From some discussion on photographic forum I noticed that for some is not very clear what is the concept of magnification of a reflex viewfinder I write percia this short article to clarify it better
An optical system, a telescope, binoculars, camera, and always consists of an objective and a viewfinder, each with its focal length and its magnification factor. The magnification of an image is always assessed in relation to the human vision: it is 1x when, through the optical system is seen as great to the naked eye.
In reflex has always been regarded as have magnification factor of 1x the objective so-called "normal" that has a field angle similar to that of the human eye, reproduces the images in the size and with a view similar to that of humans and a focal length equal to or slightly greater than the diagonal of the format, but for those longer are paintings and enlarge the image by a factor proportional to the ratio of the focal length normal (50 mm for the 35mm format or the full frame) is the focal length tele. The wide angle lenses vice versa rimpicciolisce the image and the overall magnification is given then the combination of magnification of the objective and that of the viewfinder, as you know the astronomers.
In the case of the reflex the value of the magnification of the viewfinder has always been measured with the objective considered to magnification 1, i.e. 50 mm. until there were only the reflex to film these all had the same format and therefore this measure gave an indication of the viewfinder magnification.
With the introduction of reflex with different sizes of sensor, in particular, the more small, to maintain the ability to compare the viewfinder magnification would have had to measure the magnification of the image reproduced from it with a goal of reduced focal, a normal to the new format, which for the APS is about 30 mm camera lens focal lengths above this are, for the ODA of tele and enlarge the image, but the producers have continued to measure the magnification viewfinder always with the focal length of 50mm which in the case of the APS and a tele, with a magnification factor of 1.5 -1.6 (according to the size of the sensor). The magnification measured, then, is the result of the magnification of the objective more than the viewfinder.
If you don’t know what is the true viewfinder magnification is necessary to purify it of that objective that corresponds to the multiplicative factor that applies to the real lens focal length on the APS to realize more easily if they are wide angle lenses or tele.
The reason for which the producers have behaved like that and that probably did not want customers to realize they were small crosshairs of APS SLR and it seems that until today, there are managed well, even if DP Review has discovery the problem only 3 months ago.