The Avo Exposure Meter Gets a New Lease of Life

Some time ago I did a piece about an old AVO exposure meter:

 

The AVO Exposure Meter Comes Full Circle

 

These early Selenium meters seem to have retained their strength of response better than many later ones, perhaps they used a larger or purer slab of Selenium. This one responds strongly to light and is noticeably directional in operation, not quite a spot meter, but getting there. The problems are that the output from the selenium varies over time. Also they are calibrated with obsolete film speed systems - this one in the mysterious Hurter & Driffield - making it just too much hassle to try to convert to modern standards. So I finally got around to doing what I'd earlier intended – calibrate it against a modern meter. I compared the readings with my Polaris digital meter in various levels of lighting, and plotted them on a graph. There were inconsistencies, but I eventually settled on a relationship between the AVO needle position and the exposure values at 100 ISO. Then I made a scale on a piece of card which fitted neatly between the meter body and case, marking the EV's on the scale. I scanned this and used it to make cards for 200 and 400 ISO. So now I just slip in the appropriate card, and point the meter at different parts of the scene, getting a quick feel for the variation in exposure across the frame. Well, that's the theory anyway. It will come in most useful for cameras with exposure value shutters, like the Rolleicord Va. It's output could still drift, but I guess that after something like 80 years, the variation will be negligible, at least during my lifetime.

 

Why not just use the Polaris, did I hear someone ask? It's a very accurate meter, but for some reason the sensor is on the top of the meter, on the same surface as the readout. So to get a reflective reading you have to turn it away from yourself, pointing it at the scene, press the button and turn it back towards yourself to read the result. Not exactly user friendly. With the AVO I can move it around the scene, watching the needle as I go. Here are the meters:

 

I offer this suggestion free, gratis, and for nothing ...

Great suggestion, John. Thanks for posting.

Now that's a great idea John, and thanks for sharing it. The AVO meters deserve a thread of their own; they were manufactured in England by the Automatic Coil Winder And Electrical Equipment Company Limited who very sensibly opted for the AVO brand name, apparently standing for "amps, volts and ohms". You're lucky to have a working example in such nice order; I have a couple of the meters, but they show no interest in light. One is the Smethurst Highlight, reputed to be the very first meter designed to read incident light.

 

Your idea of using inserts for various film speeds was used in the fiendishly complicated French Realt meters, which had metal slides for the purpose. At the risk of hijacking your thread, I'll post a pic of these meters. Perhaps we should start a "Show your interesting meters" thread. Thanks for bringing the AVO's to the attention of the Forum. Incidentally, I agree with your criticism of the Polaris; one great advantage of the old Westons is the clarity of the dial and the fact that it's designed to be perfectly visible when in use.

 

AVO and Realt Meters

 

 

Edited July 24, 2018 by rick_drawbridge

@ John: Your card system for different films reminds me of the custom metal slides that both Sekonic and Spectra used for their professional incident cine meters. Since most movie cameras use a single shutter speed for normal filming (1/50 for most professional movie cameras at 24 fps) the only variables are film speed and f/stop. With the slide for a particular ASA the meter then reads out directly in f/stops.

That 's a great approach that addresses the differences and variation at different well .. speeds. . I certainly would not make the effort, but you reap the benefit of your work. I had my Weston IV calibrated a few years back, but age is taking it'S toll and I find the scale hard to read though as mentioned the orientation is right on. In another post someone posted a "Meter App" and I love the concept, but find the cell phone display next to impossible to read in bright light.