Easy fix for inaccurate meter?

REOS

REOS

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I’ve recently acquired a Pentax Spotmatic with a meter that is underexposing by at least a stop. I am about to load a roll of HP5 and have found that by setting the ASA to 325, the meter is pretty much spot on. Assuming no other faults with the camera, this seems like an all too easy fix. Am I missing something?
 
Yep adjust knowing the difference, they all fluctuate from each other, setting at 320 though sounds like more a 1/3 stop under
 
acrobatic_Citron said:
Yep adjust knowing the difference, they all fluctuate from each other, setting at 320 though sounds like more a 1/3 stop under
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Thanks. Knowing the principle is sound, I shall try a few more tests before venturing out. The metering needle is a tad sluggish, but seems to get there in the end.
 
REOS said:
I’ve recently acquired a Pentax Spotmatic with a meter that is underexposing by at least a stop. I am about to load a roll of HP5 and have found that by setting the ASA to 325, the meter is pretty much spot on. Assuming no other faults with the camera, this seems like an all too easy fix. Am I missing something?
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We used to call this setting an exposure index. It is what most professionals did all the time. To suit their cameras and processing.
 
REOS said:
The metering needle is a tad sluggish, but seems to get there in the end.
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Some old cameras? Panic stations when my nikon F4 wouldn't auto focus, so probally similar because of age or not used much and it gets sticky.......after continually turning knobs/switches that might have cause the problem it eventually worked (y)
 
I wouldn't worry about it. Once the battery runs out you probably won't have a realistic option for replacing them anyway.
 
Plain Nev said:
I wouldn't worry about it. Once the battery runs out you probably won't have a realistic option for replacing them anyway.
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Nah I've got an easy to get AG10 or AG12 (or maybe LR44) I used in my spotmatic SP
 
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Plain Nev said:
Well, they must obviously work then. Does it effect the metering at all?
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Most old camera electical circuits, for the exposure meter, can take a small increase to 1.5 volts and it can affect the exposure reading but just compensate (if necessary) by increasing or decreasing the iso on the camera, Some say:- the way the exposure meter reads from say 1000 to 1 sec will change slightly (with the increase to 1.5v) but film latitude easily compensates for that and if using slide film (which needs to be more accurate)....well I wouldn't use an old camera exposure meter for that.
 
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And does a 1/3 stop make that much difference compared to the inaccuracy of some older cameras' shutter speeds? Even with newer cameras, I used to bracket if I felt that a shot was not easily repeatable in the future (fragile buildings etc.) so wanted to nail it on slide. To be fair to the F65 and F80, the "correct" exposure according to the meter was usually the best!
 
All you need is a small "O" ring to fit a slightly smaller battery. An AG12 works without an "O" ring.
There are hearing aid batteries available nearer the correct voltage.

The error due to incorrect battery voltage varies with the amount of light, and the meter would probably not be totally correct even with the correct voltage battery.

However the bigger problem, because it affects things even if you use a separate light meter is that the shutter will almost certainly be slow.

Out of four I measured a few weeks ago, the best one at 1/1000 was 1/500. The others were 1/337, 1/310. 1/250 and the error gets less as the speeds reduce.

It is fairly safe to go up one F stop on 1/250, 1/500 and 1/1000, and half a stop at 1/125, 1/60 and 1/30.

Also worth remembering that despite the name, they do not use spot metering, but centre weighted, so if you have a bright sky, you still need to aim the camera down a bit to get what is below the sky line correctly exposed.
 
Sangoma said:
All you need is a small "O" ring to fit a slightly smaller battery. An AG12 works without an "O" ring.
There are hearing aid batteries available nearer the correct voltage.

The error due to incorrect battery voltage varies with the amount of light, and the meter would probably not be totally correct even with the correct voltage battery.

However the bigger problem, because it affects things even if you use a separate light meter is that the shutter will almost certainly be slow.

Out of four I measured a few weeks ago, the best one at 1/1000 was 1/500. The others were 1/337, 1/310. 1/250 and the error gets less as the speeds reduce.

It is fairly safe to go up one F stop on 1/250, 1/500 and 1/1000, and half a stop at 1/125, 1/60 and 1/30.

Also worth remembering that despite the name, they do not use spot metering, but centre weighted, so if you have a bright sky, you still need to aim the camera down a bit to get what is below the sky line correctly exposed.
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Well I often say "guys buying a very expensive exposure meter to use with an old camera is a waste of money" because as you say even with manufacturing tolerances for film, shutter speeds, lens irises etc when new can add up the wrong way if unlucky, not forgetting the camera wear over the years......and even if you use a "perfect" camera you can still get the exposure wrong for the subject, well I have a lot of experience of getting the exposure wrong from Canon T90 and Nikon F4 down to Minolta SRT101 :(o_O
 
Plain Nev said:
I'll give the LR43 a go.
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You better check on the net as they say it's AG10 or even AG9. MY Spotty SP meter died years ago so was going by memory :headbang:
 
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ChrisR said:
The Spotties used a balanced metering circuit that meant you could use a higher voltage battery without affecting the result, according to https://www.pentaxforums.com/articles/photo-articles/spotmatic-bridge-circuit.html .
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The Canon EF is another camera that can meter accurately using a 1.5V cell as oppose to the 1.35V mercury cells used in older cameras but are no longer available.

The alternatives if you must have 1.35V power for the meter are Zinc-Air hearing aid batteries at about 1.4V. Wein cells which I think are Zinc-Air cells but quite expensive or one of several adapters that incorporate a schottky diode to reduce voltage from 1.5V to 1.35V. The battery form factor can also be problematic but o-rings can often be used to keep the battery in position.
 
Plain Nev said:
I'll give the LR43 a go.
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That is the same as the AG12 and it works fine without an "O" ring, the battery cover keeps it central enough even though it is smaller diameter than the original.
 
Sangoma said:
That is the same as the AG12 and it works fine without an "O" ring, the battery cover keeps it central enough even though it is smaller diameter than the original.
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Well I thought my spotty sp used a LR43 (AG12) but on looking at the net and they are saying AG10 or AG9.....if it's AG12 it's nice to know my memory hasn't let me down. h'mm just worked out why the AG12 works as the spotty meter will still work when the battery is placed incorrectly i.e. + to "screw in cap" the manual says + facing into the camera.
edit: Cleaning surfaces and have got my spotty SP working again with AG12 battery.
 
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Just checked my Spotmatic and the battery I am using is an SPX400 which is equivalent to Energiser 387s. This is a 394 battery with a plastic collar to locate the battery in the battery chamber. The chemistry is silver oxide and mine has been in the camera for over 5 years, admittedly with infrequent use but still appears to be going strong.
 
Well the LR43 didn't do it. The battery compartment isn't quite deep enough. :confused:
 
Plain Nev said:
Well the LR43 didn't do it. The battery compartment isn't quite deep enough. :confused:
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That's odd, the AG12 fits fine, and I'm sure I saw the LR43 listed as equivalent.

I do have more than one version of the Spotmatic, I will get them out later and check.

I did get some smaller batteries about 1/4" diameter and used them with an O ring, but I can't remember if that was on a Spotmatic or a Zenit.
 
Plain Nev said:
Well the LR43 didn't do it. The battery compartment isn't quite deep enough. :confused:
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H'mm well my SP works with the LR43 if the +ve is facing the screw in cap, which as mentioned before is contrarily to the manual which says "+ve is to the top of the camera".
The battery housing on mine is all metal, looking at some pictures of other spotties and the housing is plastic ?????? whether someone altered the polarity in the past (to take LR43)......I don't know.
The ASA is set for 320 (for 200 ISO film) to agree with seperate exposure meter.
 
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The old mechanical shutters were famous for being inaccurate. back in the day I once spent a day in my then local camera shop. They had an "open day" (yes really) and some company brought in testing gear to free test customers cameras, just for fun they tested a few new cameras from the stock. Some were a stop out. The testing people said that was normal.
So any old camera with worn springs and a sticky iris... well say no more.
 
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swanseamale47 said:
The old mechanical shutters were famous for being inaccurate. back in the day I once spent a day in my then local camera shop. They had an "open day" (yes really) and some company brought in testing gear to free test customers cameras, just for fun they tested a few new cameras from the stock. Some were a stop out. The testing people said that was normal.
So any old camera with worn springs and a sticky iris... well say no more.
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It's a wonder (many years ago) why I had some VG results with Kodachrome and Agfa slide film, and the poor ones I can blame the camera :)o_O
 
Just had a quick look.

The Spotmatic F I have an AG12 in, it is the one in the picture with the black plastic battery holder.

The other Spotmatics I have AG3s with O rings.

The ordinary Spotmatics (ie not the F) have the white plastic battery holder, which from the badly out of focus photo you can see it has a taper at the bottom, which prevents the AG12 from fitting.

pbat.jpg

Another point that needs making, is the statement that it make no difference which way up the battery is fitted is only true when the meter is centred.
But the direction the needle moves when changing shutter speed or aperture with reverse when the battery is reversed.

And no difference when it is centred is theoretical, because ageing of the resistors in the circuit may change that too.

If you use an AG12 on the F, and install the battery correctly, you need an O ring to prevent the battery shorting on the battery cover lip.
 
Sangoma said:
But the direction the needle moves when changing shutter speed or aperture with reverse when the battery is reversed.
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That's not much of a problem (as you know) as taking an exposure:- you alter the aperture or shutter speed till the needle is central. For me I can't be bothered reading + or - in the window to work out whether to increase/decrease aperture, same for shutter speed as you can see which way the needle is going in what you decide to change.........Same for other cameras I own ;)
 
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It would be a bit of a problem for me, because the whole point of the meter is knowing if you're under or over. Yes, you can hit the mark regardless, but you might want to compensate for the background light. There's no need to make it any more difficult than it is.
 
Plain Nev said:
It would be a bit of a problem for me, because the whole point of the meter is knowing if you're under or over. Yes, you can hit the mark regardless, but you might want to compensate for the background light. There's no need to make it any more difficult than it is.
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Oh well I can't see your problem as if e.g. a scene is lit with different light then just take a meter reading of some thing nearby like grey, green grass even deeper blue sky and that would give an overall (near as dammit)correct reading...the tricky problem is a scene in deep shadow and bright light e.g. a white house and half is in deep shadow as you know meters can't read white or towards black correctly and would need a few shots at different settings. In portraiture with person not facing the sun and the person has a white face, no problem if he/she is wearing the colours above to take a meter reading.
Sorry if I'm teaching grandma to suck eggs and you know all this anyway and just mentioned this in case you hadn't thought of above.
 
excalibur2 said:
Oh well I can't see your problem as if e.g. a scene is lit with different light then just take a meter reading of some thing nearby like grey, green grass even deeper blue sky and that would give an overall (near as dammit)correct reading...the tricky problem is a scene in deep shadow and bright light e.g. a white house and half is in deep shadow as you know meters can't read white or towards black correctly and would need a few shots at different settings. In portraiture with person not facing the sun and the person has a white face, no problem if he/she is wearing the colours above to take a meter reading.
Sorry if I'm teaching grandma to suck eggs and you know all this anyway and just mentioned this in case you hadn't thought of above.
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I wouldn't bet on it. ;)
 
Sangoma said:
Just had a quick look.

The Spotmatic F I have an AG12 in, it is the one in the picture with the black plastic battery holder.

The other Spotmatics I have AG3s with O rings.

The ordinary Spotmatics (ie not the F) have the white plastic battery holder, which from the badly out of focus photo you can see it has a taper at the bottom, which prevents the AG12 from fitting.

View attachment 391979

Another point that needs making, is the statement that it make no difference which way up the battery is fitted is only true when the meter is centred.
But the direction the needle moves when changing shutter speed or aperture with reverse when the battery is reversed.

And no difference when it is centred is theoretical, because ageing of the resistors in the circuit may change that too.

If you use an AG12 on the F, and install the battery correctly, you need an O ring to prevent the battery shorting on the battery cover lip.
Click to expand...
My recently acquired Spotmatic has the same battery chamber as the white one you have pictured here and it took me quite a while to find a suitable battery; no one on the Pentax Forum recognised it. Help came in the shape of Cameratiks in Edinburgh who recommended the Energiser 392. It looks absurdly small for the chamber, but doesn’t rattle around once the lid is on and appears to work fine.

image0.jpeg
 
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REOS said:
My recently acquired Spotmatic has the same battery chamber as the white one you have pictured here and it took me quite a while to find a suitable battery; no one on the Pentax Forum recognised it. Help came in the shape of Cameratiks in Edinburgh who recommended the Energiser 392. It looks absurdly small for the chamber, but doesn’t rattle around once the lid is on and appears to work fine.

View attachment 392083
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That is the same as an AG3.

I use an O ring to prevent any chance of the cattery cover lip shorting it when fitting.
 
Sangoma said:
That is the same as an AG3.

I use an O ring to prevent any chance of the cattery cover lip shorting it when fitting.
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Thank you. Makes complete sense. I’m off to buy some O rings.
 
If you have an old Wein Cell's knocking about you can nick the O ring off that.
 
Unfortunately I’ve never used a Wein Cell. I was put off by reports that they didn’t last long so opted for The Small Battery Company’s adapter, which may or may not prove to be cheaper in the long run.
 
He's a wise man that Sangoma. The O Ring trick worked for me. :cool:
 
I must dig out my Spotties and play with batteries!
 
I've not really used my Spotmatic since. But initially it seems pretty accurate too. A tad high, perhaps, but no big deal.
 
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