split image focusing

[dxphoto]

Does anyone how these things work? Why the image is split when it is not in focus. How about those microprisms. what's the mechanism behind it. Thanks.

 

[phaedrus]

Both focussing aids function by the same principle. They consist of two (split image) or multiple prisms (microprisms) placed side to side but with opposing bases (thick parts). The height of these two prisms reaches equal height at the level of the focussing plane (film plane).

 

[AgX]

Split image wedge:
The image is seperated into two by the two wedges and displaced to opposite directions. The orientation of these two images depends whether the wedges are in front of rear of the focal plane.
In case that part of the two wedges where they intersect (a line) is in the focal plane no displacement takes place.

The images will appear sharp enough as long the distance of the wedges is not too far off the focal plane; I assume the half-images will get off the wege before.

The larger the angle of the wedges the more pronounced will be the displacement. The large that angle the larger the aperture of the lens must be to avoid that one of the wedge images turns dark.
(Actually I yet have not understood why only one turns dark… When rotating the camera by 90° the other wegde darkens. However if you only rotate by 45°, both wedges stay light the same time.)


Microprism:
It is a an array of many wedges in the form of tiny tetraeders or pentaeders, with their bases all in one plane.
Wheras the effect of the single pair of wedges is to enhance vision due give a distinct vernier effect, the microprism array scatters the image. This scattering is much more pronouncecd (depending as above on the slope of those pyramids) than the blurr occuring on the ground glass.