[stevebrot]

I was looking at the Wikipedia article for the Pentax Spotmatic this morning and noticed that it has been flagged for missing citations. I also noticed that it is rather poorly written and organized (apologies to any forum members who have contributed to that wiki). I don't have adequate knowledge to do the edits myself, but I can make a few suggestions:

• Break the article into sections (General, history, models, legacy or some-such)
• Correct factual errors. I may be wrong, but I don't believe that any of the Spotmatic line ever supported center-weighted metering, despite numerous sources on the Web that say otherwise.
• Provide the missing references

Any takers?


Steve

[aoeu]

I agree that this needs help. Printed references posted on line will help.

[titrisol]

Is this any better?
Pentax Spotmatic - Camerapedia

[arnold]

Is this any better?
Pentax Spotmatic - Camerapedia

Sorry I didn't see the original, but question the battery information. My understanding is that a bridge circuit will allow 1.5volt batteries to be used.

[titrisol]

Yes you can use silver or even alkaline batteries

[stevebrot]

Sorry I didn't see the original, but question the battery information. My understanding is that a bridge circuit will allow 1.5volt batteries to be used.

You are correct. I reviewed the meter schematic for both the SP and SP II last night and they are equivalent*. Neither design should be voltage dependent. That is the problem with wikis and the Web in general. Inaccurate information takes on a life of its own and soon becomes what I refer to as "Internet Truth". The SP battery voltage restriction and meter type (average vs. center-weighted) are two good examples.


Steve

* Service manuals on pentax-manuals.com

[titrisol]

As I prepare to edit those pages, let's get some facts that are wrong:
- Batteries, originally 400 PX, but canbe replaced with any 1.5V
- Metering is average (not centerW)
- ???

[stevebrot]

As I prepare to edit those pages, let's get some facts that are wrong:
- Batteries, originally 400 PX, but canbe replaced with any 1.5V
- Metering is average (not centerW)
- ???

As a reference for the battery issue, you can use the service manuals on Pentax-manuals.com. I don't know what reference you can use for the metering system.


Steve

[titrisol]

Well, Herbert Kepler's book states about the metering system; etc

[wombat2go]

The circuit diagram is on last page, fig 9 of the service manual for product 23102
and also on page 57 of Keppler.
He says "All meters in Pentax cameras measure the total brightness of the entire ground glass."
These were 2 cadmium sulphide light dependent resistors, one each side of viewfinder.
It is not a pure Wheatstone bridge because the meter is biased to +3 microamp to center the needle.
So there will be some sensitivity to battery voltage - I don't know the resistor values but probably a fair amount of battery sensitivity
because the CdS ldrs will be sinking about that order in the lower light and smaller apertures.
The service manua specifies Mallory RM640 which was a 1.35V mercury cell and says the battery must be replaced at 1.30 volt to get correct exposure.

[stevebrot]

says the battery must be replaced at 1.30 volt to get correct exposure.

This is true and I take that to be the minimum potential needed to for accurate metering. The user manual says no less that 1.3v.

There is a another, equivalent schematic on the National Camera Technical Training SPII service manual (p. 23). It is essentially the same, but has better functional labeling and component representation and does not include the flash sync circuits.

http://www.pentax-manuals.com/markroberts/nat_cam_spii.pdf

As for the issue of low-light and small apertures, that is the bane of all stop-down TTL metering systems. The low sensitivity threshold for the meter may be reached in much brighter light than if the aperture were not in the way! This was (curiously) called the low limit for meter coupling even though it had nothing to do with coupling per se. Pentax included a chart in the user manual showing the combinations of aperture, shutter speed, and ASA that were beyond and above the meter's capability.


Steve

[titrisol]

Made a few changes and added an infobox to the right
Still have to break article into sections
suggestions welcome

[wombat2go]

added an infobox to the right

Titrisol, That is very good.

About the use of silver oxide batteries, they have a plateau of 1.55V compared to the 1.32 V of the original mercury cells.
Ref 5 is not citing silver oxide as a replacement as far I I read, and the diagram P23 shows it is not a balanced bridge, so will be affected by battery variation.
Perhaps the use could be qualified with a comment that accuracy might be affected especially at low apertures/low light.

I have seen some reference to a shottkey diode being put in a container with a AgO cell but I don't know if that will fit in a Spotmatic.
That would be a more accurate option if it is possible.

By the way I don't have direct experience with the Spotmatic because don't have one here. Perhaps I need one now!
However I restored a 1960's HoneyWell Pentax Exposure Meter by re-designing its electronics to slightly more modern tech - I put article on Apug DIY about that.
Unencumbered by an aperture, it works from EV18 to EV3; I just took a EV4 reading in the office, it said , at iso 400, f/8 and 1 second, and the exposure was very good on the K-01 at that setting

[stevebrot]

Perhaps the use could be qualified with a comment that accuracy might be affected especially at low apertures/low light.

The question is whether the effect is outside the spec'ed meter sensitivity range. I suppose I could do a little testing since I have both a Spotmatic II with silver cell and an accurate hand-held meter with great low-light sensitivity. EV 1 (100) at sensor is pretty easy to reach with most stop-down systems.

By the way I don't have direct experience with the Spotmatic because don't have one here. Perhaps I need one now!

Good idea.


Steve

[stevebrot]

Ok...here are some numbers based on a blank wall reading in my home. The published low end of the Spotmatic's meter range is 1s for f/16 @ ASA(ISO) 100. All readings at ASA(ISO) 100 and f/16.

Gossen Luna Lux (SPD cell) : 1s

Spotmatic II (CdS cell) : 1s
Pentax KX (SPD cell) : 1s
Ricoh XR-2s (CdS cell) : 2s
Pentax Super Program (SPD cell) : 1s
Ricoh XR7 (SPD cell) : 1.5s
Pentax K10D (SPD cell) : 1.5s
Pentax K10D (SPD, stop-down metered) : 2s *

* Thrown in for giggles. Stop-down metering on the K10D is known to be unreliable.


Steve