Several years ago, the idea of downsizing your primary camera system from a full frame DSLR to a crop or even (gasp) to a micro four-thirds mirrorless system would be considered ridiculous. Smaller sensors were suited for entry-level, low image quality cameras, according to many enthusiasts, and only a large, heavy, full frame DSLR could be considered for serious photography.
The introduction of technologically advanced mirrorless cameras and high quality lenses has created an environment where a photographer can produce exceptional images from small sensors as well as full frame sensors, or can utilize a combination of both as conditions require.
Advantages to full frame sensors over smaller sizes still remain, although the differences are much less significant than in the past. These include enhanced low light performance and greater dynamic range. Larger sensors produce larger image files, which have more digital information. There are many more specialized lenses, such as very long, fast sports and wildlife telephotos, available for full frame cameras. Achieving extreme bokeh is easier with larger sensors.
However, the advantages that most modern mirrorless camera systems engineered for crop and micro four-thirds sensors have are compelling.
: Without the necessity of a mirror and reflex housing, the cameras are smaller and lighter, sometimes dramatically so. The lenses and accessories designed for these smaller systems are also smaller and lighter.
Lenses are engineered to produce an image circle, which corresponds to the sensor it is projected on; an f/2.8 lens designed for a micro four thirds system will have a much smaller inner diameter and overall size than one which must cover a full frame sensor. [Note that this applies to mirrorless full frame cameras as well as to full frame DSLRâ€™s; the lenses of necessity will be similar in size]. It becomes entirely feasible to transport two bodies and five or six lenses in a modest backpack.
: Image files produced by modern small sensor cameras are very high quality, with more than enough digital information to produce large prints, yet the file sizes are much more manageable. I personally do not miss rendering and storing folders full of 50MB+ size image files of my patio tomatoes, especially when in practice these images are certainly not twice as good as my current 25MB compressed files.
: The electronic viewfinders [EVF] found on many mirrorless cameras have many unique advantages over optical viewfinders. With an EVF the photographer gains the ability to preview the exact lighting and depth of field of the image, can achieve more precise focus through either detail enlargement or by utilizing colored highlights to display the in-focus areas [focus peaking], can directly monitor the effects of presets and film simulations, and can amplify the signal to provide a view of very dark or heavily filtered scenes.
: A very welcome feature for photographers who frequently shoot in low-light environments where tripods arenâ€™t practical or permitted is in-body image stabilization of the sensor. This enables sharp images of static subjects at much lower shutter speeds than are normally utilized for hand-held shooting, on non-stabilized lenses. Smaller sensors, with their lower mass, are easier to stabilize than larger sensors; micro four-thirds sensors can achieve an incredible 6 stops of stabilization.
Having travelled the road from crop sensor DSLRâ€™s to high megapixel full frame DSLRâ€™s to crop sensor mirrorless cameras, I can say that the gains far outweigh the losses, and the current systems of small sensor mirrorless cameras are worthy of very serious consideration by any photographer.