Pentax 67 TTL Meter Coupling Range

ksulliva01 · 03-04-2023, 05:46 PM

Hi,

First post here as I just acquired my first Pentax! (Pentax 67 late model).

I have a question about the 67 TTL Prism meter. Just loaded some 800 iso film for the first time. I noticed that with the iso set to 800 on the camera, when I select a shutter speed of 1 second, the meter just turns off (needle instantly drops to the bottom). At 1/2 second and faster, at iso 800 the meter seems to work properly at all slower shutter speeds. As soon as I set film speed to 400 or slower, meter works properly.

Is this due to the inherent limits of the meter's coupling range? Are there certain exposure values that cause it to basically shut off?

I found a manual describing the meter's coupling range which I think answers my question, however I find it kind of strange that some of the values in the shaded area still register with the meter, while certain values just seem to make it quit.

Is this normal behavior? Appreciate any insights!

Attached Images

leekil · 03-04-2023, 08:13 PM

Perhaps you have an older prism meter, and they later expanded its usable range?

ksulliva01 · 03-05-2023, 08:06 AM

I have the later model TTL prism. Everything about the meter seems to function correctly, I just find it strange that certain combinations of ISO and shutter speed (seemingly regardless of aperture setting) just cause the meter to shut off. The faster I set the iso, the faster the shutter speed at which it occurs. For example:

400 ISO: Speeds of 1 second to 1/1000 are selectable
800 ISO: Speeds of 1/2 second to 1/1000 are selectable
1600 ISO: Speeds of 1/4 second to 1/1000 are selectable

Since it seems to correlate in stops, I'm assuming that's just normal behavior, but having a hard time wrapping my head around why this would be. Right now in my office, at iso 800, F11, the meter is calling for a shutter speed of 1 second. As soon as I turn the dial to 1 second, the meter just instantly drops all the way down as though it turns off. So, I turn the shutter speed to 1/2 second and the meter comes back to life registering 1 stop underexposed.

Is this just the limit of an old metering system like this?

photoptimist · 03-05-2023, 10:34 AM

The manual shows the range of ASA and shutter speed combinations that offer trustworthy meter readings. As the manual says, the meter might seem to work outside the range, but it cannot be trusted.

Because this is analog meter, its behavior beyond limits of the range are not as binary as with a digital system. You may have gotten lucky and have a meter that is reasonably accurate a stop or two outside the official range. However, you may also discover that this camera's meter is only accurate outside the official range if the battery is perfectly fresh, the temperatures are warm, the planets are aligned, etc.

In other words, be careful if you try to use the meter outside the range.

tuco · 03-05-2023, 10:44 AM

Yes, it is a basic metering system on this camera. A more sophisticated metering can be found on the late model Pentax 67II.

There is the Pentax 6x7 (non MLU and MLU version), Pentax 67 and Pentax 67II generations of the camera with two different TTL prisms. The TTL for the 6x7 and 67 are the same except for cosmetic branding differences. And 67II has a different TTL prism all together. You can circumvent the limited metering capabilities with a Pentax one-degree spot meter or use a light meter app on a smart phone.

ksulliva01 · 03-06-2023, 12:43 PM

Thank you for all of the thoughtful responses. I decided to shoot my first two rolls of film simply using the camera's TTL meter, just so I have some baseline going forward. Hopefully the planets were aligned (aren't analog devices fun?). Of course an external spot meter or phone app will provide more accurate lighting evaluation. I'm still wondering from those who may have one, if it is normal behavior for the meter to just shut off at certain combinations of ISO/Shutter Speed as mine does. Of course the combinations that cause this behavior are well outside the coupling range outlined in the manual, so not really a real-world issue, but I'm curious about the quirkiness of the behavior.

photoptimist · 03-06-2023, 05:45 PM

Originally posted by ksulliva01

I'm still wondering from those who may have one, if it is normal behavior for the meter to just shut off at certain combinations of ISO/Shutter Speed as mine does. Of course the combinations that cause this behavior are well outside the coupling range outlined in the manual, so not really a real-world issue, but I'm curious about the quirkiness of the behavior.

It's not actually quirky, it's physics.

All those combinations on the border of out-of-range have the same very low number of photons per second. Those combinations are on the edge of being just enough to stimulate the sensor to output a usable reading. That lower left blue zone represents light levels indistinguishable from total darkness as far as the meter is concerned.

03-04-2023, 05:46 PM	#1
ksulliva01 New Member Join Date: Mar 2023 Posts: 3	Pentax 67 TTL Meter Coupling Range Hi, First post here as I just acquired my first Pentax! (Pentax 67 late model). I have a question about the 67 TTL Prism meter. Just loaded some 800 iso film for the first time. I noticed that with the iso set to 800 on the camera, when I select a shutter speed of 1 second, the meter just turns off (needle instantly drops to the bottom). At 1/2 second and faster, at iso 800 the meter seems to work properly at all slower shutter speeds. As soon as I set film speed to 400 or slower, meter works properly. Is this due to the inherent limits of the meter's coupling range? Are there certain exposure values that cause it to basically shut off? I found a manual describing the meter's coupling range which I think answers my question, however I find it kind of strange that some of the values in the shaded area still register with the meter, while certain values just seem to make it quit. Is this normal behavior? Appreciate any insights! Attached Images

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03-04-2023, 08:13 PM	#2
leekil Pentaxian Join Date: May 2016 Photos: Albums Posts: 2,003	Perhaps you have an older prism meter, and they later expanded its usable range?

03-05-2023, 08:06 AM	#3
ksulliva01 New Member Join Date: Mar 2023 Posts: 3 Original Poster	I have the later model TTL prism. Everything about the meter seems to function correctly, I just find it strange that certain combinations of ISO and shutter speed (seemingly regardless of aperture setting) just cause the meter to shut off. The faster I set the iso, the faster the shutter speed at which it occurs. For example: 400 ISO: Speeds of 1 second to 1/1000 are selectable 800 ISO: Speeds of 1/2 second to 1/1000 are selectable 1600 ISO: Speeds of 1/4 second to 1/1000 are selectable Since it seems to correlate in stops, I'm assuming that's just normal behavior, but having a hard time wrapping my head around why this would be. Right now in my office, at iso 800, F11, the meter is calling for a shutter speed of 1 second. As soon as I turn the dial to 1 second, the meter just instantly drops all the way down as though it turns off. So, I turn the shutter speed to 1/2 second and the meter comes back to life registering 1 stop underexposed. Is this just the limit of an old metering system like this?

03-05-2023, 10:34 AM	#4
photoptimist Pentaxian Join Date: Jul 2016 Photos: Albums Posts: 5,123	The manual shows the range of ASA and shutter speed combinations that offer trustworthy meter readings. As the manual says, the meter might seem to work outside the range, but it cannot be trusted. Because this is analog meter, its behavior beyond limits of the range are not as binary as with a digital system. You may have gotten lucky and have a meter that is reasonably accurate a stop or two outside the official range. However, you may also discover that this camera's meter is only accurate outside the official range if the battery is perfectly fresh, the temperatures are warm, the planets are aligned, etc. In other words, be careful if you try to use the meter outside the range.

03-05-2023, 10:44 AM	#5
tuco Pentaxian Join Date: Jul 2009 Location: Pugetopolis Posts: 11,026	Yes, it is a basic metering system on this camera. A more sophisticated metering can be found on the late model Pentax 67II. There is the Pentax 6x7 (non MLU and MLU version), Pentax 67 and Pentax 67II generations of the camera with two different TTL prisms. The TTL for the 6x7 and 67 are the same except for cosmetic branding differences. And 67II has a different TTL prism all together. You can circumvent the limited metering capabilities with a Pentax one-degree spot meter or use a light meter app on a smart phone.

03-06-2023, 12:43 PM	#6
ksulliva01 New Member Join Date: Mar 2023 Posts: 3 Original Poster	Thank you for all of the thoughtful responses. I decided to shoot my first two rolls of film simply using the camera's TTL meter, just so I have some baseline going forward. Hopefully the planets were aligned (aren't analog devices fun?). Of course an external spot meter or phone app will provide more accurate lighting evaluation. I'm still wondering from those who may have one, if it is normal behavior for the meter to just shut off at certain combinations of ISO/Shutter Speed as mine does. Of course the combinations that cause this behavior are well outside the coupling range outlined in the manual, so not really a real-world issue, but I'm curious about the quirkiness of the behavior.

03-06-2023, 05:45 PM	#7
photoptimist Pentaxian Join Date: Jul 2016 Photos: Albums Posts: 5,123	Originally posted by ksulliva01 I'm still wondering from those who may have one, if it is normal behavior for the meter to just shut off at certain combinations of ISO/Shutter Speed as mine does. Of course the combinations that cause this behavior are well outside the coupling range outlined in the manual, so not really a real-world issue, but I'm curious about the quirkiness of the behavior. It's not actually quirky, it's physics. All those combinations on the border of out-of-range have the same very low number of photons per second. Those combinations are on the edge of being just enough to stimulate the sensor to output a usable reading. That lower left blue zone represents light levels indistinguishable from total darkness as far as the meter is concerned.