C cameras tend to have zoom lenses that start at 18mm, which also correspond to 28mm equivalent field of view.
If you are used to video recording with a GH2 at 14mm, you will be disappointed with the GH3 at 14mm, though. Since it no longer corresponds to 14mm in video mode, but 15mm. This is calculated as 14mm times 5710/5287, with 5710 being the multi aspect sensor diagonal at 16:9, and 5287 being the normal Four Thirds 16:9 sensor diagonal measured in pixels. See this illustration:
Most Micro Four Thirds standard zoom lenses start at 14mm, which is common for the basic kit zooms. But it's not so impressive, and when the non-multi-aspect-sensor of the GH3 makes it into 15mm in video mode, it can be limiting. With a non-multi-aspect-sensor, you may see the need for the Lumix X 12-35mm f/2.8 or the Olympus 12-50mm f/3.5-6.3, which start at a the wider 12mm focal length.
On the other hand, if you don't like to use the wide angle end of the zoom, but care more about tele, then the GH3 is going to be better for you, of course, as the tele effect of the lenses increase slightly in video mode, as compared with the GH2. On the GH3 in video mode, the kit zoom lens corresponds to 45mm in the longest setting, rather than 42mm on the GH2.
If you like to take photos in the 4:3 aspect ratio, then there is no difference whatsoever between the GH2 and the GH3. The resolution is exactly the same.
However, the GH1 and GH2 had the option of using the 3:2 and 16:9 aspect ratios for photos as well, while still retaining the same diagonal field of view. If you intend to use the images in this format, then you would use the lens image circle more efficiently with these cameras, and the GH2 will give you better resolution to boot. Here is a comparison table:
|4:3||4608x3456 (16MP)||4608x3456 (16MP)|
|3:2||4752x3168 (15MP)||4608x3072 (14MP)|
|16:9||4976x2800 (14MP)||4608x2592 (11MP)|
These differences are not that important, surely, but there is in fact a significant difference.
On the other hand, with a camera like the GH3, it is best to stick with the 4:3 format when photographing, and crop the image later, if needed. And that saves time and hassle while photographing, which is not the worst thing you could do.