Strictly speaking, a reflex camera is simply one that allows the user to view the image composed by the light arriving directly through a camera lens, i.e. the light which will actually produce the image of your photograph.
The reflex part of the designation refers to the fact that the light passing through the lens is reflected upwards by a mirror so that it can be seen through the viewfinder. The image seen through the viewfinder is thus what is seen by the lens.
That factor contrasts with a viewfinder camera where what is seen is the image composed by light arriving directly through the viewfinder, not the light passing through the lens. The image seen in this way will not be the same as that seen by the lens.
Why does this matter? This is a more complex question for many factors influence the answer. Some of the more pertinent yet easy to understand are:
* Because the lens and the viewfinder are in different places, what you see in the viewfinder will not be the same as what the camera sees. In technical terms, the light from the lens is directed by the lens to the focal plane - i.e. in a film camera, that is the film surface, in a digital camera, that is digital sensors.
* Viewfinders do not adjust for the nature of the light entering the lens of the camera. - This gives the user artificial and much less sensitive information as to how the camera is 'seeing' the subject matter.
A 'for instance' is that the viewfinder image will not change in sharpness and clarity, regardless of how close or distant the user is from the subject matter.
All lenses have what is known as a focal length and for practical purposes here, that can be regarded as the point at which the lens will produce the sharpest image on the focal plane.
The greater the distance in front of and behind that point of sharpest focus the less sharp the image will be.
Viewing though a reflex camera, i.e. through the actual lens, allows the user to see how sharp the image is at any particular distance. Viewing through a viewfinder camera does not.
I have to stress that, as I intimated earlier, this explanation is intentionally a simple one and many more factors are relevant, including the aperture through which the light arrives, (a smaller aperture providing a sharper image and a larger aperture a less sharp image), distortion of light within the lens itself and more.
One other factor that is worth mentioning is that the image received through the camera lens is similar to that received by your eye. - It is reversed or, if you like, upside down. Our human brains right this image so that we see things as they are.
Most reflex cameras use a system that combines a mirror, a pentaprism and a focusing screen, though there are other forms. These items function together to both correct the inverted image so that it is seen in the correct orientation. In addition, when the shutter release is pressed the focusing screen pivots upwards to allow the light to fall onto the light sensitive surface, i.e. the film or digital sensors. The result is that other than for the split second during which the photograph is taken, the user can see direclty through the lens at the subject being photographed.
I apologise if this explanation has not achieved its purpose, which is to help you understand the difference and advantages of a reflex camera. If I have simply added to your confusion I am sincerely sorry.
