Micro-geometrical optical principle of split-image focusing screen: mutual deflection of double prisms

Why is the horizontal line in focus is flat, and defocus when the horizontal line will be knife-cut into two-way staggered?

• • microstructure * * : the split-image focusing screen is not flat ground glass in the center, but consists of two tiny * * semicircular beveled Prism Wedges * * biting into each other. The two prisms have completely opposite tilt angles (one to the upper left, one to the lower right) .

• • • Ray deflection and parallax overlap * * :
• beams of light from the left and right sides of the exit pupil of the lens pass through the two oppositely tilted prisms;
• • • In Focus * * : when the Focus is just on the surface of the focusing screen, the two beams converge at the same point, the refraction angles are complementary after passing through the prism, and the image edges of the upper and lower semicircles are seamlessly occluded
• • • Out of Focus * * : when the Focus is on the front or rear of the focusing screen, the two beams are separated on the surface of the focusing screen, and the prisms with opposite tilt angles deflect the light laterally in the opposite direction, the lines are torn and staggered in the middle. The photographer simply rotates the focusing ring until the staggered lines are reassembled into a straight line, representing absolute focus.

# # 2. Why does a smaller aperture (below f/5.6) darken one half of the split image?

This is purely a result of the geometrical aperture of the light and the physical obstruction of the pupil space of the human eye:

1. • • The Critical Angle of refraction of a PRISM * * :

• the wedge angle of a split-image prism is precisely designed so that the Cone of Light must have an adequate opening angle (usually at an aperture of * * f/4.0 or f/2.8 * * or more) .

2. • • The cone of light narrows at small apertures * * :
       When using an f/5.6 or F/8 lens (such as a 70-to 300-mm telephoto zoom lens end, or a retro lens f/8, or a depth-of-field preview by pressing the f-stop Lever) , the lens aperture shrinks dramatically and the exit cone becomes extremely narrow.
3. • • The outgoing beam is off-axis * * :

• when a narrow beam of light is refracted through an oblique prism, the angle of refraction is too large for the light to reach the outside of the viewfinder eyepiece's field of view and enter the photographer's pupil!
  With no photons entering the retina, the half-cracked Prism that the human eye sees appears to be a complete Prism Blackout.

Three practical micro-manipulation techniques for removing half a black spot

When you have to manually focus with a small aperture lens, the following physical tweaks can greatly alleviate the blackness:

1. Pupil Shifting * * :

• when the top half of the split image turns black, move your eye behind the eyepiece * * very slightly up or down 1-2 mm * * ;
• as the pupil of the eye moves slightly, just enough to intercept a critical beam of refracted light, the half-black image is instantly filled with light and brightened!

2. • • assisted by a microprism ring and peripheral ground glass * * :
       Many focusing screens have a Microprism Ring and frosted glass around the split image. The tolerance of the microprism to small aperture is slightly higher than that of the split image. When the split image is black, the microprism area is focused. When the microprism is defocused, the microprism will produce a mosaic ripple with intense flicker, and the flicker disappears and becomes a clear image at the moment of focusing.
3. • • full aperture focusing principle * * :

• all modern film dslrs are equipped with an automatic full aperture lever. Even if you're shooting landscapes with an F/16, keep your camera wide open (e.g. , f/2.8 or F/4) when you're composing your shots When you press the shutter release, the camera automatically closes the aperture to expose the image, which doesn't affect the view.

A guide to selecting a replaceable focusing screen for long focal lengths and macros

Nikon F 3/FM2, Canon F-1/EOS-1V, Pentax LX and other advanced body support free-to-plug replacement focus screen:

• • • A/K screen (standard split-image microprism) * * : for daily 24 mm ~ 105 mm, f/1.4 ~ f/3.5 wide aperture lens the main hang.
• • • B screen (pure ground glass full screen) * * : no central crack ring, no central black spots. Extremely suitable for 300 mm f/4 + super telephoto shooting, reentrant lens or macro lens below f/5.6.
• • • E-type screen (gridded building matte screen) * * : with horizontal and vertical nine-square scale lines, no crack image interference, dedicated to building perspective correction and landscape levelling.

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