One is of course that the sensor is smaller than most other system cameras, with the exception of the Nikon 1 system, which has an even smaller sensor. Another reason is that the register distance is shorter than for DSLR systems.
But one major reason is that the system relies on software correction of the sensor output. This includes correction of Chromatic Aberration (CA) artefacts and vignetting.
Most of the Micro Four Thirds lenses need geometric distortion correction applied for the output images to become rectilinear. This is done totally seamlessly by the camera and software, both for JPEG and RAW images. So the user never notices that the image, as seen by the camera through the lens, is not rectilinear in the first place.
This is in contrast to older DSLR systems. In these systems, there is an optical viewfinder, in which the users sees exactly what the sensor sees, through the lens. With a DSLR system, the lens must be rectilinear, otherwise, the user will be appalled by the geometric distortion when using the camera.
Here is an illustration of two basic kinds of distortion: Pincushion distortion (left) and barrel distortion (right):
In reality, the geometric distortion might very well be more complicated than what is illustrated by these simple models.
In this article, I look at another type of correction employed in two of the newer lenses, geometric distortion correction. By looking at the RAW file in a third party program, I can extract the uncorrected image, and compare it with the out of camera (OOC) JPEGs. For a good reference, I photographed a wall with square tiles:
After processing the RAW image to find the true geometry of the underlying image, I superimposed the two, using red lines for the uncorrected geometry, see below.
I included the appropriate adjustment needed. The adjustment numbers in percent refer to the "Lens Distortion" filter in The Gimp, an image processing software. Of course, to become rectilinear, some lenses might require more complicated adjustment than the simple model given by the "Lens Distortion" filter. So these figures are just intended to be approximate relative indicators of the degree of distortion. A positive figure indicated barrel distortion, while a negative figure indicates pincushion distortion.
This lens features some barrel distortion in the wide end, and no distortion correction in the long end. However, even after the in-camera distortion correction, there is some residual barrel distortion in the wide end. This is the most pronounced at close focus distance, but can also be seen with infinity focus, in my experience. It is not uncommon that the geometric properties change slightly with focus for internal focus lens designs.
See the review of the lens for a real life example illustrating the barrel distortion in the wide end.
Also in the long end, the out of camera JPEG images show some distortion. They tend to have a bit of pincushion distortion.
Lumix X 12-35mm f/2.8 @ 12mm: -14%
Lumix X 12-35mm f/2.8 @ 35mm: 0%
It is perhaps unexpected to see that this lens has less barrel distortion correction in the wide end than the basic version of the lens, the Lumix G 14-42mm f/3.5-5.6 kit zoom lens. Given the smaller size of the power zoom version, one would expect that more optical compromises have been made.
Just like with the Lumix X 12-35mm f/2.8 lens, there is some residual barrel distortion even in the corrected image at 14mm. There is also some residual pincushion distortion in the long end.
Lumix X PZ 14-42mm f/3.5-5.6 @ 14mm: -15%
Lumix X PZ 14-42mm f/3.5-5.6 @ 42mm: +5%
I'm a bit disappointed to see that there is noticeable barrel and pincushion distortion in the images produced by the premium Lumix X 12-35mm f/2.8 zoom lens. For such an expensive lens, one would have expected more perfect images.
On the other hand, this is not really any real problem. If you need to have absolutely rectilinear images, then you can apply a bit of lens distortion post processing. Also, I guess this comes down to a compromise between having a traditional focus mechanism, which is slower, but more resistant to distortion at closer focus distances, and internal focus, which is much faster. The Lumix G 20mm f/1.7 pancake lens has the traditional focus mechanism, and quite some people dislike it for the slow and noisy autofocus.
Summary of other lenses
And here is a summary of the adjustments to all the lenses I have tested. The percentage in the table refers to the Gimp image processing Lens Distortion filter value needed to make a rectilinear image: 0% means no correction, a negative value means barrel distortion, and a positive value means pincushion distortion.
|Lens||Focal length||Relative distortion correction|
|Leica DG Summilux 25mm f/1.4||25mm||-8%|
|Lumix G 20mm f/1.7 Pancake||20mm||-11%|
|Lumix G 14mm f/2.5 Pancake||14mm||-16%|
|Lumix G 14-42mm f/3.5-5.6||14mm||-18%|
|Lumix G 14-42mm f/3.5-5.6||30mm||0%|
|Lumix X PZ 14-42mm f/3.5-5.6||14mm||-15%|
|Lumix X PZ 14-42mm f/3.5-5.6||42mm||+5%|
|Lumix X 12-35mm f/2.8||12mm||-14%|
|Lumix X 12-35mm f/2.8||35mm||0%|
|Lumix G 7-14mm f/4||7mm||-17%|
|Lumix G HD 14-140mm f/3.5-5.6 II||14mm||-16%|
|Lumix G HD 14-140mm f/3.5-5.6 II||50mm||0%|
|Lumix G HD 14-140mm f/4-5.8||14mm||-17%|
|Lumix G HD 14-140mm f/4-5.8||30mm||-4%|
|Lumix G HD 14-140mm f/4-5.8||50mm||-1%|
|Lumix G 45-200mm f/4-5.6||45mm||+1%|
|Lumix X PZ 45-175mm f/4-5.6||45mm||0%|
|Lumix X PZ 45-175mm f/4-5.6||100mm||+5%|
|Lumix G 100-300mm f/4-5.6||100mm||0%|
|Olympus M.ZD 45mm f/1.8||45mm||0%|
|Panasonic Leica Lumix DG Macro-Elmarit 45mm f/2.8 1:1 Macro||45mm||0%|
|Lumix 8mm f/3.5 fisheye||8mm||0%|
|Sigma 30mm f/2.8||8mm||0%|