Above is a chart showing sensor sizes in the various cameras. Medium format probably isn't relevant to most of us.....only a very serious photographer. I think you can think of the sensor in terms of film......the larger the film....the more date it captures. The smaller the sensor...the less data it is capable of capturing. As you can see cell phones and point and shoot cameras have pretty small sensors. Are they good enough for your uses? That depends.....they are capable of pretty good pictures....see cell phone pictures and my original pictures at post 1. Those pictures are good.....but they have one thing in common.....good light.....except for the moon shot. That is where small sensors fail. Poor light or enlarging the photograph which will soon become pixilated.
As you move up the sensor size you move up in the amount of data to be captured and the pictures can be enlarged while still looking sharp. Once you move beyond the cell phone, point and shoot and bridge cameras ( a point and shoot with a zoom lense) you get into camera bodies that allow various lenses to be attached. These cameras also begin to offer image stabilization and a lot of controls. The advantage of being able to change lenses is that the camera can now be set up for about any type of photograph you wish to take. Wide angle lense, 14mm for landscapes or the interior of a house where you want to see the entire width of the room. Or, a wide angel to zoom lense that goes from 12mm to 85mm. The advantage here of course is a lens that offers a good deal of flexibility......the downside....a loss of light unless a very expensive lense.
It seems the next thing that begins to effect cost is quality of construction of the body and the lenses......just like everything else in life. Plastic is cheaper. Durable metal frame bodies that are water and dust proof add more to the cost. The quality of construction is simply more durable and expensive. Very important if you are a professional.
Just like firearms it is difficult for one camera to cover the bases. I keep a Walther P99 handy for house security while I carry a Smith bodyguard but neither is good for bird hunting and the finest .22 is suitable for deer hunting. I wandered off into the Everglades the other day to take pictures of whatever. There were gators, birds and while I was able to capture some pretty good pictures this is where a camera with a fairly large sensor and a long lens with good glass would be ideal. I would not have minded strapping on a big DSLR with a foot long lens for that hike. But I would mind carrying that camera everyday just in case something happened to catch my interest photography wise. I should point out that the mirrorless cameras are considerably smaller than some DSLRs......but that all becomes moot when you screw on a 300 or 400mm lens. You now are carrying a 12 gauge shotgun.....not a pocket pistol.
So where does all this leave a newbie.....confused....
If you just want to be able to capture a picture or make a movie any time something interesting pops up.....use your cell phone. You don't need a low end point and shoot although they are considerably cheaper than a cell phone for work in dusty, wet, dirty areas.
Some mirrorless cameras are pretty small like the Olympus OM series. 1/3 to 1/2 the size of a Big Canon or Nikon. The full frame Sony mirrorless are a little larger than the Olympus but still considerably smaller than the big DSLRs. Sony and Canon and Nikon and other do make smaller DSLRs. These are not pocket size but are considerably smaller and lighter than the big ones. These will be in the $600 range with a kit lens. I don't think there is really any way to wrap your brain around this without visiting a camera store and actually handling the cameras. Most will have lenses on them and you will understand really quickly what all this size and weight business is about. You will also be able to look at the lenses available. You will see what a 35mm or 50mm prime lens offers. Excellent light catching capability in a 1" to 2" long lens that keeps the camera as compact as possible. Just the lens for a walk about street camera. Then you will see what a 12" lens that is 3" or 4" in diameter will do size and weight wise to any camera.
In the end......it boils down to what are you going to do with this camera. Right now I'm thinking....cell phone and no point and shoot. Then I'm going to drop the idea of the Canon 80D....simply too large....when I think I can get the same quality of pictures from a much smaller DSLR or a mirrorless camera. Remember, I can screw on just about any lens I want to the smaller body. I don't make my living with a camera and I have no interest in blowing up my photos to 2', 5' or 8' sizes. In fact, I don't really like photos as art work as strange as that may sound. Paintings....oil, acrylic, water...yes...photos, no. But that is just me.
I think I am interested in the smaller and cheaper Canon DSLRs or the Sony cropped mirrorless. Then I will spend the savings on good lenses that fit my needs. For the photographs I'm interested in...I don't think I will see any difference in full frame vs cropped. I found the Olympus a bit too small and a number of times I had my thumb or other parts of my hand pressing buttons that didn't need to be pressed while I was photographing. I took a thousand or more shots so I have a pretty good feeling for that camera. Finally, I would suggest you pay close attention to how well you can grip and handle the camera. Most DSLRs excel at grip surfaces. If you have large hands, I don't, some of the smaller cameras might be a bit too small. This isn't a problem for one or two shots......but if you are flying down the swamp in an airboat....you need a easily gripped camera with the controls readily at hand. 1917