Spotmatic F: Modern batteries with higher current ISO compensation for accurate meter?


Link Posted 01/05/2015 - 19:01
Just been reading that with the modern batteries that the increased current will lead to over exposure if using the meter. Now to compensate for this should I as I have seen suggested adjust the ISO/ASA rating higher on the camera? I.E. I have 100 ISO film, set the camera ISO to 200 to get a more accurate reading?

I may be utterly wrong of course. And I should be able to Sunny 16 by now of course, but it's good for a guide.
K5, K200 and several film Pentax cameras!


Link Posted 01/05/2015 - 19:12
You should be fine with those cameras that take silver oxide cells, such as the ME Super, MX and later. The ones that take mercury 1.35V cells should, according to the literature, be fitted with Wein cells that are also 1.35V. Don't forget these cells need activation before use. remove the tab and wait 30 minutes before using.

Alternatively, just change the ISO/ASA setting to give the exposure you require. If your camera under-exposes, reduce the value. If it over exposes, increase the value.
Best regards, John


Link Posted 01/05/2015 - 20:19
I've been using the 1.5V 625 cells. Generally fairly accurate but maybe slightly over exposing. Will use a digital or the Super A to double check.
K5, K200 and several film Pentax cameras!


Link Posted 01/05/2015 - 20:29
It depends on the film, for mono or colour print I wouldn't worry about it, for slide film maybe do some tests and adjust accordingly. The meters in these old cameras (Spotmatics) were crude anyway and the metering cells have probably lost some sensitivity by now as well.
PPG Flickr


Link Posted 01/05/2015 - 23:13
I don't think you should have any problem with the Spotmatic F meter, when using SR44 cells. These are the same ones used in the K and M series cameras.

Info from Yahoo Spotmatic group.
Spotmatic Battery Information
The most experienced members of the Spotmatic group have compiled the information below and many of them have been using Spotmatics since they purchased them brand new in the sixties and seventies. This information is being provided so that you do not have to ask about it as a discussion topic on the forum. It has already been discussed and answered many times and there is nothing more to add so please do not ask again. All you need to know is contained in this document.

For battery cap information go to the bottom of this document.

In relation to batteries, there are 5 groups of Spotmatic cameras:
1. SP Series. This includes SP, SP500, SP1000, SPII and SPIIA.
2. SP F
3. Electro Spotmatic and ES
4. ESII.
5. SL. This camera has no meter and, thus, no battery.

SP Series
The 1.35-volt mercury (mercuric oxide) battery originally used in the Spotmatic SP is no longer available due to environmental protection laws.
Replacement batteries available today are 1.55-volt but the Spotmatic uses a "bridge" circuit in its meter, making it insensitive to battery voltage variations. So, despite what you may have read - No meter recalibration is necessary as all Spotmatic SP series cameras can handle the increased voltage.

The alternatives are:
Spend about $30 for an adapter from CRIS Camera Services (Camera Repair by CRIS Camera Services: Digital Camera Repair: Canon, Fuji, Nikon, Pentax, Ricoh, Sigma) and a battery to insert into it. But this would be a waste of money as it is definitely not required.
Spend about $10 for a Wein Products MRB400 cell (modified Zinc-Air cell). They are quite expensive and have a limited life (about 6 to 12 months) unless you remove the battery after each use and seal up its air hole (it needs air to work). This is not a recommended solution.
Spend about $3.00 for one of the many 1.55-volt silver oxide batteries available over the Internet, at Radio Shack, most drugstores, camera stores and even some supermarkets. They will last you for about three years if our experience is typical. Make sure you insert it the right way for proper polarity, and take up the excess space in the battery box with a rubber O-ring from the hardware store, that will cost you about 49 cents. You might have to bend up the battery contact in the battery box to make good contact.
Silver oxide batteries that work are: Mallory PX-400 or RM-400-R, Varta V394, Maxell SR936W or SR936SW, Renata 394, or any type 392 equivalent.
Do not be tempted to use alkaline battery equivalents as they have a much steeper discharge curve. The silver oxide batteries have a flat discharge curve similar to the old mercury batteries.

Mallory PX625, Duracell PX625A, Excel Z625PX, Eveready E-625N.
The Spotmatic F uses a dual coil meter movement that, like the bridge circuit, cancels out battery voltage variations on the level we are dealing with here. So it is immune to the difference between 1.35v and 1.5v, too. The original battery for the Spot F was a mercury type. In this case, Pentax used the identical circuit in the KM and K1000, without any electronic changes. They substituted the silver types with NO ALTERATION at all.

Electro Spotmatic and ES.
6-volt Eveready #544, Duracell 28L Lithium.

Eveready S76E, Mallory MS-76H, in fact any 76-type battery works -A76, S76, MS76.

Read more at:

My Spotmatic metering works fine with SR44's, compared to my DSLR meter and a hand held meter. The older, pre-Spotmatic meters do need the Wein cells to meter correctly.


Link Posted 02/05/2015 - 06:13
Very interesting. Thanks for that! So the SPF should operate ok. Grand. I don't expect perfect metering and the shutter speeds are probably a little off anyway (one day I must get it serviced, but am wary of posting it as it does have sentimental value).
K5, K200 and several film Pentax cameras!


Link Posted 02/05/2015 - 07:52
The meter in my SP1000 is not accurate. It tends to under-expose by a stop but it's not consistent either, so I don't use it and stick to sunny-16. The meter in my SP500 works fine but I rarely use it either.

Wein cells are a waste of time and money - they expire in no time even if not activated and they cost a fortune. Don't bother.
Pentax hybrid user - Digital K3 & K200D, film 645 and 35mm SLR and Pentax (&other) lenses adapted to Fuji X digital
Fan of DA limited and old manual lenses


Link Posted 02/05/2015 - 08:13
The only thing about that Spotmatic group info is that it comes across as quite tetchy and dismissive. I've read the discussion re the Spotmatic bridge circuit before, and it makes me wonder how the previous 20 years or so of people saying the opposite can be reconciled with this.

Of course, the only way to be sure is to try it and compare results. Whether or not the meters are still accurate enough to make this meaningful is another matter!

Best regards, John


Link Posted 03/05/2015 - 07:46
With comparison to the Super A lightmeter (this has had a Harrow Technical CLA in the last few years so should be ok) the Spotmatic F gave similar results. Well as close as can be expected really.

Quite pleased as both cameras are really nice to use and hope the results turn out ok!
K5, K200 and several film Pentax cameras!


Link Posted 25/05/2015 - 00:19
I use SR44s and LR44s in my two Spottie Fs and I have no problem of under/overexposure.

My SP1000 still has the original battery I bought the camera with and works well even if I tend to use it meterless and stopping down the lens is a boring operation for me.


Link Posted 25/05/2015 - 13:31
I used silver oxide cells in my Spotmatic for several years without problems. My Spotmatic is one of the original ones from the 1960s and the metering system is a null-balance one, similar to a Wheatstone bridge. One adjusts aperture or shutter speed to bring the needle to the centre, where no current is flowing in the bridge, therefore battery voltage is irrelevant. I had to get used to the reverse polarity compared to the original mercury cells, but the voltage has no effect because one is bringing the needle of the meter to show balance between positive and negative.
Do the later models of Spotmatic use a different metering method?
A few of my photographs in flickr.
Lizars 1910 "Challenge" quarter-plate camera; and some more recent stuff.
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