[Baard-Einar]

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I have a Spotmatic SP500 and I put a SR936 battery on it

There's an issue, though. The needle of the lightmeter moves up when I put a slower shutter speed, but it doesn't move if I open the lens.
So the lightmeter only seems to be sensitive to the shutter speed - but even so, it has a weird behaviour - at 1/8s, the needle is at the center, but if I move to 1/4th, the needle immediately jumps down to the bottom

What is the issue here?

To meter you need to push the meter switch upwards. Then check to see if the lens stop down, either by looking in the viewfinder to see if the image gets darker. It not you migh have a problem with the aperture mechanism in the lens.

[fog.photo]

To meter you need to push the meter switch upwards. Then check to see if the lens stop down, either by looking in the viewfinder to see if the image gets darker. It not you migh have a problem with the aperture mechanism in the lens.

The stop-down mechanism on the lens does work - the pin can be pushed in with the lens off the body and the viewfinder does get darker if I step the lens down

However, the viewfinder always gets darker whether I use the automatic or manual mode on the lens

[Baard-Einar]

The stop-down mechanism on the lens does work - the pin can be pushed in with the lens off the body and the viewfinder does get darker if I step the lens down

However, the viewfinder always gets darker whether I use the automatic or manual mode on the lens

Does the Auto/manual switch work when the camera is off the body? Place it in automatic, look through the lens. Is it open or does it close down as you turn the aperture ring?

[Papa_Joe]

The stop-down mechanism on the lens does work - the pin can be pushed in with the lens off the body and the viewfinder does get darker if I step the lens down
However, the viewfinder always gets darker whether I use the automatic or manual mode on the lens

The auto mode on the lens keeps the aperture open, when you stop down. In the manual mode of the lens, the aperture will close when you stop down.
To meter with the SP500 you have, as has been said, to push up the meter switch. The lens will then close the aperture in auto mode, so that the meter reading is always taken with the lens stopped down.

So far theory. Now, to your issue. You wrote, the SP 50o does not respond to the aperture being opend up.

The meter should always respond to the amount of light falling through the lens. The viewfinder should get lighter or darker, when you turn the aperture ring on the lens. Provided the meter is switched on of course. If the amount of light does not change, you have an issue with the aperture mechanism. If the amount of light does change, you have an issue with the metering system.


However be sure, that the meter switch is really al the way up to the top. Only then, the meter will be switched on properly. Also if the lens is in auto mode and the meter switch is not pushed up all the way, the lens might not stop down fully, such you might get a false reading i.e. f 11 instead of f 16. To test the meter I would suggest to switch the lens to manual mode, as then lens will work the aperture independent of the body an you will avoid issues with the stop down pin not working properly.


If the lens is stopping dow properly while metering and the meter does not respond, you probably have issues with the electric components of the meter. As you wrote, the meter jumps immedatedly down when changing from 1/8 to 1/4 I would guess this is the case.

[mikeavison]

If Rayallen and his Spotmatic group are correct that the Spotmatic F uses a dual coil meter then the 1.55 V battery should work fine. This would be equivalent to a system using a balanced bridge circuit (e.g. a bridge circuit where the correct reading coincides with zero current flow through the meter.) This is the case because in a dual coil meter (compared with the more common single coil and spring meter) the force pushing the needle down is provided by a current obtained from the cell (via resistors), and the force pushing the needle up is provided by the current from the cell flowing through the CDS light sensor. Ultimately both these forces scale linearly with cell voltage so any variation in cell voltage is cancelled out. Incidentally I once had a lovely GMC Blazer who's fuel gauge was dual coil and the advantage in this case was that when you turned off the ignition both the up and down forces on the needle went to zero so the gauge read the correct level when you jumped in, even with the ignition off. In the case of a conventional single coil and spring meter the up force on the needle comes from the input current (depending on cell voltage) and the downward force depends on a spring , independent of cell voltage, so changes in cell voltage do not cancel out.

Since everyone seems to agree that the bridge circuit for the the group of cameras including the Spotmatic SP, SP500, SP1000, SPII and SPIIA is not balanced (the needle is in the mid position when 50 micro A is flowing), then this circuit will be sensitive to battery voltage and some adjustment is likely to be necessary to get the most accurate possible exposure when gong from a 1.35V cell to a 1.55V cell. It is a shame that Pentax did not fit a centre-zero reading meter then no adjustment would have been necessary.

I have just done a quick comparison between

• Spotmatic 1000 camera (with silver oxide cell 1.55V)

• Minette selenium cell light meter (very old and suspect)

• Sony digital bridge camera (cheaper type of digital)

• Fujifilm XT-100 mirrorless camera (higher quality model than the Sony)

The results were that the Lightmeter and Fuji agreed within 1/2 stop, The Sony and the Spotmatic indicated there was one stop more light (so would have led to a one stop underexposure on film).

This is not very conclusive. However if you halve the ASA setting on the Spotmatic dial (as widely advised on this forum and elsewhere) then it agrees with the Fuji and the selenium cell lightmeter, leaving only the "consumer" grade Sony camera out of line. Make of that what you will.

I may rig up a calibrated power supply later to confirm and quantify how much error is introduced by going from a 1.35V supply to a 1.55V supply, on my Spotmatic 1000. If I do I will report back.

Or I might just pick up a second hand Spotmatic F haha.

Thanks to everyone for their contributions, especially Rayallen and the Spotmatic group.

Mike Avison

Mike Avison

[Roger1954]

Hi Everyone

Like many of you I have been having trouble finding replacement batteries for my Pentax SP cameras.

I did a lot of searching and came up with a battery that fits and works fine for the SP range. These are not for the Spotmatic F which takes a different battery. These are for the SP, SP11 etc. The SP cameras used to take a 3.5v battery. The Renata batteries are 1.55v. I do believe that after reading other posts on this forum that the SP range has a voltage regulator and can use the 1.55v batteries without any issue. It appears to be correct as I am having no problems with the SP meters in cameras where I have installed them.

The battery is the Renata 390 it has a code name of SR1130SW. The battery size is 11.6mm X 3.1mm. It is a 1.55 volt silver oxide battery. If fits perfectly without any adapters and works as it should. For UK and EU buyers you can get them here on Amazon for £2.97 for two. I am sure they would work out cheaper if you buy a pack of ten. I bought two only as I wanted to see if they fitted and worked properly.

Renata Silver Oxide Swiss Made Watch Battery - 390 (SR1130SW): Amazon.co.uk: Electronics & Photo

In the US and other areas you may need to track the battery type down by searching online.

I am currently testing the meters in two of my Spotmatics with these batteries. I will update the forum when I have done some tests against other meters and shot a roll of film.

Regards

Roger1954

[Baard-Einar]

Hi

THanks for sharing. I will get some here in Norway to test out. Price about 2,9€ for two.
SR1130SW Sølvoksidbatteri, 2 stk. - Biltema.no

[whojammyflip]

Why not just buy the dedicated batteries from Weincell? Scanning this thread, it seems to me that "the game is not worth the candle", trying to buy different batteries which may not work.

I've just bought one MR400 battery from the Small Battery Company. GBP 6.5 delivered. I figure that the cost of my time far exceeds the GBP 3.50 saved on buying batteries which are not dedicated. If it runs low in 3 months, I will just buy another.

https://www.smallbattery.company.org.uk/sbc_mrb400.htm

[Roger1954]

Why not just buy the dedicated batteries from Weincell? Scanning this thread, it seems to me that "the game is not worth the candle", trying to buy different batteries which may not work.

I've just bought one MR400 battery from the Small Battery Company. GBP 6.5 delivered. I figure that the cost of my time far exceeds the GBP 3.50 saved on buying batteries which are not dedicated. If it runs low in 3 months, I will just buy another.

https://www.smallbattery.company.org.uk/sbc_mrb400.htm

HI there

Wasn't a game at all. The Renata batteries that I bought fit without any adapters and work perfectly with no problems. You can buy 2 for a few pence shy of £3.00. There isn't anything gained by paying £5.00 more per battery for zinc air batteries that start to drain as soon as you install them and continue draining whether you are using the camera meter or not. The Renata silver oxide batteries work as good as any, better than most and are reliable and long lasting.

Don't take my word for it buy a couple for £2.97 and try them for yourself.

Renata Silver Oxide Swiss Made Watch Battery - 390 (SR1130SW): Amazon.co.uk: Electronics & Photo

Read more at: https://www.pentaxforums.com/forums/8-film-slrs-compact-film-cameras/135112-...#ixzz7isBoTF00

[mikeavison]

HI there

Wasn't a game at all. The Renata batteries that I bought fit without any adapters and work perfectly with no problems. You can buy 2 for a few pence shy of £3.00. There isn't anything gained by paying £5.00 more per battery for zinc air batteries that start to drain as soon as you install them and continue draining whether you are using the camera meter or not. The Renata silver oxide batteries work as good as any, better than most and are reliable and long lasting.

Don't take my word for it buy a couple for £2.97 and try them for yourself.

Renata Silver Oxide Swiss Made Watch Battery - 390 (SR1130SW): Amazon.co.uk: Electronics & Photo

Read more at: Asahi spotmatic sp battery recommendation - Page 3 - PentaxForums.com

Hi Roger, have you checked the accuracy of exposure. My SP1000 over reads by about 1 stop (so under exposes) with silver the oxide cell, So I just set half the ISO.

[Roger1954]

Hi Roger, have you checked the accuracy of exposure. My SP1000 over reads by about 1 stop (so under exposes) with silver the oxide cell, So I just set half the ISO.

Hi Mike

I am putting a roll of film through one of my SP1000s and also testing the meter against a full frame digital camera, both with 50mm 1.8 lenses on. I will update you with the results of the testing once I have had the film developed. If like you I find that the meter consistently overexposes by 1 stop I can live with that by adjusting the ISO like you have..

TBH I wouldn't totally rely on the old selenium cells in the SP or any other vintage camera that is close to 60 years old to be consistent. I have found information that says the selenium cells don't age unless exposed to the air which sometimes can happen if what they are sealed in deteriorates. I will do some tests as I have a number of SPs that I have collected over the years. I will update you when I have done.

Roger

[whojammyflip]

HI there

Wasn't a game at all. The Renata batteries that I bought fit without any adapters and work perfectly with no problems. You can buy 2 for a few pence shy of £3.00. There isn't anything gained by paying £5.00 more per battery for zinc air batteries that start to drain as soon as you install them and continue draining whether you are using the camera meter or not. The Renata silver oxide batteries work as good as any, better than most and are reliable and long lasting.

Don't take my word for it buy a couple for £2.97 and try them for yourself.

Renata Silver Oxide Swiss Made Watch Battery - 390 (SR1130SW): Amazon.co.uk: Electronics & Photo

Read more at: Asahi spotmatic sp battery recommendation - Page 3 - PentaxForums.com

I've just bought some, and will revert with a comparison of the two.

The Weincell has a rubber ring around it, which can move up and down the battery - I had to reverse the polarity, so that the + terminal faces the body and the - terminal faces the cap....metering is spot on. Surprisingly senstive too, so that I am metering 1/4s f4 @100 indoors....the Sekonic L208 is getting towards the edge of that, so that it nearly cannot measure that light value. Awesome piece of kit. It looks like the Spotmatic I have here has barely been used, the Super Takumar 55/1.8 is spotless. Looking forward to putting in some bulk film and taking some shots.

[mikeavison]

Hi Mike

I am putting a roll of film through one of my SP1000s and also testing the meter against a full frame digital camera, both with 50mm 1.8 lenses on. I will update you with the results of the testing once I have had the film developed. If like you I find that the meter consistently overexposes by 1 stop I can live with that by adjusting the ISO like you have..

TBH I wouldn't totally rely on the old selenium cells in the SP or any other vintage camera that is close to 60 years old to be consistent. I have found information that says the selenium cells don't age unless exposed to the air which sometimes can happen if what they are sealed in deteriorates. I will do some tests as I have a number of SPs that I have collected over the years. I will update you when I have done.

Roger

HI Roger, that is good I look forward to hearing about your results. Just a couple of points to note, like most SLRs of that era the whole SP range used cadmium sulphide (CdS) photo-resistors as sensors , not selenium cells. Selenium cells are mostly found on rangefinder and viewfinder cameras either around the lens or on the top panel near the viewfinder. You can identify them at a glance because they virtually always have a plastic compound lens in front of them. Selenium cells generate their own current so many such cameras had auto-exposure mode but did not require any battery - pretty neat. But the selenium cells themselves are much larger than a CdS photo-resistor so could not realistically be incorporated into the pentaprism of an SLR. In my experience CdS photoresistors do not degenerate like selenium cells do. To be honest i have never read anything that confirms or refutes this, it is just experience. So I think that with a constant and correct voltage supply from a battery, all your SPs will operate just fine. Later SLRs used photodiodes which also no not usually degenerate like selenium cells.

BTW you said "overexpose" in relation to the silver oxide battery but in mine the meter over-reads which leads you underexpose the film. As you know for negative film underexposing by one stop gives less satisfactory results than over-exposing by one stop.

Finally (and you might well disagree with me) I am not very confident about using a digital camera as a lightmeter. The reason being that as long as the picture is fine the makers of the camera do not really need to worry about what actual exposure factors are indicated (since you do not transfer those factors anywhere else.) In other words the sensor might be a bit more sensitive than the ISO setting indicates and the meter might have a corresponding under-read that yields perfect results. But if you transfer the meter readings to a different sensor (film camera) it will give the wrong exposure. I don't know for a fact that this occurs but I am cautious - it might. If I was wanting to test an SP I would much rather wait for a sunny day and use sunny 16 rule, (unless a proper light meter of known good accuracy is available.)

Good luck with your test exposures, I hope it goes well and look forward to reading about it.

[Roger1954]

I've just bought some, and will revert with a comparison of the two.

The Weincell has a rubber ring around it, which can move up and down the battery - I had to reverse the polarity, so that the + terminal faces the body and the - terminal faces the cap....metering is spot on. Surprisingly senstive too, so that I am metering 1/4s f4 @100 indoors....the Sekonic L208 is getting towards the edge of that, so that it nearly cannot measure that light value. Awesome piece of kit. It looks like the Spotmatic I have here has barely been used, the Super Takumar 55/1.8 is spotless. Looking forward to putting in some bulk film and taking some shots.

Hi there

Let us know what you make of the Renata batteries. I have just read a post by Mikeavision saying that the 1.5v batteries will make the meter in the SP overexpose. I did some tests and this is correct but it is consistent. I have done as Mike has by adjusting the ISO by one stop. Camera has 200 ISO film in but is set to 100 ISO. Now the metering matches 2 other cameras that I have tested the SP against. I changed the other cameras to centre weighted metering for the test. The cameras I used are a Nikon D700 and older Nikon F80. All with 50mm 1.8 lenses.
With the Pentax SP set with the ISO reduced by one stop, all 3 cameras expose exactly the same. I would be happy to make the adjustment on the SP to reduce the ISO by one stop and have the silver oxide batteries that will last a very long time.

Let us know how you get on and if you need to make the same compensation.

PS. Sounds like a very nice SP you have. The 55 1.8 lens is awesome. I use my one on my Nikon DSLR's, an older Olympus EM-1 M4/3 mirrorless and the older Fuji XT-1 with adapters. The images are stunning with that 55mm 1.8. Have you tried yours on any digital camera ?

Regards

Roger

---------- Post added 10-30-22 at 05:37 AM ----------

HI Roger, that is good I look forward to hearing about your results. Just a couple of points to note, like most SLRs of that era the whole SP range used cadmium sulphide (CdS) photo-resistors as sensors , not selenium cells. Selenium cells are mostly found on rangefinder and viewfinder cameras either around the lens or on the top panel near the viewfinder. You can identify them at a glance because they virtually always have a plastic compound lens in front of them. Selenium cells generate their own current so many such cameras had auto-exposure mode but did not require any battery - pretty neat. But the selenium cells themselves are much larger than a CdS photo-resistor so could not realistically be incorporated into the pentaprism of an SLR. In my experience CdS photoresistors do not degenerate like selenium cells do. To be honest i have never read anything that confirms or refutes this, it is just experience. So I think that with a constant and correct voltage supply from a battery, all your SPs will operate just fine. Later SLRs used photodiodes which also no not usually degenerate like selenium cells.

BTW you said "overexpose" in relation to the silver oxide battery but in mine the meter over-reads which leads you underexpose the film. As you know for negative film underexposing by one stop gives less satisfactory results than over-exposing by one stop.

Finally (and you might well disagree with me) I am not very confident about using a digital camera as a lightmeter. The reason being that as long as the picture is fine the makers of the camera do not really need to worry about what actual exposure factors are indicated (since you do not transfer those factors anywhere else.) In other words the sensor might be a bit more sensitive than the ISO setting indicates and the meter might have a corresponding under-read that yields perfect results. But if you transfer the meter readings to a different sensor (film camera) it will give the wrong exposure. I don't know for a fact that this occurs but I am cautious - it might. If I was wanting to test an SP I would much rather wait for a sunny day and use sunny 16 rule, (unless a proper light meter of known good accuracy is available.)

Good luck with your test exposures, I hope it goes well and look forward to reading about it.

Hello Mike

Yes the cells in the SP are cadmium.

I did some tests and the results are the same as you found. You have to reduce the ISO by one stop lower than the film in the camera to compensate for the reading difference.

I did the tests and found as you have that changing the ISO by one stop corrected the problem and is consistent. I have done as you have by adjusting the ISO by one stop. Camera has 200 ISO film in but is set to 100 ISO. Now the metering matches 2 other DSLR cameras that I have tested the SP against. I changed the other cameras to centre weighted metering for the test. The cameras I used are a Nikon D700 and older Nikon F80 all with 50mm 1.8 lenses.
With the Pentax SP set with the ISO reduced by one stop, all 3 cameras exposure readings are exactly the same. I would be happy to make the adjustment on the SP to reduce the ISO by one stop and have the 1.5v silver oxide batteries that will last a very long time.

Re: Using a digital camera as a light meter.

I found that using the cameras that I have noted above in centre weighted metering mode instead of matrix gave the same image reading of 125th at f5.6 across all of the cameras including the SP. I did notice that when I changed to matrix metering on the digitals it gave a reading of 60th at f5.6.

I am going to test a roll of transparency using the compensation of reducing the ISO on the SP. I will take the same images on the Nikon D700 DSLR and compare the results. I will update you on here when I have had the film developed. I will look into how I can share the images on here so that you can see the results.

Regards

Roger

[whojammyflip]

OK, so a couple of Renata 390 batteries just arrived, from Amazon, and they don't work in my Spotmatic at all. The needle doesnt move whatsoever. I'd highly recommend the Weincell if you have a camera body like mine. At the end of the day, its only been 3 quid to test the idea that the Renata battery works, and it concretely does not work in my camera. In contrast, the Weincell works perfectly.

The + terminal on the Renata has to face outwards, as the plastic insert inside my Spotmatic prevents turning them around the other way. In the picture beneath, I have the rear of my camera (note the plastic housing inside, which is bowl shaped), a Renata 390, a Weincell 400, and the battery cover to my camera body. The camera is number 2779604, so its a version 1 and mid series, according to the van Oosten book.