In the last few years mirrorless cameras have gone from being fringe players in the camera market to being industry leaders with a significant market share.   But what is the difference between a mirrorless camera and a DSLR?  That's a question a lot of people have aksed and there seems to be some confusion about the answer.  So let's take a look at some of the differences of each type of camera and some of their strengths and weaknesses.  

The most obvious difference is that one has a mirror and one doesn't.  Why does this matter?  Because the mirror affects how the camera focuses.  There are three types of autofocus, phase detection, contrast detection, and hybrid.  A DSLR cameras uses phase detection and mirrorless cameras uses contrast detection.  Some newer mirroless cameras like Sony's just announced A7rII uses hybrid detection.  On a DSLR, phase detection takes place when the mirror reflects the image on to a seperate sensor which compares two reflected images to find the distance. On a mirrorless camera, contrast detection is done on the image sensor of the camera, so focus is determined by the light striking the sensor and the processor uses focus peaking to determine optimal focus. The third type, hybrid focus, uses phase detection lenses on the image sensor to find the distance and then fine tunes the focus using contrast detection by the processor. Of the three types of focus, phase detection is the fastest which gives the autofocus speed advantage to the DSLR cameras.  Contrast detection tends to hunt for the proper focus making it slow and not as responsive as phase detection.  Is this a deal breaker?  No, for most casual photographers and even some pros the hunting by the contrast detection is not much of an issue for everyday photography just a bit annoying. The new hybrid focus systems look promising.  Each generation of the hybrid system gets faster and better.  But there are still some technical hurdles to overcome, so hybrid systems still lag behind phase detection. So if you're photographing anything fast or you need to track a moving object, you would do well to stick with a DSLR.  

Another major difference is in lens selection.  DSLR cameras have been around for a long time so there a lot of lenses available from both the manufacturer and third party vendors.  Mirrorless on the other hand is still relatively new so the selection of lenses is slim(but growing) and telephotos are hard to come by.  The possible game changer is lens adapters which allow you to use either Canon or Nikon lenses on a mirrorless body.  The main problem with lens adapters is that they don't allow you to autofocus non-native lenses or if there is autofocus it is so slow you might as well stick to manaul focus.  The new A7rII hybrid autofocus seems to be taking steps to solving this issue but I feel using Canon or Nikon lenses on a mirrorless body might still be one or two generations away from being practical.  So if you need to use a good telephoto or prefer a large lens selection stick to a DSLR.    

The biggest difference between a DSLR and a mirroless camera is size.  Without a mirror and a pentaprisim, a mirrorless camera is smaller and lighter than a DSLR and are far easier to carry around.  A mirrorless camera with an APS-C image sensor is about the size of an old point and shoot but it has all the same functions as a DSLR.   Even the full frame mirrorless cameras are much smaller and lighter then their DSLR counterparts.  Such size and portability is very appealing to casual photographers and pros.  Anyone looking for a good travel camera that easy to carry around, should definitely take a look at the available mirrorless cameras.

Another difference is mirrorless cameras use an electronic viewfinder versus an optical viewfinder in a DSLR.   When you look through a mirrorless camera's electronic viewfinder it shows you what the sensor sees.  So what you see is exactly what the sensor will photograph.  Electronic viewfinders are also brighter. Overall electronic viewfinders are very nice and I like them.  There are few trade offs,  the most notable is they drain battery life.  Another is there is a slight lag when you take a photo and the viewfinder blacks out.  There is some debate as to how useful they are in low light situations but I think they work fine.  In comparison, a DSLR's optical viewfinder shows you a reflected image of what is seen through the lens.  The optical viewfinder also crops the image slightly so you're not seeing the whole scene.  So you have to be mindful of the edges when you frame your photograph.  But again if you need to track moving objects or photograph anything really fast, you can't beat an optical viewfinder.  The reason is you never lose sight of your subject through the viewfinder, even if you're shooting continously because there is no lag or blackout.  

The last major difference is that in my opinion mirrorless cameras are better suited for video.  The reason they are better goes back to the mirror or lack there of.  When a DSLR shoots video the mirror on the camera goes up and stays up meaning it can't use phase detection to focus.  This meant you couldn't autofocus when you used older DSLRs to shoot video.  New DSLRs solve this problem by switching to contrast detection so that the camera can focus when the mirror is up.  But in my opinion they lag behind the mirrorless cameras which don't have a mirror and already use contrast detection. Yes, some movies and tv shows use DSLRs to film scenes.  They do this because the lenses on the DSLR are better.  But to overcome the focusing issue they use very expensive camera rigs with follow focus rings.  These rigs are not very practical for the casual shooter, or the pro who shoots primarily stills.  So if you want a good all around camera that can shoot good still picutres and good video I would use a mirrorless.   

As for image quality, neither type of camera has an advantage over the other.  Digital sensors are very good today and you will get very nice pictures no matter which type of camera you choose.  So these are the some of the differences between a mirrorless and a DSLR camera.  Both types of camera will take excellent pictures and bring much photographic joy.  Which one you choose will ultimately be dictated by your photographic needs and goals.