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What is the meaning of the reflected image of a double lens?

A double lens reflex is a very simple configuration consisting of a pair of identical lenses, one to provide the image shape and the other to provide the viewfinder and focus image (Fig. 6). The two lenses are usually mounted on a common plate or are connected by gears. A viewfinder is used / And the focal focus is the size of the film and when the vision lens is focused to clarify the image of the viewfinder lens, the photographic lens also becomes focal.
This leads to a very slight difference for subjects or objects whose area exceeds about 2-3 m (6-8) feet, but when focusing closer, the difference - the parallax error - becomes clear. Since the common arrangement of the two lenses is one on top of the other (the viewfinder on top) a close-up image, if framed well on the viewfinder, will lose some of the top, which may result in missing heads from the image.
The reflected image of a double lens is more complex because it contains internal calibration (automatic exposure due to parallax). In the absence of this calibration, this must be taken into account.

There are few mirror images of dual lenses that are interchangeable because the two lenses must be changed for the device to work. Likewise, we find that the effect of filters or filters, close-up lenses, etc., mounted on the capture lenses, can only be noticed when a similar accessory is attached to the vision lens. Which makes the device expensive and impractical. The presence of parallax error prevents close-up photography from being actually performed .