Pentax focusing screen pattern

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iso rivolta Contributing Member • Posts: 818
Pentax focusing screen pattern
9

Dear all,

Recently I became interested in looking at the structure of the focusing screen in my K-3 II. The default screen is Natural-Bright Mate III, the same as inside K-3 III.

To get images of both the matte part of the focusing screens I used the system below, where I got two cameras with their lenses focused at infinity and I illuminated the optical viewfinder with a green LED flashlight. I did this to avoid color moire in the resulting images.

Canon 600D with Tamron 90mm macro and Pentax K-3 II with FA 50mm macro, both focused at infinity. Green flashlight illuminated here Canon's OVF.

To get a good resolution (magnification), the camera whose focusing screen is photographed should have a wide-angle lens mounted, while the camera photographing it, a telephoto lens.

Pentax K-3 II screen seen at low magnification. 50mm macro mounted on the Pentax, Tamron 90mm on the Canon

Pentax K-3 II screen seen at high magnification. To see the fine pattern, I had to mount the Rokinon 8mm fisheye on the Pentax.

The structure of the matte field of the Pentax K-3 II screen looks to have a pseudo-random pattern, likely done by laser (laser matte). The pattern is very fine and this should be good for getting a high resolution in the OVF.

100% crop of the above

Now, to see the back side of the focusing screen, usually having a Fresnel field, the lens on the camera being photographed should be focused slightly below infinity.

Fresnel field visible on the back of Pentax K-3 II focusing screen. The brackets and center mark are on the other side of the screen, so out of focus. The 50mm macro was mounted on the Pentax.

Interestingly, the center circle (spot metering circle) has no Fresnel pattern. This should make manual focusing more accurate in this region.

100% crop of the above image

Canon EOS 600D (EOS Rebel T3i / EOS Kiss X5) Pentax K-3 Pentax K-3 II
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