[pathdoc]

Some models of Helios 44-3 are non-standard, having a focus ring that extends too far back. Mamiya SX lenses have issues too, and late Fujica IIRC. Anything that has extra bits for open aperture metering...

Informative. Thank you.

---------- Post added 02-09-15 at 15:46 ----------

NEVER had a single undersexposure

Shouldn't that be "oversexposure"? Best typo ever.

[Cuthbert]

There is no issue with any lens that has a standard M42 mount. Some models of Helios 44-3 are non-standard, having a focus ring that extends too far back. Mamiya SX lenses have issues too, and late Fujica IIRC. Anything that has extra bits for open aperture metering...

Sorry to correct you but I bent the pin of an Helios 44m when I was trying to screw it on one of my SPF.

Clever! The next logical step would be to transmit data (and somehow, power) the other way. "Now, introducing the world's first M42 autofocus lens with program aperture control!" Or "Our new micro 4/3 adapter allows saving your Praktica lens's aperture setting into the EXIF data." Now THAT would be handy!



Something to be aware of, if I ever get one of these. What's the issue, and am I better off getting a different mark of Spotmatic?

This is the tab of a Fujinon lens:



This is the electric system of the high end Prakticas:

[pathdoc]

This is the electric system of the high end Prakticas:

This is probably what a Pentax mount would have looked like if they had stuck with screw mounts through to the Pentax-A era and beyond! The possibilities look interesting.

[Cuthbert]

This is probably what a Pentax mount would have looked like if they had stuck with screw mounts through to the Pentax-A era and beyond! The possibilities look interesting.

Pentacon invented this system in 1969, probably Pentax and other had a good look at it in later years.

[boriscleto]

Sorry to correct you but I bent the pin of an Helios 44m when I was trying to screw it on one of my SPF.

The pin of the 44m seems to hit the meter tab about half way on. The lens never screws all the way in. Never tried it before, so the 44m doesn't have a standard mount. No big loss as the 44m doesn't have the swirly bokeh anyway...

I suppose I could trim a little off the aperture pin...

[g026r]

Don't know about the F, but I have had some issues with my Spotmatic and regular m42 lenses. The one that sticks in my mind was the '50s CZJ Tessar, which would clip the back of the lens on the mirror when shot at infinity focus.

(The problem was the metal collar around the rear element that sits back past the mount when the lens is attached to the camera.)

[Ontarian50]

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.

[titrisol]

Well, I learned to take pictures with a Spotmatic, and my dad used silver batteries even in the late 70s on it.
In the early 00s I got hold of a couple of them for next to nothing, and have used silver oxide batteries with them.
I checked the ligtmeters to a Nikon FE and later to Digicams and always was within 1/2 stop of each other which is more than good enough

I think the sell-off during the digital boom was due to many factors, one of them was tha tpictures didn't look as vivid and as sharp as the digital ones more than anything.
By the early 2,000 the price of a camera went from a couple of months salary to a couple of weeks, and many people were happier with smaller cams that fit in their pockets.
Plus Spotties were 30yr old by then!
Ahhh good times

[pathdoc]

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.

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.

And then what happens? If the thing will still work properly apart from having no metering capability, I'm prepared to take my chances and guess the settings on Sunny Sixteen type rules.

[stevebrot]

Earlier references to a "bridging circuit" in Spotmatics making them impervious to battery voltage errors doesn't ring true with me.

Pentax Spotmatic meter circuit



It is not a true bridge because the circuit is not balanced when the needle is centered. For what it's worth, both my Spotmatics (SP, SPII) meter accurately at 1.5 volts, though to be honest, I have never done a comparison at EVs near the low end of the meter sensitivity.


Steve

[Cuthbert]

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.

Perhaps my previous post wasn't clear, I have THREE SPFs, ALL working. One was underexposing but it was a problem with the CdS, not the battery.

Two of them are Asahi and one Honeywell, and the reading is the same....for what I'm aware the circuit of the SPF is the same as the one used on the K1000 and KM, so it's very reliable.

Pentax Spotmatic meter circuit

Attachment 280846

It is not a true bridge because the circuit is not balanced when the needle is centered. For what it's worth, both my Spotmatics (SP, SPII) meter accurately at 1.5 volts, though to be honest, I have never done a comparison at EVs near the low end of the meter sensitivity.


Steve

Interesting, is the R3 the variable resistence used to set the ASA? However from what it's written in the middle the needle moves up and down with the current (ampere), not with the voltage, that's the reason why 1.2 or 1.5 Volt won't make any difference, or at least this is my understanding of this circuit.

For the K1000 I found this:

[stevebrot]

Interesting, is the R3 the variable resistence used to set the ASA?

No, I don't think so. It is my understanding that the ASA and shutter speed use the same variable resister. It is my understanding that R3 is used to trim the circuit, though I am not certain.

Yes, the meter measures current and not voltage, but the needle is centered* at 3 μA current through that leg of the circuit. As a result, the bridge is mildly unbalanced.


Steve

* The springs are slightly stronger on one side of the needle than the other.

[stevebrot]

Here is an old thread on the MF Lens forum discussing the Spotmatic meter and the ultimate fix...custom trimmed resistors!

International teamwork: Fixing Spotmatics


Note some familiar user names on the thread


Steve

[monochrome]

Here is an old thread on the MF Lens forum discussing the Spotmatic meter and the ultimate fix...custom trimmed resistors!

International teamwork: Fixing Spotmatics


Note some familiar user names on the thread


Steve

Good grief - is there anything you don't know?

[stevebrot]

As a result, the bridge is mildly unbalanced.

From the thread referenced above:

...Just a small note to say that the total current flowing through the meter circuit is a lot larger than through the coil. In fact the coil is centered at 3uA but the total current varies between 24uA (no light, meter set to 1s) and 260uA (lots of light, meter at 1s). With the needle centered, which is what matters, I measured ~60uA at low light and ~90uA with lots of light. So the circuit is close to a balanced bridge, and the current flowing through the coil is quite irrelevant...

Steve