... "camera" is a box ... on one side is something that can record light, film in the olden days, in these times some sort of sensor ... opposite the film/sensor is either a pinhole or a lens ...
... fancy slr ("single lens reflex) or dslr (digital single lens reflex) cameras have a mirror between the lens and the film/sensor ... this mirror allow the user to view directly through the lens ... since the mirror is quite obviously in the way of taking a picture, it must fly up out of the way after the shutter button is pressed ...
... because of the space necessary for the mirror, the distance between the film/sensor and the lens mount must be rather long ... this is the "flange-to-film/sensor" distance ...
... my new fuji is a "mirrorless" camera (i hate the term, it's actually a Digital Viewfinder camera, but those initials were already taken and the various companies couldn't come up with anything they agreed upon) ... there is nothing between the lens and the sensor ... i view through a rather nice digital viewfinder ... that viewfinder could be anywhere, even separate from the camera, but fuji decided to use the classic slr form when designing their x-t1 ...
... because there is no mirror, the x-t1 can be extremely thin ... the flange-to-sensor distance of the x-t1 is almost 1/2 that of slr/dslr cameras with their flipping mirrors ...
... soooooooooo ...
... so, this is the best part ... because lenses for other cameras have to be further away from the film/sensor—because they have much greater "flange to film/sensor" distance—it is possible to use an adapter to put them on the fuji ... all needs be done is machine a little tube that is fuji on one end, the respective lens mount on the other, and which is exactly the correct length so that the fuji's flange-to-sensor measurement plus the adapter tube equals the proper lens-to-film/flange measurement of the camera involved ...
... here you can see the fuji x-t1 ... in order to make the camera "thick" enough so that the zuiko lens works properly, i've added a cleverly machined tube that locks into the fuji and holds the zuiko lens ...
... simple ...
... of course, with this combination i must focus the lens manually, just as we did in the olden days ... but, in return for this bit of extra work, i can purchase absolutely fabulous glass from dozens of manufacturers for 1/4th-1/10th the cost of comparable modern lenses ... (the lens pictured cost $29 on eBay ... it's in brand-new condition ... to equal its optical quality/performance with a modern autofocus lens, i would have to come up another $100-300) ...
... not a bad deal, and, as i've said, if it worked for henri cartier-bressen who am i to complain ...