Forgot Password
Pentax Camera Forums Home
 

Reply
Show Printable Version Search this Thread
12-19-2013, 01:17 PM   #1
Site Supporter
stevebrot's Avatar

Join Date: Mar 2007
Location: Vancouver (USA)
Photos: Gallery | Albums
Posts: 26,189
Spotmatic Wikipedia Article

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

12-19-2013, 06:29 PM   #2
Site Supporter




Join Date: Sep 2010
Posts: 456
I agree that this needs help. Printed references posted on line will help.
12-19-2013, 08:43 PM   #3
Pentaxian
titrisol's Avatar

Join Date: Dec 2007
Location: In the most populated state... state of denial
Photos: Gallery | Albums
Posts: 1,094
Is this any better?
Pentax Spotmatic - Camerapedia
12-20-2013, 02:54 AM   #4
Site Supporter
arnold's Avatar

Join Date: Aug 2012
Location: Queensland
Photos: Gallery | Albums
Posts: 3,193
QuoteOriginally posted by titrisol Quote
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.

12-20-2013, 07:15 AM   #5
Pentaxian
titrisol's Avatar

Join Date: Dec 2007
Location: In the most populated state... state of denial
Photos: Gallery | Albums
Posts: 1,094
Yes you can use silver or even alkaline batteries
12-20-2013, 11:11 AM   #6
Site Supporter
stevebrot's Avatar

Join Date: Mar 2007
Location: Vancouver (USA)
Photos: Gallery | Albums
Posts: 26,189
Original Poster
QuoteOriginally posted by arnold Quote
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
12-20-2013, 12:29 PM   #7
Pentaxian
titrisol's Avatar

Join Date: Dec 2007
Location: In the most populated state... state of denial
Photos: Gallery | Albums
Posts: 1,094
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)
- ???
12-20-2013, 09:16 PM   #8
Site Supporter
stevebrot's Avatar

Join Date: Mar 2007
Location: Vancouver (USA)
Photos: Gallery | Albums
Posts: 26,189
Original Poster
QuoteOriginally posted by titrisol Quote
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

12-21-2013, 08:54 AM   #9
Pentaxian
titrisol's Avatar

Join Date: Dec 2007
Location: In the most populated state... state of denial
Photos: Gallery | Albums
Posts: 1,094
Well, Herbert Kepler's book states about the metering system; etc
12-21-2013, 11:10 AM   #10
Site Supporter




Join Date: Jan 2010
Location: Michigan
Photos: Gallery
Posts: 2,072
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.
12-21-2013, 09:30 PM - 1 Like   #11
Site Supporter
stevebrot's Avatar

Join Date: Mar 2007
Location: Vancouver (USA)
Photos: Gallery | Albums
Posts: 26,189
Original Poster
QuoteOriginally posted by wombat2go Quote
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
12-21-2013, 10:16 PM   #12
Pentaxian
titrisol's Avatar

Join Date: Dec 2007
Location: In the most populated state... state of denial
Photos: Gallery | Albums
Posts: 1,094
Made a few changes and added an infobox to the right
Still have to break article into sections
suggestions welcome
12-22-2013, 07:52 AM   #13
Site Supporter




Join Date: Jan 2010
Location: Michigan
Photos: Gallery
Posts: 2,072
QuoteOriginally posted by titrisol Quote
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
12-22-2013, 11:11 AM   #14
Site Supporter
stevebrot's Avatar

Join Date: Mar 2007
Location: Vancouver (USA)
Photos: Gallery | Albums
Posts: 26,189
Original Poster
QuoteOriginally posted by wombat2go Quote
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.

QuoteOriginally posted by wombat2go Quote
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

Last edited by stevebrot; 12-22-2013 at 11:25 AM.
12-22-2013, 12:42 PM   #15
Site Supporter
stevebrot's Avatar

Join Date: Mar 2007
Location: Vancouver (USA)
Photos: Gallery | Albums
Posts: 26,189
Original Poster
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

Last edited by stevebrot; 12-22-2013 at 01:02 PM. Reason: Added KX to the mix
Reply

Bookmarks
  • Submit Thread to Facebook Facebook
  • Submit Thread to Twitter Twitter
  • Submit Thread to Digg Digg
Tags - Make this thread easier to find by adding keywords to it!
article, spotmatic, wikipedia
Thread Tools Search this Thread
Search this Thread:

Advanced Search


Similar Threads
Thread Thread Starter Forum Replies Last Post
For Sale - Sold: Black Spotmatic F, Silver Spotmatic F, Pentax AP, H1a, SV MightyMike Sold Items 36 02-11-2013 07:21 AM
Wikipedia Help Urgent benjikan General Talk 7 07-18-2011 04:34 PM
Wikipedia and the Blindingly Obvious Mike Cash General Talk 13 12-10-2009 03:56 AM
A quick thought about Wikipedia Steve Beswick General Talk 6 11-17-2009 07:09 PM
Spotmatic F Vs Spotmatic ES 3RiC Pentax Film SLR Discussion 14 05-26-2009 06:08 AM



All times are GMT -7. The time now is 04:59 PM. | See also: NikonForums.com, part of our network of photo forums!
  • Red (Default)
  • Green
  • Gray
  • Dark
  • Dark Yellow
  • Dark Blue
  • Old Red
  • Old Green
  • Old Gray
  • Dial-Up Style
Hello! It's great to see you back on the forum! Have you considered joining the community?
register
Creating a FREE ACCOUNT takes under a minute, removes ads, and lets you post! [Dismiss]
Top