Biasing the ASA to compensate for 1.5v Battery

[Doug Knutsen]

Gerald: Thanks for the reply. I'm glad to know I'm not risking damage to my beloved Yashicas. I AM going to be more careful about recommending Batteries Plus in the future.

Doug

 

[Ryuji]

What I am referirng to are those cameras that have a match needle meter design and use a button battery like the 675. For such cameras a zinc-air or silver oxide cell is usually required.

1. In match needle meters the current going through the meter is unchanged as the voltage is changed. So the difference is the power dissipated by the resistor network. The ratio of the dissipated power is the ratio of the voltage squared, or 23% more. Are you seriously arguing that this difference can burn the circuit?

2. How does silver oxide help in the voltage difference part compared to alkaline cells?

 

[Ryuji]

The only batteries currently available in the US for camera use are alkaline, silver oxide, zinc-air and the older carbon-zinc types.

This is not true. Several lithium cells are available. Some of the lithium cells are in the size of two SR76's stacked up, and it's called CR1/3N. It's a very useful battery.

Lastly and most importantly, alkaline and carbon-zinc batteries cannot be used because of the steep voltage change during discharge.

This is not true. They can be used, as long as the voltage and/or the meter is adjusted. It's just that the meter reading is less accurate when the battery is halfway in its life.

 

[Ryuji]

Ironically Zinc air cells have the highest capacity [mAh/kg] of ANY commercially available battery technology available today.

While zinc air may have high energy density, its lifespan is limited by the other factor. My Konica Autoreflex T's never required battery change since I put in a fresh CR1/3N 10 years ago. My Konica Hexar never required battery change since I put in 2CR5 many years ago. My Auto S2's never required battery change since I put in alkaline cells when I repaired them many years ago. (I don't know if there is 625-sized silver oxide cells but I couldn't find any at that time.) Auto S2 doesn't even have a power switch and I make no effort to get last drop out of the battery. But with zinc air, you'll have to tape the hole or something when not in use to get several months of use. That's a bit too much work for me. (Opinions vary on this one. I'm lazy.)

 

[Ryuji]

Using OM cameras with slide film I do find a just noticable underexposure using 1.5v batteries, although it's only very slight.

If you mean Olympus OM-1 or OM-2, you shouldn't have any exposure error due to the battery voltage difference since they have a match needle meter.

AE cameras don't use match needle. The first AE mechanism was Konica's lock-needle design and this (and most other) type of meter is susceptible to voltage difference, particularly when the light level is high.

 

[naturephoto1]

Ryuji,

I am not sure if having a match needle requires no adjustment is correct. As far as I know, my Rollei 35S which originally used either a 1.35V Mercury PX 13 or a PX625 now requires the usage of a 1.5V PX625 A alkaline battery unless an adapter is used. Everything that I have read and Krikor the Rollei repairman has indicated and has made the adjustment to the meter to compensate for the variation due to the voltage difference. The adapters do the same thing so that the meter not be adjusted, but allows for the usage of a different 1.5V alkaline battery.

Rich

 

[Ryuji]

I am not sure if having a match needle requires no adjustment is correct.

To be precise, match needle meters in which the needle rests at the center (zero-in position) when the battery is removed makes correct exposure reading regardless of the small voltage difference due to battery replacement. Olympus OM-1 and Pentax Spotmatic are examples of this type of cameras.

 

[PeterB]

But with zinc air, you'll have to tape the hole or something when not in use to get several months of use. That's a bit too much work for me. (Opinions vary on this one. I'm lazy.)

I agree with you that a few months is not very useful, but these Wein zinc air cells I mentioned above claim to last 1 year. I think the manufacturer may have achieved this by reducing the oxygen port hole diameters on the cell's surface.

Ultimately as you say it comes down to what we are willing to put up with. Most motor drive (i.e. non manual advance) cameras will need fresh batteries at least every few months if used regularly and most of us have put up with that !

regards
Peter

 

[Woolliscroft]

If you mean Olympus OM-1 or OM-2, you shouldn't have any exposure error due to the battery voltage difference since they have a match needle meter.

And yet I do.

David.

 

[jmailand]

After reading this site I got my Konica C35 to work with 675 hearing aid batteries. Should help other cameras too. http://rick_oleson.tripod.com/index-111.html

James.

 

[Gerald Koch]

I did forget lithium batteries in my list of available batteries. I guess they are just too new-fangled for me.

My point about not using alkaline batteries is that the steep discharge curve of these batteries makes them more and more inaccurate with use. The discharge curve for silver oxide cells are quite flat in comparison making them a better choice.

Match needle meters use a bridge circuit and are less dependent on battery choice.

There are several sites on the web which discuss camera battery replacement and I would suggest those interested google to find them. At least one of them has a warning that using the wrong battery can damage the camera.

A simple solution when a silver oxide battery would be a good replacement is to lower the voltage by using a germanium 1N34A diode. This diode is cheap less than a dollar. Again their are web sites describing this modification.

 

[OldBikerPete]

Mercury batteries are 1.35V, Alkaline batteries are 1.5V (fresh). That's a difference of 11% or less than 1/10 stop if the meter error is linearly dependant on battery voltage (and for CdS cells there should be insignificant non-linearity).
Have you ever done an instrumental comparison between Alkline discharged to 1.35V and fresh ones. I'm betting that you'll barely see the meter needle move.

 

[Gerald Koch]

Mercury batteries are 1.35V, Alkaline batteries are 1.5V (fresh). That's a difference of 11% or less than 1/10 stop if the meter error is linearly dependant on battery voltage (and for CdS cells there should be insignificant non-linearity).

The response of the meter to voltage depends on the circuit. I have seen posts on the web saying the error can be 1/2 to 1 stop when substituting an alkaline or silver oxide battery for the mercury battery.

 

[Jon King]

A fact or two

All through this thread I've been reading personal experiences and assumptions, many about the OM-1. None of those OM-1 experiences match up with mine. I have one of my OM-1's with me at work today, so all the responses finally got me off my rear and into the electrical lab to see how the meter reading would actually vary with voltage.

I meter Tri-X at 250. With a silver oxide battery in the OM-1, matching the shutter and aperture readings to my Nikon F3 and Sekonic 508 need an ISO setting of 40.. a 2 2/3 stop difference! Set this way, I get negatives that are as well exposed as I produce on any other camera I have, limited only by my metering skill.

I set up on the bench, I varied the voltage to the OM-1, and metered the same area, adjusting ASA until the needle was centered in the +/- area (the OM-1 is not truly a match needle, in the way an SRT-201 is). A look at the OM-1 meter schematic (http://olympus.dementia.org/Hardware), shows that it is not a bridge circuit, so it there is no reason it shouldn't be sensitive to battery voltage.

Here are my readings for this camera:

VBAT ASA
1.60V 32
1.50V 50
1.40V 80
1.35V 100
1.30V 112* (100 was low, and 125 was high, by equal amounts)
1.20V 160

The OM-1 measurements seem to show about a 1 to 1 1/3 stop shift between the mercury voltage of 1.35 and a silver oxide voltage of 1.5 to 1.55.

This OM-1 appears to have an extra f-stop and a bit shift even at the mercury battery voltage range of 1.30 to 1.35. The meter coupling system has strings and springs and pulleys, so I'm guessing that something in that mess is also off.

What is also quite clever is the OM designers apparently designed the meter response to be less sensitive to battery voltage around the mercury battery voltage range. The camera works fine as designed, but was never optimized to have a higher batter voltage range.

My conclusions? If I ever tear this OM-1 down, I'd try to fix my meter error of a stop, and probably add a germanium diode in series, as is suggested several places around the internet, to get my drop close to the original design voltage. Together, those steps should get me close the original calibration of the meter. Most likely, I'll just keep Silver Oxide batteries in it, remember that ISO 250 is 40 and happily make pictures with it.