Camera's internal metering off by 1 stop

[BetterSense]

I just bought an ugly-condition Pentax ME Super. I shot a roll of print flim through it for testing and they came back ok. But this morning I stepped outside and it was serving me a shutter speed that my gut told me was too fast for the light conditions.

I don't have a light meter but I decided to test all my cameras together. I tested my Pentax Program Plus, ME, and this ME Super in question, as well as my Niken F801, by pointing them at a blank brick wall. I used the same lens set at f/16 on all the Pentax cameras and I set the Nikon to f/16 and Center-weighted. After correcting for the film speeds in the cameras, they all gave a shutter speed of 30 at EI 400. Except the ME Super was showing 60.

This is enough evidence for me to consider just setting the film speed one stop slower that true EI, to bring the ME Super in alignment with the other cameras. Is there any problem with doing this? I know some cameras have off-the-film metering or something so I don't want to outthink my camera. What can cause a camera's meter to be off? They all have fresh Silver-Oxide batteries in them.

 

[BobNewYork]

As long as the one stop difference is across the range of lighting conditions and subject brightnesses that's fine. It just means that Tri-X in your ME Super is EI 200. I use different EIs for the same film in different camera bodies all the time; but take some real shots to confirm it.

 

[BetterSense]

Right. I suppose it would take detailed testing. Because the case may also be that the shutter speeds in this camera are slightly slower than indicated, canceling out the discrepency in the indicated metering. Although with an electronic shutter I don't think that's likely.

Are external light meters accurate enough (note: I didn't say "precise enough"), on absolute terms, to use as a calibrator for camera meters? I would venture that light meters themselves vary from brand to brand and model to model a fair bit. But people that work on cameras must have something absolute to test on...not a brick wall and a bunch of other cameras.

 

[BobNewYork]

Shutter speeds can still be off as can aperture settings. In the past I've actually taken two, high-end hand held meters fresh from the box and found they differed by a full stop from each other - presumably a half stop in each direction. Doesn't really matter though, your personal EI just takes everything into account - meter, processing, shutter, aperture etc. I'd shoot a roll in the ME Super as is and see how your negs are - if they're thinner than your used to halve the EI. You can fine tune as you go.

I use one hand held meter to "calibrate" my other meters, (in camera and hand held) and just adjust the "others" to it. In theory this doesn't account for differences in shutters and aperture settings - in practice I get consistent negs. Except when I screw up, of course !!

 

[Mike Kennedy]

I just got a pentax ME from apug member David and read in the manual that the exposure composition dial must be set to 1x for normal shooting.Is your ME Super set up correctly?

Mike

 

[Ralph Javins]

Good morning, BetterSense;

Ah, the old mercury battery thing strikes again. Yup, your fresh Silver-Oxide battery has a nominal voltage of about 1.50 VDC. The old PX-625 mercury batteries had a nominal voltage of 1.35 VDC. Both of them have a nice flat discharge curve, but that 10% voltage difference is what is giving you that one stop difference in your light meter reading. You can continue to use the "cut the ASA speed in half" work around, or you can have a camera tech (or you) install a Schottky barrier diode in the circuit from the battery to the meter to provide a fairly constant 0.15 to 0.20 VDC drop in the voltage to bring your light meter back into normal operating range. The light meter circuit can be recalibrated to work with this, if there is enough of a difference left for you to noitice.

Also, there is the option of buying one of the Silver-Oside battery holders that has a Schottky barrier diode installed in the holder that will do the same thing for you. A little more expensive, but there is no modification to your camera. One of the sources is the MR-9 from CRIS Camera in Arizona, www.criscamera.com, but there are other sources also. You can even make one.

 

[Chan Tran]

Your shutter speed may slow and the effect cancels each other out. You would have to test the shutter accuracy too but although I can test the shutter speed in manual, testing them on auto is more difficult.

 

[mudman]

Ralph - Correct me if I'm wrong, but the ME came out after the switch to Silver-oxide.

 

[Chan Tran]

The ME run on 2 silver oxide batteries. MS76 or LR44 or equivalent.