A shorter register distance means that wide angle lenses can be constructed simpler. With a long register distance, typically for a DSLR camera, you need a complicated retrofocal optical design to make wide angle lenses. With a shorter distance, the lens design becomes simpler, and you can make smaller, lighter, and less expensive wide angle lenses.
Here is a good example. On the left, there is the Yasuhara Madoka 180 7.3mm f/4 (my review), designed for the Sony E-mount mirrorless system. On the right, is the Lensbaby 5.8mm f/3.5 with an adapter for Sony E-mount. Both are circular fisheye lenses for the APS-C sensor size. But the lens on the right is designed for use on DSLR cameras, with a longer register distance.
Beyond being designed for mirrorless and DSLR, respectively, the lenses are a bit different. The Lensbaby covers a slightly wider field of view, and has a marginally larger maximum aperture. The Yasuhara lens is more distorted in the edges, and has the better optical performance. But by and large, the lenses do the same thing. And there is a huge difference in the size. Which lens would you rather bring along?
And that is the power of mirrorless systems: To allow for small, well performing lenses. Here is a selection of very small lenses that are available for mirrorless cameras. They are all quite good, considering the price, size and weight:
From left to right (click to see my reviews): Yasuhara Madoka 180 7.3mm f/4 circular fisheye lens (Sony E-mount), Samyang 7.5mm f/3.5 full frame fisheye (Micro Four Thirds), Nikkor 10mm f/2.8 wide angle (Nikon 1), Lumix G 14mm f/2.5 wide angle (Micro Four Thirds), Nikkor 18.5mm f/1.8 fast normal lens (Nikon 1), Lumix G 20mm f/1.7 fast, wide normal lens (Micro Four Thirds).
To illustrate the field of view of the various lenses, here is the same scene rendered by them (click for larger images):
|Lens||Length||Diameter||Weight||Lens elements/groups||System crop factor||Equivalent focal length|
|Yasuhara Madoka 180 7.3mm f/4 circular fisheye lens||43mm||61mm||200g||7/6||1.5||11mm|
|Samyang 7.5mm f/3.5 full frame fisheye lens||48mm||60mm||197g||9/7||2||15mm|
|Nikkor 10mm f/2.8 wide angle lens||22mm||56mm||77g||6/5||2.7||27mm|
|Lumix 14mm f/2.5 wide angle lens||21mm||56mm||55g||6/5||2||28mm|
|Nikkor 18.5mm f/1.8 fast normal lens||36mm||56mm||70g||8/6||2.7||50mm|
|Lumix 20mm f/1.7 fast wide normal lens||26mm||63mm||87g||7/5||2||40mm|
As far as I know, this is the first circular fisheye lens made specifically for mirrorless cameras. Unfortunately, it is only available for Sony E-mount.
They claim to be in the process of releasing it for Micro Four Thirds as well, however, I don't think that will happen, as the lens has an image circle slightly larger than the Four Thirds imaging sensor. And who wants a partially circular fisheye lens?
The lens is well made, and the image quality is good. On the negative side, the image is quite compressed along the edge, more than with other fisheye lenses.
This is one of the awesome deals within Micro Four Thirds: A compact fisheye lens at a good price, with very good image quality. Good fisheye lenses used to be very large and very expensive, but with mirrorless cameras, it is now available for the consumer market.
There is a competing lens from Panasonic, the Lumix G 8mm f/3.5. While it does feature autofocus, it is a lot more expensive, and not really any better optically. Autofocus can be useful on a fisheye lens if you are taking close up pictures of moving objects, for example, cute pictures of pets. But otherwise, one would normally just leave the focus close to infinity.
When Nikon introduced the Nikon 1 system in 2011, this was their "prestige" lens. Unlike most other Nikon 1 lenses, it has a metal bayonet mount. Most other Nikon 1 lenses have a chromed plastic bayonet mount.
It has a very good optical performance, and is compact and well made.
This lens has a somewhat poor reputation, probably due to being sold cheaply from split Lumix GF2 and GF3 kits. But in fact, it is quite good, and it is still the smallest Micro Four Thirds lens (disregarding the basic camera body cap toy lenses).
It is shown here with a 46mm-37mm step down ring as a basic hood.
Unlike the Micro Four Thirds format, Nikon was sure to launch a cheap, fast normal lens early in the Nikon 1 system history. The lens is affordable, light, small, and the optical qualities are good.
Many enthusiasts expect to be able to pick up a relatively inexpensive 50mm equivalent lens when they buy into a system. Micro Four Thirds can be a disappointment in that respect, as there is only the quite expensive Lumix/Leica 25mm f/1.4 (my review). Olympus also offer a normal lens, the M.Zuiko 25mm f/1.8, but it, too, is rather expensive.
The very first Panasonic Micro Four Thirds prime lens, it was an instant classic when it was launched in 2009. It is known for the very good sharpness already wide open.
On the negative side, the autofocus is somewhat slow and noisy. But with recent Micro Four Thirds cameras, the focus performance is rarely a problem.
With the shorter register distance (flange focal length), comes the possibility to design more compact lenses, especially wide angle lenses.
Also, some mirrorless lenses are designed to require in camera geometric distortion correction, which further allows them to become smaller, while retaining the good image quality.
All this gives us a range of very compact, light, and non-obtrusive lenses, surely good tools for any enthusiast photographer.