PX625 Alkaline Battery Tests: Observations and Questions

["C.M. Fortunko" <fortunko@boulder.nist.gov>]

Friends,

I have just come back from an afternoon of testing the performance of PX625
form-factor batteries in two Leica cameras: M5 and SL2, equipped with 50mm
Summicrons with proper hoods.

My experimental equipment was: Gossen LunaLux meter, Pentax Spotmatic
(analog) meter, Leica M6 (recently tested by Ernst Hartmann of Leica), M5,
SL2, and PX625 (mercury) and PX625U (alkaline) batteries from Varta. I also
had a tripod and two new gray cards from Kodak.

With the sun behing a big cloud I obtained the following results, assuming
100ASA film speed:

Pentax Spotmatic                  1/125sec, f8+half stop
Gossen LunaLux (Reflection Mode)  1/125sec, f5.6+half stop
Gossen LunaLux (Incident Mode)    1/125sec, f5.6+half stop
M5 (PX625 battery)                1/125sec, f5.6+half stop
M5 (PX625 U battery)              1/125sec, f8
SL2 (PX625 battery)               1/125sec, f5.6+half stop
SL2 (PX625 U battery)             1/125sec, f8
M6                                1/125sec, f8+half stop

What struck me right away is that the PX625 U alkaline batteries apppear to
produce meter readings within less than half a stop under diffuse, natural
light conditions. The results were identical to those I obtained last night
under artificial lighting.

The second thing that struck me was the difference between the M6
(supposedly in calibration) and the Pentax Spotmatic and the rest of the
measurements. They were a full stop off. This called for additional
investigation!

I waited patiently for the sky to clear. This took place at approximately
3:00PM, or 1500 military time. (I used my trusty mechanical watch, although
the NIST atomic clock is nearby.)

I then applied the "sunny sixteen" rule, which should hold since the sun is
a well-regulated and universal light source. Sure enough, my Gossen LunaLux,
as well as the M5 and SL2 meters directed me to 1/125sec at f16. On the
other hand, the Pentax Spotmatic and M6 meters were telling me to set at
1/125sec and f16. I anticipated this results, but wanted direct proof. Also,
I was testing the linearity of the various metering systems.

I just could not believe that the M6 could be in error. After all, DIN
stands for Deutsche-Invented Number and cannot be wrong. My Pentax has a
Zone VI scale, although I bought it used and don't know if it actually has
Fred Picker's improvements inside.(You know, like Intel inside, etc.)

Well, I went to test the M6 against a brand-new M6 at Jones Drug and Camera.
(Yes, in Boulder they sell Leicas in drug stores.) Unfortunately, the new M6
gave me identical meter readings to my "reference" M6!

I then went to Mike's Camera, which sells both Pentax meters: analog and
digital. Here, I had more luck. I discovered that my Spotmatic was one stop
off (really, 1 EV off) relative to the new meters, which are being sold as
new. This made me feel much better, but not fully satisfied.

My preliminary conclusion is that the M5 and SL2 cameras appear to work OK
with the PX625 U alkaline batteries. Resetting the dial is preferable to
sending the cameras to Leica NJ. I don't know what will happen as such
batteries age. The PX625 (mercury) are known to age gracefully and die
without warning. What are the discharge characteristics of alkalines?

One of the LUGgers pointed out that the Leica meters may have some sort of a
regulator inside. This may be so. I am tempted to open up the SL2 this
evening. However, I may not be qualified for performing such operations.
Next week, I will call John van Stelten.

However, questions remain. For example, why is the meter of the M6 one stop
off. Could it be that it is not calibrated to the 18%, gray-card standard. I
have heard that some camera companies do this to insure a higher percentage
of good photos and, therefore, greater customer satisfaction. Clearly, more
work needs to be done in this area to resolve this question. I have been
brainwashed to believe that the M6 is an instrument. Therefore, I have come
to expect that its meter would be calibrated to the 18% standard.

Also, it may be good to run a "round robin" test comparing the performance
of the PX625 with PX625 A and PX625 U as well as the new zinc-oxide (Wein)
battery. 

I would appreciate all thoughts you may have on this matter. Please feel
free to comment. I am not fragile.

Also, if anyone has a bad MR4, please let me have it for an autopsy.

Pax et Looks,

Chris