You ask yourself - what is a four thirds sensor?
The four-thirds sensor is significantly larger than most of the sensors in point and shoot cameras and thus produce higher image quality. Photos from this kind of sensor can be enlarged easily to 8 x 10 inch prints or even 11 x 14 inch prints.
These sensors also are slightly more square than some of the other sensors. This is not terribly important, but you should be aware of it simply because it does affect print shapes. If you want to produce a photograph that is much wider than it is tall, you will be cutting out (cropping) a portion of the image from the sensor. If you intend to enlarge this oblong photo, be aware that print size could be restricted somewhat.
Overall though, these sensors produce very good photographs.
What else is important about mirrorless cameras?
1) They tend to be smaller and lighter than many cameras that produce high-quality images.
2) They are quiet as the shutter is almost silent.
3) These cameras are designed so that the lens can be removed quickly and replaced with another lens.
4) These cameras tend to be better-built; they are more sturdy, have higher quality guts, and long-lasting shutters. You still do not want to drop one, but these cameras will take harder, longer use.
The four advantages above make mirrorless cameras attractive because they can be very portable, very flexible (can make a wider range of types of photographs) and will stand up to hard use and last. Some have viewfinders and some do not. The viewfinder will always be an electronic image that is very much like the image being captured by the sensor. The electronic image seen is usually not quite as clear as would be seen by the eye alone. However, these viewfinder images are very useful. Sometimes the LCD screen can be difficult to see in bright sunlight and the viewfinder helps compose the photograph. Mirrorless cameras can be more expensive, partly because additional lenses can be purchased for use with the camera body.
These cameras are very good for long-distance hikers or anyone who wish to carry their camera equipment but do not want to carry a heavy bag or backpack.
Next lets take a look at APS-C sensor cameras.
Cameras that use the APS-C sensor tend to be Single Lens Reflex (SLR) cameras which use a mirror to send the view of the image being made to a viewfinder. You see exactly what is being photographed. While these cameras have large LCD screens as well, the viewfinder gives an excellent view of the photograph to be made. When the shutter is released, the mirror moves out of the way allowing the light to reach the sensor. This happens very quickly and you won't really notice. However, the sound of the camera taking a photograph can be louder than expected.
These cameras are usually larger than mirrorless cameras. They also have removable lenses and offer choices of other lenses that can be mounted onto the camera body. These camera make excellent images that can be enlarged.
They are the choice of advanced photographers and semi-professional photographers (even some pros use this type camera). They can range from reasonable to very expensive in cost.
The APS-C sensor is larger still than four-thirds sensors and are slightly more oblong. This makes for a camera that is very flexible for lots of different types of photography. These cameras tend to have lots of features that can be missing on other camera types. Think of these cameras as camera systems.
By that I mean users will buy the camera, several lenses, an electronic flash unit, tripod, filters, and remote shutter device. Thus a whole system of camera equipment, used together to create photographs.
Lastly, there are full-frame cameras. These cameras (almost always SLR cameras) are large and heavy and have a wide range of features. The full-frame sensor is larger than APS-C sensor. In fact, the sensor is the same size as older 35 mm film camera film frames are. In your mind, to visualize the size of the sensor - think of the 35 mm slides your Father took. Those are the same size as the full-frame sensor. There are even a few digital cameras that have a larger sensor (similar to the 2-1/4 inch film cameras).
Full-frame digital cameras are expensive (starting around $2,000.00). Their lenses are bigger and also very expensive. Usually only professional photographers can justify the expense of these camera systems.
In part three of this series, we will evaluate all this information and help by providing guidelines to use in choosing a camera.