The SEI Photometer
A Legend Among Spot Meters

sorcadmin's picture

Few people realize that photographic spot meters date back some 2/3 of a century. The very first was built by Arthur Dalladay, editor of The British Journal of Photography, in about 1935; he described it in the BJP Almanac of 1937 on pages 127-138. This meter still exists, in the possession of a subsequent editor of the Journal. Within a decade or so, there appeared two commercial meters based on the same principle. One was the "Ainger Hall" meter, manufactured initially by the Bowen Instrument Company and then later (to special order only) by one of its inventors, W.G.H. Turl. The other, with numerous refinements, was the SEI (Salford Electrical Instruments) Photometer. By 1948 the SEI Photometer was being distributed by Ilford Ltd. and (in the U.S.A.) by The Zoomar Corporation.

All were comparison photometers, based on the Bunsen grease spot photometer. These rely on the ability of the human eye to distinguish minute differences in brightness between adjacent areas: in this case, a "comparison spot" of adjustable intensity, and the chosen subject tone. They consist, in effect, of a telescope with the comparison spot on a photometric cube in the middle of the field of view.

The light source in the SEI is a bulb with a frosted top, powered by a 11/2v battery. The bulb is underrun for maximum battery and bulb life. It rarely if ever "blows," but eventually, the light output falls so low that it is impossible to get the calibration illuminance high enough. A small photoelectric cell in the bulb chamber feeds a moving-coil ammeter on the top of the Photometer, and a rheostat in the base is used to control the bulb to set the meter to the one mark, "Standard Brightness." This means that if you can get a new, frosted bulb and solder it into place in the bulb holder--and I'm working on it--you should be able to calibrate the meter to work with it. The Ainger Hall/Turl meter was much simpler, and had to be calibrated to a candle flame in a darkened room.

The bottom 11/2" (38mm) of the 8" (20cm) instrument forms a rotating unit which drives a pair of opposed continuous neutral density wedges between the bulb and the photometric cube. This allows the brightness of the comparison spot to be varied. Apertures are engraved on the rotating base, together with a rotating collar to set the film speed. Mine is calibrated in British Standard Logarithmic (B.S. Log) units from 10-50. This is the same as A.S.A. (Log) and therefore can be converted to (original) A.S.A. (arithmetic) readings. These correspond pretty closely to ISO speeds, so the ISO arithmetic range is 0.75-8000, an impressive range even today. For quick reference, D.S. Log to ISO equivalents are 14* = 50, 17* = 100, 20* = 200, 23* = 400, 27* = 1000, and 32* = 3200.

Two separate index marks are provided for the speeds, 20Þ (62/3 stops, 100:1) apart. The black index mark is used for determining exposures for negatives (shadow readings), the white, for transparencies (highlight readings). There is no index for reading gray cards or mid tones or any other drivel, because the designer understood what he was doing and expected users to have attained a similar level of technical understanding. The instruction book does however give reflectances of several key tones, including white snow, fresh white paint (80 percent), slightly weathered white paint (65 percent), old weathered white paint (60 percent), diffuse highlight on a "normal" face tone (Caucasian--remember we are talking about 50 years ago: 30 percent), and diffuse highlight on a "bronzed" face tone (20 percent).

Shutter speeds are printed on the body of the instrument, to be read off against the aperture scale on the rotating base. Life is however complicated by the fact that there are three measuring ranges, selected via a color-keyed three-position switch that moves neutral density filters into the light path.

Each range is 100:1. The red base scale is used for luminances of 0.01-1 foot-Lambert. (A foot-Lambert is one lumen per square foot; more modern units of luminance include the nit or candela/square meter and apostilb or 1/pi nits.) The white middle scale reads from 1-100 foot-Lamberts. The highlight scale--nominally blue, but faded closer to green on my meter--reads from 100-10,000 foot-Lamberts. The total measuring range across the three scales is therefore 0.01-10,000 foot-Lamberts, or one million to one.

Making Readings
To take a reading for shooting negatives, place the 1/2Þ photometric spot over the darkest area in which you want texture and take a reading, with the film speed set against the black index mark. For reversal film, where exposures are keyed to highlights, you place it over the brightest area in which you want texture, and use the white speed index mark.

Now press the button on the base, and turn the base until the spot disappears--which it does, in quite a magical way, when the two tones (chosen tone and spot) are identical. If the spot cannot be made light enough or dark enough, change ranges as described earlier. The color of the spot is controlled by another pair of filters, one for artificial light, one for daylight: not a perfect solution, but quite adequate in practice.

As well as for conventional exposure determination, the instruction book gives information on copying, using the SEI as an enlarging exposure meter, using the foot-Lambert scale (use the screw on the tapered collar as an index), and determining reflection factors. There were also accessories for densitometry, though I have never seen them. As well as being a sublime tool, it is a superlative gadget.

What, then, are the disadvantages of this paragon among spot meters, with its enormous range, tiny measuring angle, easy close focusing, high precision, and unparalleled build quality? Broadly, they fall into three groups: commercial, convenience, and antiquity.

From a commercial viewpoint, the most obvious is that all those switchable filters, sliders, rheostats, calibration meters, and the like mean that it was terrifyingly expensive to make. This must have contributed to its demise, as far as I know in the 1960s. Furthermore, it relies on a bulb which is a consumable rather than a component.

Moving on to convenience, it's slow to use, requiring two hands, and while the view on the red scale is big and bright, and the view on the white scale is adequate, the very heavy ND on the blue scale makes things pretty murky. Another practical problem is that the image in the viewfinder is upside down.

The scales on the meter are very clear and easily legible, but there are a lot of them, and the shutter speeds are idiosyncratic, with many marked only by lines. On the blue scale, for example, you have 1/100, line (1/125), line (1/160), 1/200, line (1/250), 1/320, line (1/400), 1/500, line (1/650), line (1/800), 1/1000, line (1/1250), 1/1500... The red scale is real fun with such readings as 2h. 47", 2h 13", 1h. 45"...13'20", 10'40", 8'10", and more.

It's up to you to choose the nearest speed on your shutter, but with 1/3 stop aperture settings from f/1 to f/32 and well over 100 shutter speeds, running from the aforementioned 2 hours, 47 minutes to 1/500,000 sec, you have plenty of choice. Such precision takes quite some time to read and interpret.

The third category of drawbacks relate simply to its age. The bulbs are no longer available, though this may be surmountable as noted. The obsolete speed scale is a nuisance, but you can easily use the "crib" above, and it looks as if it should be possible to slip off the transparent protective cover, fit a new ISO scale (the scale itself is on a strip of paper), and then refit the cover.

For reasons I have never been able to fathom, even the original instruction book tells you to remove the paper sleeve from the battery--it won't fit otherwise--before cleaning the contacts with emery cloth and smearing them with petroleum jelly. The original U2 size is similar to the modern D cell and I found that a Sanyo D cell worked, after I had stripped off the wrapper and (still more carefully) the glue which would otherwise have stuck the cell fast in the meter--though the locating hole in the top of the battery compartment would allow you to poke it out if necessary.

Finally, the incredibly complex assortment of mechanisms may be damaged: I had to solder the finger back on the coil-wound calibration rheostat in the base. To a considerable extent, though, the SEI is so beautifully made that most parts can be made up and fitted by a sufficiently skilled repairer.

Is it usable today? Yes, subject to the earlier concerns about parts and repairs. Is it still the finest spot meter ever made? Quite possibly, if you are not in a hurry: it must be the meter par excellence for fine art landscape photographers, and reputedly, refurbished examples are still sought after in Hollywood. Modern meters are a lot more convenient, and unless you particularly need (for example) to be able to take readings at 6" without accessories, they are probably a better bet. The mystique of the SEI also ensures that prices of working meters stay high: rarely under $200-$300, quite often twice as much. Then again, a $45 Casio quartz watch is likely to keep better time than an antique Breguet mechanical watch at 100 or 1000 times the price; but both are more than accurate enough, and which would you rather own?

Share | |