Question about Canon Photography

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I recently had my Canon FTb(n) 35mm camera fully refurbished and it's ready for another 40 years of service. But, how do I power the light meter? The camera uses 1.35 volt PX625 mercury oxide batteries which, as best as I can tell, no longer exist. What power options do I have, if any? Mercury cells were preferred for meter use because they maintained a relatively flat output voltage over the life of the battery, something that's needed for accurate light meter readings. They also lasted a long time (years), as a Cadmium Sulfide (CdS) light meter would only draw power when light hit it. In the dark, it didn't. Do I have any options (other then buying a hand held light meter :-þ )?

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  • Dad Cat Sep 12, 2010

    Obertelli gave me, what appears to be, an exhaustive list of options for powering my mercury battery dependent 35mm camera. Frankly, I'm surprised (but pleased) at the depth of this guru's knowledge.

    If I have to pick a nit, it would be Obertelli not including more information about why he (or she, I'm not sexist) believes Frans De Gruitjer product is technically superior to the MR9 adapter. I would still like to know.

    Not thinking of everything that might be pertinent to my question, I should add two more pieces of information.

    1. I live in the US.

    Does the ease of obtaining an MR9 adapter outweigh the benefits of Mr. De Gruitjer superior product?


    2. I shoot transparencies almost exclusively. Given this type of film's relatively narrow exposure latitude, I need the most accurate meter information I can get.

    Would an SR9 be good enough or would it be worth the trouble and expense to get a battery adapter from Mr. De Gruitjer?


    I would appreciate Guru Obertelli responding to my nit and answering my two follow-up questions so I'll have as complete an understanding of the pluses and minuses of my options as possible.

    But, on balance, I would have to rate Obertelli's response to my questions as EXCELLENT.
    I would give this guru a score of 97 out of 100.

    If my nit were addressed and my two follow up questions answered, I'd be inclined to give the three missed points, for a perfect score.

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  • Master
  • 3,006 Answers

You have a few options:-

1. If you shoot negatives, then just use the cheapest unstable voltage lithium equivalent (LR9/V625U). Negative film and the printing process has a wide exposure latitude and so most users won't notice any fault.

2. Use the silver oxide equivalent (S625PX) which has a stable output voltage of 1.55v which is higher than the 1.35v original. Again you can either rely on exposure latitude, or if you shoot transparencies you have the additional option of compensating by up to one or two stops as necessary by adjusting the film speed dial. As the output voltage is constant, so is the compensation for the life of the battery. Or you can get a camera technician to recalibrate the meter to suit the higher voltage. Compensating has a downside though and it's that you're effectively shifting the EV scale so you may lose some of the exposure range which your meter can cope with.

3. Use a Weincell MR625 Zinc/Air battery. This has the correct voltage but they don't last as long and cost more than the other batteries listed so far.

4. Use a CrisCam MR9 adaptor. This is a one off purchase of a voltage reducing adaptor which houses a silver oxide cell. You can move it from camera to camera, and as the voltage has been reduced rather than compensated for you retain the full EV range of your meter. If you live in North America it's cheaper to buy directly from the US manufacturer, but they charge really silly money for shipping overseas.

5. Although I have an MR9 adaptor, I've switched to using an excellent and technically superior alternative which is custom made to order by Frans De Gruitjer in the Netherlands. It's about half the price of the MR9 adaptor, has superior output characteristics which mean it's also suited to cameras with a high current drain. Or you can make your own, from scratch, using very comprehensive instructions he provides or make one using a kit he provides. If you buy them ready-built, then postage is included if ordering within Europe and they have a five year warranty. For current details on prices and availability, contact Frans at battery.adapter@orange.nl . There's no language barrier as he speaks perfect English. Unlike the CrisCam unit the adapter is supplied without a battery but you can obtain the correct SR44's easily elsewhere. Frans also allows purchasers three months to return the adapter for a refund for any reason at all.

Please note that many of the links are to a UK battery supplier. I'm not connected with them in any way, but if you're in the UK/Europe I highly recommend them. However, my reason for linking to them is solely for the comprehensive information they provide.

Good luck whatever you decide, and I trust that you will return the favour by taking a brief moment to rate my answer.

Posted on Sep 10, 2010

  • Obertelli
    Obertelli Sep 12, 2010

    Thanks for the kind feedback, dacat1, my ego has been thoroughly pampered!

    The technical superiorities of the De Gruitjer adapter over the CrisCam version are that the latter does not handle current drains higher than 200 microamperes very well and that it's design means that it's slightly taller than the PX625. The current drain characteristics may not be a problem on cameras which use the battery solely to power the meter (eg. Olympus OM1/OM1n) but Canon were one of the earliest manufacturers to cut the number of mechanical components to save money by using electronics. I honestly don't know what the current drain of your FTb is, but I prefer to have a truly universal adapter. The CrisCam unit fails in this respect as it can be highly inaccurate on models such as the Nikon F. The CrisCam model also places it's electronics in the bottom of the adapter beneath the Silver Oxide SR44, whereas De Gruitjer's design has the circuitry around the battery to maintain the original height. The difference is not great, just +0.38mm (0.015") but it's still enough that it won't correctly fit some battery chambers, especially the ones with unsprung contacts which depend on the battery being held in contact solely by screwing down the battery cover. The link to De Gruitjer's article in my original answer provides far more detail and also some output graphs, but be careful how you read the graphs! The author provides a few caveats regarding their interpretation.

    If the CrisCam adapter fits your camera, and the camera does not draw more than 200 microamperes then there is no problem especially if you never anticipate an need to use the adapter in other PX625 situations. But if you email De Gruitjer for the cost of shipping to the USA you may find that it's worth ordering a few and selling the excess to other US residents, especially Nikon F series owners.

    As for exposure accuracy, the most accurate replacement for the mercury cell is the zinc-air Weincell, but you absolutely must ensure that the air holes are not blocked and you will have to drill holes of a similar area to those on the cell in the battery cover. They don't last as long as the silver oxide cells, and don't achieve full voltage for a few minutes after replacement or even longer in cold weather when they may not achieve full output at all. To be fair, the silver oxide and adapter option is also temperature sensitive, but seems to be far less so. I shoot transparencies on my Olympus 35RC, Vivitar 35ES, Pen FT and OM1n and so far have been delighted with the results of the De Gruitjer item (on the 35ES I soldered the components in line with the battery wires as a permanent fixture). As I scan all of my transparencies I get a comparable exposure latitude to that of negative film, so I tend to overexpose slightly to maintain shadow detail and recover the blown out highlights with software afterwards. It's incredible how much detail I've found even in old and badly-faded negatives which were originally badly overexposed. But to bring matters back to the point, only the original mercury cell is 100% correct, but I find that the silver oxide cell with a voltage reduction (preferably Dutch) is by far the best compromise and that the minor exposure errors are not a problem if images are digitised afterwards.




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