Originally posted by Cuthbert The fact is that you're completely wrong, physics is not an opinion....and I've taken DOZENS of rolls of film that prove that. NEVER had a single undersexposure, not even with slide film.
Some "underexposed" shots:
The battery used is a Vart PX625A 0% mercury and lead.
Essentially a M42 lens without the aperture blade can be metered just stop down. Just three companies in the M42 era were able to invent a system to meter wide open: Pentax, Fuji and Praktica.
Pentax had the additional blade, Fuji invented an external pin that engaged with a slot in the bodies of high end cameras and the DDR guys used a ingenious electric system with resistances or capacitors that could be read by the body. All these systems are different and the SPF is also incompatible with some M42 lenses like the Soviet Helios.
Since you're using a V625A battery, I have to assume you are using a Spotmatic F - the only one, in my recollection that used that battery. The F has a much different circuit layout to the earlier Spotmatics - unfortunately many of those circuits are prone to failure (well, they're pretty old now) so you are fortunate to have one of the still operational ones.
But the majority of Spotmatics employ the smaller VX400 type battery, and the early 1960s circuitry was adjusted for 1.35V. It's electronics, not physics, at work here. As I said before, it's a simple matter (and I've done it many times) to compare readings between 1.35v and 1.5v cells in the same Spotmatic - about 1.5 stops difference.
Every now and again, you will come across a camera that reads just about spot on with a 1.5V battery, despite no likelihood of having been recalibrated in recent memory. In that case, it turns out the meter cells have weakened close to the right amount to be brought back in line with a stronger battery. In that case, however, you're likely on borrowed time, as the cells aren't getting any better.
On a sadder note, before the digital boom, many families were ditching their treasured old SLRs, including Spotmatics because "the pictures weren't as good as they used to be". Turns out what was happening, was they were going to the store to buy a new battery and were given a 1.5v equivalent by the sales clerk - both unaware that 1.35v mercury batteries were taken off the market. Because the battery looked the same, and was sold as an equivalent, they popped it in and went on vacation. Because they were shooting colour negative film, the resulting underexposure gave them prints with more grain and weaker colour than they expected. Not gruesome, but not what they were used to.