This really excellent condition 150mm viewfinder is a very desirable companion for the 150mm lens on your Mamiya 7 or 7ii camera. These are almost impossible to find – there are plenty of 150mm lenses out there offered for sale and very very few finders.
This example is boxed, and cosmetically very near mint with just one small paint rub, but there is slight haze inside that has very little effect on use. Not quite perfect but perfectly usable.
This finder makes use of the 150mm lens much easier – especially if you use this lens a lot. The very small image in the rangefinder window is not so easy to work with!
There are essentially two parts to the viewfinder. The rear rotating ring next to the rubber eyecup is simply a dioptre for eyesight correction. The front ring with the distance scale is basically a parallax compensator for use at closer distances to make sure that the camera is pointing exactly at the intended target. You focus the lens as usual using the rangefinder in the camera. You find that the focus distance is 5m (or whatever). You then set 5m on the 150mm viewfinder and the 150mm finder with correctly align the camera onto your subject. As the focusing distance gets closer it has a greater effect as the parallax error is greater.
The 150mm viewfinder has nothing at all to do with focusing. It gives you a larger image to look at than the camera viewfinder, but its real purpose is to make sure that the camera is pointed exactly where it should be.