OM1n lightmeter overexposing

[elerion]

I got an OM1n about half a year ago. Great camera. The shutter is accurate at all speeds. Lightmeter, though, is overexposing by around two stops (readings are low). As it is a fully manual camera, lightmeter is optional. But I'd have like it to be at least reasonably ok, for those moments requiring fast response.

The camera had a silver oxide 1.55V battery when I bought it. The expected behaviour would have been to get higher readings, not lower (as it requires 1.35v mercury battery).
I installed a germanium diode, as some other people have reported to do.
I now get even more overexpose images (close to three stops lower readings in some ranges).

It seems that the lightmeter was faulty at the time I bought the camera.
But I've read that CdS sensors rarely go bad.
Could it be something else? Anything I could try before giving up the meter?
I tried with three different lenses.

Thank you.

 

[Hilo]

I need to check my OM1, which I do not have with me now. But I think the silver oxide 1.55 is fine, but you may need to install a ring, or adapter, in the battery compartment. I believe the older batteries were larger.

Even if your metering is 2 stops off, you can set the ASA dial for it to correspond correctly.

germanium diode - this I have never heard off

 

[BMbikerider]

Germanium Diode. Perhaps this is another name for the voltage reducing insert that can be used where a camera that requires a 1.35v mercury battery, can happily use the silver oxide or lithium 1.5v cels.

 

[Hilo]

A picture of my OM-1. Is that ring inside the compartment your diode? Btw, how do you check that you are over-exposing?

 

[elerion]

A picture of my OM-1. Is that ring inside the compartment your diode? Btw, how do you check that you are over-exposing?

I don't use any ring. The silver-oxide battery is smaller than the original one, but it still makes contact.

I know I would be overexposing because I compare the readings with those of several lightmeters, and another film camera with built-in exposure meter.

Two/three stops is just too much offset. It also seems to vary a little bit with light intensity.

 

[wiltw]

Following measurements were done in EV10 ambient light, as measured with Minolta Autometer Vf (I mention this because I know the amount of error is dependent both upon battery voltage and level of light being measured.

• OM-1 with PX625 mercuric oxide with 1.34V output (yes, I still have one of those!) reading 18% gray card = baseline reading (0EV)
• OM-1 with PX625A (alkaline) with 1.48V output reading 18% gray card = -0.5EV
  (camera thinks more light is present than when measured with genuine PX625

Since the amount of error is dependent both upon battery voltage and level of light being measured, it is not possible to use a simple offset via an ISO value faster/slower by a fixed amount.

 

[Hilo]

This I found on the net. Sounds worth it to me !

Camtech, http://www.zuiko.com/, offers an adapter that adjusts both the size of the battery chamber and the voltage of a 1.5V battery (386 etc.). I believe the adapter costs around $30.

 

[Hilo]

And: http://www.ebay.com/itm/MR-9-PX625-...655221?hash=item2ef8fa65b5:g:Y~MAAOSwiDFYMG9~

 

[Trangia]

This I found on the net. Sounds worth it to me !

Camtech, http://www.zuiko.com/, offers an adapter that adjusts both the size of the battery chamber and the voltage of a 1.5V battery (386 etc.). I believe the adapter costs around $30.

I don't think the MR( does anything else except allow the smaller/modern batter to fit securely in the old style battery chamber. John specifies that he installs a diode for the voltage drop.

http://www.zuiko.com/web_5__20150924_033.htm

"All our OM-1/1N repairs include conversion to the 357 silver oxide battery (MS76, KS76, SR44W, G-13, 303). Meter circuit mod includes a diode for stepping the voltage down to 1.35V. The 625A or 625U battery that is sold to unsuspecting buyers is not a real replacement for the mercury 1.35V cell. First, the 0.2V difference causes a 2-3 stop error in the OM-1 / 1N meter. Since it is alkaline, the voltage starts to fall as soon as you start to use it. Any calibration that is done with that battery is almost immediately lost. The zinc-air 1.35V battery (really 1.4V) will cause a 1/2 (one half) stop meter error and, once activated, that battery goes dead in a few months whether you use it or not . If the meter does not work with these batteries, it is most likely because the meter was already defective before the battery was replaced. If you do not want to invest in an overhaul to get your camera converted to the 1.55V silver oxide battery, you can buy an MR9 adapter, which drops into the battery box, and uses the 1.55V 386 silver oxide battery."

And in the description from the linked ebay listing:
"Q: Will this convert the voltage of a battery?
A: No - the adapter will not convert a voltage - it only converts the physical size of the battery"

I am not an electrician, but perhaps the original poster's issue is that the diode was installed improperly/backwards current flow? Is this possible?

 

[MattKing]

The Camtech conversion solution changes the size of the battery compartment and adds circuitry to convert the 1.55 volt voltage from a smaller silver oxide cell to the 1.3 volts that the OM-1 meter expects (because that is what the original mercury cells supplied).
An MR-9 adapter does the same thing, but allows the camera to remain unmodified, and the adapter to be moved from camera to camera.
The eBay listing is for an adapter that allows you to use the smaller hearing aid zinc air batteries. Their voltage and discharge characteristics are similar to the original mercury cells that the camera was intended for. That eBay adapter has no circuitry - it just allows the smaller cells to fit. The Zinc air hearing aid batteries are quite inexpensive, but they don't last nearly as long as the silver oxide cells that fit into the MR-9 adapter, or the original mercury cells.
I have both an MR-9 and the zinc air adapters, and they both work well.

 

[Ricardo Miranda]

The Camtech conversion solution changes the size of the battery compartment and adds circuitry to convert the 1.55 volt voltage from a smaller silver oxide cell to the 1.3 volts that the OM-1 meter expects (because that is what the original mercury cells supplied).

Correct.
What he does is to add a "germanium diode" just outside the battery compartment.

For Hilo and BMbikerider here is what a germanium diode is:
https://commons.wikimedia.org/wiki/File:Germanium_Diode_1N60.jpg
It is also known as a Schottky diode.

And here is how to modify an OM-1 to use Silver Oxide 1.55V battery:
http://olympus.dementix.org/Hardware/PDFs/OM1DiodeVer2_1C.pdf

The Zinc air hearing aid batteries are quite inexpensive, but they don't last nearly as long as the silver oxide cells that fit into the MR-9 adapter, or the original mercury cells.

Considering that most ZA675 are sold in packs of 6 they do work for over a year when you consider the 6.

Lightmeter, though, is overexposing by around two stops (readings are low).

Back to your question.
Of the NINE OM-1 I use, one is doing exactly the same with a 1.4V ZA675.
As it is pretty much constant across the the EV spectrum from Low to High Light, I just adjust the ASA dial to a higher ASA or just add 2 stops to whatever reading it tells me.
I suggest you find a competent technician that knows how the OM-1 lightmeter works.
If you can't find one in Spain, consider sending it to Miles Whitehead in the UK or John Hermanson in the USA.
http://www.mwcamerarepairs.co.uk/

 

[Hilo]

Like Ricardo, I find changing the ASA dial the better thing to do. I have had a Beattie screen in my OM1 for ages and I put a sticker inside the camera how to deal with 400 and 125 ASA, those being all I use.

 

[thuggins]

I got an OM1n about half a year ago. Great camera. The shutter is accurate at all speeds. Lightmeter, though, is overexposing by around two stops (readings are low). As it is a fully manual camera, lightmeter is optional. But I'd have like it to be at least reasonably ok, for those moments requiring fast response.

The camera had a silver oxide 1.55V battery when I bought it. The expected behaviour would have been to get higher readings, not lower (as it requires 1.35v mercury battery).
I installed a germanium diode, as some other people have reported to do.
I now get even more overexpose images (close to three stops lower readings in some ranges).

It seems that the lightmeter was faulty at the time I bought the camera.
But I've read that CdS sensors rarely go bad.
Could it be something else? Anything I could try before giving up the meter?
I tried with three different lenses.

Thank you.

I'm not sure where you've seen that CdS sensors rarely go bad. Every CdS light meter I have has gone bad. The selenium ones are still working great after 50+ years. That being said, I have had better experience with cameras that light meters.

Over time the resistance of the meter circuit can increase, which would cause the low reading you are seeing. If you added more resistance to the circuit, that will certainly exacerbate the problem (the diode you installed is just a type of resistor). It is definitely worthwhile getting it repaired.

 

[albada]

My experience agrees with Tim's: CdS sensors often (or usually) lose sensitivity. I've seen OM-1s range from 1 to 5 stops loss, and hand-held meters also usually have some loss. I'll usually measure the light with both the camera and a known-accurate meter, and compensate by changing the ASA on the camera.

Mark Overton

 

[elerion]

I already set the ASA dial to 1600 for 200 film, and check for +2EV instead for 400 film. It's consistent enough for not ruining any exposure.

I already modified the camera to work with 1.5 V silver-oxide battery, so there's no need to buy an adapter as some of you suggested. Thanks for the advice anyway.

Based on your comments, I conclude the lightmeter is not working fine, and thus, the modification has broaden the offset. Not big deal, as I can undo the modification anytime, and shoot up to ISO 640-800 setting dial to 1600 and adding 2 stops on the reading.

It could be worse

 

[jlbruyelle]

Correct.
What he does is to add a "germanium diode" just outside the battery compartment.

For Hilo and BMbikerider here is what a germanium diode is:
https://commons.wikimedia.org/wiki/File:Germanium_Diode_1N60.jpg
It is also known as a Schottky diode.

A correction is badly needed here: a Schottky diode is not at all the same thing as a germanium diode. A Schottky diode uses a Schottky barrier which is a metal-silicium junction, whereas a germanium diode is an ordinary PN junction diode, just with a different semiconductor (germanium instead of silicium).

Both devices have very different voltage/current characteristics and temperature coefficients, although many unseasoned authors only see that they have roughly the same dropdown voltage at an extremely low current. What you want here is a constant 1.35V voltage like the original mercury battery for which the OM-1 circuitry was designed, and no type of diode will give you that - especially if you consider that alkaline batteries don't have a constant voltage in the first place. Silver oxide batteries do have a constant voltage, but a diode will not drop it down to the correct value, so the calibration is still lost. However, the Schottky diode will cause less variation than the Ge diode, so it is a better choice - although still not a good one. You will find all the supporting data and technically correct explanations in this article: http://www.butkus.org/chinon/batt-adapt-us.pdf.

The only option that will give you the same constant 1.35V voltage as a mercury battery, is a Zn-Air battery in an unmodified camera. This is the only case that will not require recalibration and will not cause linearity errors. Note that Zn-Air batteries have more capacity than mercury, but last less time in intermittent use because of the chemical reaction going on while as long as they are in contact with air. You can make them last longer though, by plugging the holes with a piece of adhesive tape while not in use. Fortunately, Zn-Air batteries are not all that expensive, providing you use a type intended for hearing aids (with a size adapter that you can make yourself, or buy on the bay) instead of those prohibitively expensive Weins.

 

[Ricardo Miranda]

a Schottky diode is not at all the same thing as a germanium diode. A Schottky diode uses a Schottky barrier which is a metal-silicium junction, whereas a germanium diode is an ordinary PN junction diode, just with a different semiconductor (germanium instead of silicium).

Thank you for the clarification.
As I don't have any education in Electronics I had assumed they were the same.
And it makes sense that an engineer friend of mine always mentioned Schottky diode in relation to the OM-1.

The only option that will give you the same constant 1.35V voltage as a mercury battery, is a Zn-Air battery in an unmodified camera. This is the only case that will not require recalibration and will not cause linearity errors. Note that Zn-Air batteries have more capacity than mercury, but last less time in intermittent use because of the chemical reaction going on while as long as they are in contact with air. You can make them last longer though, by plugging the holes with a piece of adhesive tape while not in use. Also note that Zn-Air batteries are not all that expensive, providing you use a type intended for hearing aids (with a size adapter that you can make yourself, or buy on the bay) instead of those prohibitively expensive Weins.

That part I am very aware of it and the use of a ZA675 with a small plumbers O ring to centre it is in use in all my OM-1.