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30
Using The Light Meter/Exposure Calculator (A2B and A2F)
I. First, you must set the Tabs to the proper film speed.
Determining Film Speed: To set the Film Speed Tab, you must translate the film
speed on the film cartridge, which is in ISO(ASA), to the film speed written on the
camera, which is in Weston. Weston ratings are about 80% of ISO ratings; use the chart
below to approximate your Weston rating. (If you are using film faster than 100 ISO, set
the Film Speed Tab to 100 ISO/72 Weston and you will compensate for it in Step V.)
ISO Weston
15 12
25 20
50 40
100 72
Table 5-1: Equivalent ISO and Weston film speeds
The Aperture Tab sits on top of the Film Speed Tab, but the two are not permanently
attached. Slide the Aperture Tab down onto the Film Speed Tab until your Weston rating
is the largest number exposed on the Film Speed Tab. Make sure that this setting does
not get changed.
II. Move the Tabs together on the Slide so that the arrow on the bottom of the Film
Speed Tab (stamped on top of the number 12) is lined up with the current lighting
conditions. Your options are BRIGHT, AVERAGE, CLOUDY, or LIGHT INT (used for
lit interiors).
III. Hold the camera with your elbows bent at a 90 degree angle and look through the
Extinction Meter. Move the Slide until the Brightness Pointer lines up with the dimmest
rectangular window you can see. If you are uncertain of which is dimmest, choose the
brighter one.
IV. All of the calculations on the Extinction Meter are now complete. Look at the
Shutter Speed Plate number to the left of the Aperture Tab to see which shutter speed
should accompany which f-stop. The following two examples demonstrate the use of this
system. (On later A2Bs, the top shutter speed is not 200, but 150. Use whichever
number is on your Extinction Meter.)