Apr 11 2012

Olympus OM-1 metering

Olympus OM1

There is plenty of good information about the Olympus OM-1 and the other cameras in this series on the Internet so I don’t feel that I need to cover that ground.  I will however take a moment to discus the metering and how it is measured and presented to the user.  

The OM-1 has TTL metering (Through the lens) that is provided by two Cds cells placed internally around the eyepiece.  A certain portion of the light passing through the viewfinder prism falls on these cells causing their internal resistance to drop.  This effects the circuit causing the meter needle to move.  Other parts of this circuit are the ASA dial, on off switch and of course the shutter speed and aperture linkage. All these variables contribute to the position of the meter needle. This meter requires batteries to operate but the camera it self is entirely mechanical allowing you to shoot without the meter. 

The meter itself is located on the left hand side of the viewfinder and gives an indication of proper exposure for an average scene.  As you make changes to the aperture and shutter the needle moves correspondingly.   As the lens remains at it’s maximum aperature  prior to actually taking a picture these changes are transferred from the lens to the camera via a mechanical linkage.  In addition to centering the needle you can determine +-1 EV of adjustment in half stops by comparing  the needle to the markings in the viewfinder.  I’ve created this animation to show these.

 Olympus OM1 meter animation

My camera has an interesting focus screen that does not have the standard central focus micro-prisms or split image but is essentially a matte screen with a finer circular matte at the center and frame lines that are well within the image.  I believe this was the all matte type that someone has customized with frame lines for either astrophotography or as Ted Samzadeh of Omnilargess has suggested for medical imaging.  In any case the fine matte screen provides an exceptionally clear and unobstructed view. 

Olympus OM1 viewfinder


Here are some of my ubiquitous sample images from my OM-1 taken with the Zuiko 35-70 F3.6 or the 28mm F 3.5 Zuiko as these are the only two lenses I currently own.  Duncan Turner of  DLT Photographic has now lent me a Vivitar 55mm macro in OM mount so that should be an interesting lens to try out with this camera.