Modern Classic SLRs Series :
Canon New F-1 - The New Split in Focusing Screen

 
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The New F-1 posseses one of the rare few brightest viewing comfort in its finder among most cameras in the early eighties. These are development effort and thus, credit has to be given. Nothing is better than archiving these important research work for future quick references for someone to refer to.

Cross section.jpg New Split.jpg
Since all interchangeable focusing screens of the New F-1 system are produced through either the laser matte or bright laser matte techniques, the image may be focused at any point within the viewfinder.However, seven of the available screen types additionally feature center focusing aids. The following introduces two unique rangefinder concepts developed by Canon for the New F-1.

The ultra-bright New Split screen - How these was achieved ?

Magni View.jpg
It was a modification of the basic split-image type, Canon calls this rangefinder the New Split. In a regular split-image focusing aid, ranging is accomplished by aligning the upper and lower halves of the image.


Increasing the steepness of the angle of the two opposing prisms effectively increases the ranging accuracy of this focusing screen. But there are also drawbacks: the steeper this angle becomes, the more likely the chance of half of it blacking out in the stopped-down mode or when using slower lenses which is a common phenomenon for most of us. The accompanying diagram showing a cross section of the New Split illustrates how two different angles can be used to eliminate this problem, and maintain ranging accuracy at the same time. The steeper of the two angles is 10°50'; the gentler sloping angle is roughly half that, or 5°40'. When a wide aperture lens is used, the steeper angle takes over for maximum ranging accuracy On the other hand, the gentle sloping prism comes into play when stopped down or when using a small aperture lens. Either way, the accuracy of the split-image ranging method is maintained, and the traditional problem of darkening is corrected.

 
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The prism layout is responsible for the New Split's performance. The refractive effect produced by the two different angles used in the microprism array combines with the diffraction grating effect so that each element of the prism picks up a certain amount of light, and passes some of it to adjacent elements. Thus, the lighting is evenly distributed over the entire microprism without the usual prism black-out at small apertures. The New Split is based on an echelette grating (a prism configuration consisting of multiple rows of microprism elements) principle. However, the elements of an echelette grating normally have a uniform angle of slope. With the New Split, each element has two angles.

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