To illustrate, here are two enthusiast cameras in the categories: Olympus OM-D E-M5 Mark II and Nikon D7200:
One difference between them, which is not easily visible in the picture, is that the register distance is much shorter for the mirrorless camera on the left: 20mm vs 46.5mm for the Nikon F mount on the right. This allows for making smaller cameras, obviously, but it also allow the designers make smaller wide angle lenses.
This is illustrated with the Samyang fisheye lenses pictured: They are functionally the same, but the fisheye lens for the mirrorless camera can be made much smaller due to the smaller register distance. For longer tele lenses, there is not so much difference for the same focal lengths, though.
The Olympus is an enthusiast mirrorless camera. Even if it is two years old, it still has pretty much state of the art image quality for M4/3.
In addition to the compact size and generous direct controls, it also has a lot of technically advanced features, like a sensor shift high resolution mode, and a very effective in-body image stabilization.
On the right, the Nikon D7200 is also an enthusiast level camera. However, the classic single reflex reflex (SLR) design means that the swinging mirror takes up more space, and the camera becomes larger. This is both bad (more camera to lug around), but also good: More space for a good grip and better ergonomic features.
We are expecting the camera to be replaced with a newer model in 2017, but this is still a very good camera. At the price, I think it is a fantastic camera: Very good photo image quality, fast, accurate autofocus, and a good user interface.
From the rear, the camera basically have the same features:
The Olympus on the left has a swiveling LCD, which I think is very useful. On the other hand, the Nikon has room for a top plate LCD display, good for quickly checking the settings.
With the small size, and the lack of a front grip, I think the Olympus E-M5II is not very easy to hold stably:
On the other hand, the Nikon D7200 has a large grip, which lets you hold the camera more stably:
This is not to say that all mirrorless cameras have small grips, of course. The Olympus OM-D E-M1 Mark II is a quite compact camera with a cleverly designed grip that provides a stable handle. I also like the grip on the more nicely priced Lumix G85.
The autofocus performance for mirrorless cameras has improved hugely during the last years. Still, I think the Nikon D7200 outperforms the Olympus E-M5II when it comes to continuous autofocus for moving objects. This is still not handled well by mirrorless cameras which rely on contrast detection autofocus (CDAF).
This makes a real difference when it comes to photographing, e.g., birds in flight or spectator sports with a long lens. In these cases, a camera like the Nikon D7200 is the better to use, and that is the reason why I bought it in the first place.
However, for most other uses, the Olympus E-M5II has an autofocus which is more than adequately fast and accurate, and I think most users will be happy with the performance.
More and more mirrorless cameras now employ on-sensor phase detection autofocus (PDAF), which overcomes some of this difference. An example is the Sony A6500, which is probably the most advanced mirrorless camera in this respect today. In the future, we can probably replace DSLR cameras even for birds in flight and other highly demanding continuous autofocus situations.
This is where a DSLR like the Nikon D7200 excels in almost all areas, except for the size of the camera, and the noise the shutter makes.
The optical viewfinder means that you see the subject truly in real time. And even if there is some time delay for the mirror to swivel up and expose the sensor, this is still very fast.
The Nikon has a larger sensor with a higher resolution. It has a truly amazing image quality for this sensor size, APS-C. Especially in lower light situations, the Nikon will give you better images.
On the other hand, the swiveling LCD display of the Olympus E-M5II can make it better to use when placed on a tripod, when photographing low angle views, like children, or even when holding the camera above your head during a concert.
Both cameras have video modes, all newer cameras do, but this is where the Olympus and other mirrorless cameras excel.
While recording videos, you can use the Olympus E-M5II most like while taking pictures, i.e., you can look into the electronic viewfinder (EVF), or use the LCD screen. The LCD screen is articulated, meaning that you can for example hold the camera at waist level, and look down into the screen to check your composition.
With the Nikon D7200, though, you can only use the rear LCD as a viewfinder while recording videos. And as the LCD screen is fixed, you need to hold the camera out in front of your head to see the screen, making it very hard to record stable videos.
Also, the autofocus during video recording is inferior with the Nikon camera, as it cannot employ the PDAF module while in video mode. Some Sony SLR models use a fixed semi-transparent mirror to overcome this, being able to use the PDAF sensor even while recording video.
Mirrorless cameras are not only smaller, they also contain fewer parts. Consider this image of a DSLR, the Canon EOS D30:
Many of the parts are not needed in a mirrorless camera:
- The mirror, obviously, is not in a mirrorless camera. It is not needed, since a mirrorless camera is basically a liveview camera, taking the image feed directly off the sensor for viewfinder purposes. This also means that no calibration is needed: The autofocus can acquire the correct focus using the sensor, rather than relying on a calibrated PDAF module reading the light reflected off the mirror.
- The pentaprism and viewfinder optics are not needed.
- Specialized light sensors and autofocus sensors are not needed, they are replaced by the imaging sensor itself
- Some mirrorless camera also do away with the mechanical shutter altogether, like the Nikon 1 J and S series.
Even if the mirror mechanism of a DSLR seems complicated, it does not add a lot to the cost of the camera: This mechanism is mature technology, meaning that it can be constructed very cheaply, and even operated with surprisingly few failures.
Mirrorless cameras are slowly eating into the traditional DSLR market: System cameras used by enthusiasts and professionals. And for good reasons: They are compact, have a good image quality and video features, and the lens lineup is getting quite respectable.
However, there are cases when a DSLR can still be the best solution, mostly when you need a very fast and accurate continuous autofocus with moving subjects.