Pentax 6x7 metered prism accuracy
Last week-end was a rather sunny and warm exception to the usual chilly weather we've had recently in Brittany. I set off with my Pentax 6x7,
and found that the meter was giving me strange readings. Compared to "sunny 16" there were 3 to 4 EV differences. I'd been used to about
1 since the camera had been serviced a couple of years ago.
Is this happening often ?
Could this be caused by a weak or inappropriate battery (I currently have 4LR44) ?
Is there a way I can adjust the metered prism finder myself ?
Have you checked pentaprism manual? You can find it online. There is a chart in the manual pointing to certain ISO Shutter combinations not metering accurately. May be your exposure was in that range, however unlikely it is worth checking.
But the battery might be the culprit.
Although the pentaprism readings are accurate in most conditions.
I have noticed that sometimes it gives weird readings in tricky situations(too much light or too dramatic lighting).
When I am in doubt:
I switch off reader point it to the ground or some place that has more homogeneous light, turn it on than point it to the scene I am metering.
I usually get better readings this way.
Another possibility : maybe the prism is not correctly coupled to the body/lens.
For the metering prism to be rightly coupled to the body/lens, you must first remove the lens, then remove and replace the prism while the lens is removed and then, only when the prism is set in place you may replace the lens ; doing otherwise (installing the metering prism when a lens is already on the body) leaves the photocell uncoupled with the diaphragm.
i situations where the meter disagrees with my brain, I go with what my brain tells me. This happens a lot, but my brain has not failed me yet.
and, if you MUST believe the meter, well, film is cheap. Bracket.
All of the above, plus: Clean the battery contacts, also be sure to clean where the (I assume they're stacked in the compartment) cells contact each other. This is the very first thing to check whenever you are getting wierd readings.
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When the battery is not functioning, the meter is designed to sense this and just won't turn on. There is also a battery-check button. But I guess you could always try warming up the battery in your pocket first. Another thing to check is if you dial is actually coupled to the meter
by the pin in the right position - this might have not been engaged if you didn't do it when you first attached the prism. I guess if your perimeter gaskets were worn out on the meter, there's a remote possibility of light leaking in through the sides. It's a good idea to double check your meter from time to time anyway using something like a spotmeter and gray card. Like any other kind of meter, they might need calibrating
by a pro from time to time.
The Pentax 6x7 metered prism is not a spot meter, not even a center-weighted meter. It meters the brightest point of the whole view to light gray; so in order to get an accurate metering in contrasty scenes, I cover the brightest area over the lens and aim to where should be light-to-medium gray. It isn't really a good meter, sometimes better trust your perception, or just use a handheld lightmeter.
I think that is incorrect. I have a sheet from them which explains it as center-weighted averaging, which should be fine for relatively evenly
illuminated scenes. I don't know about the meter on the P67II, just the older models I use. That being said, I certainly prefer using a handheld spotmeter, which will easily differentiate the kind of error the coupled meter-prism is susceptible to.
Thanks to all for your feedback. Today the meter is back to its usual 1EV offset. I will definitely order a new battery. Maybe exercising the ISO dial has helped.
One thing that might have impacted the whole thing is that I was using a polariser, but not throughout the day, so I don't think this makes a good enough explanation.
I would concur with E. Clean the battery contacts and also the prism contacts and always carry a spare battery. Even then I find there are pretty large variations from one finder to the next. At one point I had three of them so I close inspected/tested all of them and ended up selling the two least accurate ones, even they were all within 1 or 2 stops of each other. The one I ended up keeping was actually the worst looking of the lot but was the most accurate.