Magnification refers to how big the image seen in the viewfinder appears. For cameras with changeable lenses, it is specified for a normal 50mm lens focused at infinity. A magnification of 1x would be the same as you see with the naked eye. Higher magnifications give a bigger image in the viewfinder and greater focusing precision, but it can be hard to see the whole frame and in-viewfinder readings without shifting your gaze within the viewfinder.
Most viewfinders actually make things appear smaller than they are when seen with the naked eye. A viewfinder that has 0.65x magnification makes objects appear approximately two-thirds of the size they appear in reality.
Some rangefinders with interchangeable lenses are able to increase magnification in the viewfinder by up to 1.5x, to increase accuracy and ease of focusing. Adjustable magnification in the viewfinder is also employed to change the field of view (or the frame lines) to optimize for the different focal length lenses that can be mounted, either within the viewfinder or with special view-finder adapters. This is not an issue for fixed-lens cameras or SLRs.
Brightness refers to how bright or dim the image appears in the viewfinder. A bright viewfinder is important as it allows you to frame and focus your image accurately, especially in low light conditions. Brightness isn’t measured or specified in user manuals, which might only read “bright viewfinder.”
Rangefinder cameras may display a larger field of view through the viewfinder than is actually covered by the lens; i.e., it is larger than the image recorded on the film. The actual field covered by the lens is outlined in the viewfinder by bright frame lines (superimposed onto the viewfinder from the frame-illumination window). The excess image around the frame lines allows space for the lines to move within the frame to correct parallax while the frame lines allow pictures to be composed exactly to the edge of the frame. Anything that falls outside the frame lines will not be recorded in the image. A bonus of the excess is that it allows you to see moving subjects before they enter the frame.
Viewfinder Readings
Cameras that have a built-in light meter will display an exposure scale in the viewfinder on one of the four sides of the frame lines. This is most commonly found on the right-hand side, where a needle indicates the recommended aperture, EV, or an index mark against which the needle must be centered for the correct exposure. The scale is bracketed at either end by a marked zone that indicates when there is either too much or not enough light to expose the film properly. The selected shutter speed may also be visible in the viewfinder.