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Pentax Focusing Screens

focusing screen in a box

In addition to laterally reversing the image, most SLR cameras incorporate one more very important function into the viewfinder — the camera's light meter. Apparently this is the easiest and cheapest solution, but it is not necessarily the best. Because the light meter is situated between the focusing screen and the opening for the photographer's eye, it can be negatively affected by light entering through both. Replace the focusing screen with a "darker" one, and the camera overexposes. Take a picture without holding your eye close to the viewfinder's opening, and the camera underexposes. Think of it this way: the light meter's sensitivity is adjusted precisely to the mirror's reflectivity and the screen's brightness. When you replace the screen with one that transmits more light, the light meter receives more light than the film, and the picture is underexposed. Three possibilities exist for correcting this effect: recalibrate the light meter, adjust the film's ISO rating to a lower EV value, or manually overexpose each exposure. If you replace the screen with a less transparent one, you must follow the reverse procedure. The one important exception to this rule is the LX, whose light meter is in the mirror box, and is not affected by variations of the screens and viewfinders.

The quality of focusing screens has improved constantly, and each new generation is finer and looks brighter. The designations of the Pentax focusing screens use two letters and normally two digits. The first letter indicates the screen's dimensions, and distinguishes between screens with different designs, patterns, focusing aids, etc. The numbers indicate the screen age or generation. Higher numbers normally indicate more modern (and usually better) designs. Screens without numbers or with a single digit are from the first (level-10) generation.

The following sections list all Pentax cameras that support interchangeable screens and the screens that fit them.

Screens for the MX and LX

Screens for the MX and LX cameras have the same dimensions, and all have designations with first letter 'S'. Screens for the original LX (those with numbers lower than 40) seem to be about 1/3 EV brighter than the screens for the MX. Screens for the LX 2000 (those with numbers between 60 and 70, sold only for a short time in 2001) seem much brighter than the original ones, but in reality the difference is no more than 1/6 EV.

Like previously discussed, screens designed for the MX can be used in the LX without the need for adjusting the light meter. LX screens in the MX will cause a 1/3 EV underexposure, but such a small difference will be virtually unnoticeable.

Table 1. Focusing screens for the LX and MX camera bodies.
Description Screen
Name
Catalog
Number
Suited
For
Specifically Designed for... Suitable for Use with...
Matte with microprism center SA-1 30831 MX Most lenses with f less than or equal to 300 mm.
SA-21 30862 LX
SA-3 30833 MX Lenses with widest apertures between f/1.2 and f/2. Lenses with widest aperture of f/2.5 or f/2.8.
SA-23 30863 LX
SA-26 30882 LX Refractive lenses with f greater than or equal to 400 mm. Refractive primes lenses with f greater than or equal to 100 mm and widest aperture smaller than or equal to f/2.5. Most refractive zooms.
Microprism with finer microprism center SA-32 30885 LX Most lenses with f less than or equal to 300 mm.
SA-37 30884 LX Refractive lenses with f greater than or equal to 200 mm. Most lenses with f greater than or equal to 100 mm.
Matte with split-image center SB-1 30834 MX Most lenses with f less than or equal to 300 mm.
SB-21 30864 LX
Matte with split-image center and a microprism collar SC-1 30835 MX Most lenses with f less than or equal to 300 mm.
SC-21 30861 LX
SC-26 30881 LX Refractive prime lenses with f greater than or equal to 400 mm. Refractive primes lenses with f greater than or equal to 135 mm and widest aperture smaller than or equal to f/2.5. Most refractive zooms.
SC-69 30869 LX
Matte with center cross SD-1 30832 MX Macrophotography. Most lenses with f less than or equal to 300 mm.
SD-21 30865 LX
Clear-glass with center cross SD-11 30839 LX Astronomy, microscopy. Most lenses with f less than or equal to 300 mm.
Matte with circle SE 30837 MX Most lenses with f less than or equal to 300 mm.
SE-20 30866 LX
SE-25 30883 LX All lenses with f greater than or equal to 400 mm. All lenses with f greater than or equal to 135 mm.
Plain matte field SE-60 30850 LX Star lenses, mirror lenses. All lenses except those wider than 28 mm and slower than f/3.5.
Matte with circle and grid SG 30838 MX Most lenses with f less than or equal to 300 mm.
SG-20 30867 LX
Matte with grid SG-60 30870 LX Star lenses, mirror lenses. All lenses except those wider than 28 mm and slower than f/3.5.
Matte with circle and millimeter scale SI 30836 MX Most lenses with f less than or equal to 300 mm.
SI-20 30868 LX

SFXn/SF1n and SFX/SF1

Three screens were available in addition to the standard one: FD-41 (Scale matte Screen for Photomicrography and macrophotography), FG-40 (AF sensor field surrounded by a grid) and FI-40 (AF sensor with a horizontal and a vertical millimeter scale). Both screens are well suited to scientific, architectural, and macrophotographic work.

Z-1p/PZ-1p, Z-1/PZ-1, Z-5/PZ-5

Screens for all these bodies have the exact same dimensions, but 60-series screens are about 1 EV brighter than the 50-series. All screens have matte backgrounds and different markings.

Table 2. Focusing screens for the Z/PZ-series bodies.
Description Screen
Name
Catalog
Number
Suited
For
AF sensor field (the standard screen) FF-60 unknown PZ-1p
FF-50 unknown Z-1/PZ-1, Z-5/PZ-5
AF sensor field with a 6 mm grid FG-60 unknown PZ-1p
FG-50 unknown Z-1/PZ-1, Z-5/PZ-5
AF sensor field with vertical and horizontal scales FI-60 unknown PZ-1p
FI-50 unknown Z-1/PZ-1, Z-5/PZ-5
AF sensor field with golden section FK-60 unknown PZ-1p
AF sensor field and spot-meter field FS-60 unknown PZ-1p
FS-50 unknown Z-1/PZ-1, Z-5/PZ-5

MZ-S

There are two screens available for this body: the standard GF-60 (plain matte) and the GG-60 (matte with grid lines).