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Spotmatic F Metering Problem
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PostPosted: Tue Nov 25, 2008 8:10 pm    Post subject: Spotmatic F Metering Problem Reply with quote
I set a lamp onto a plain wall and took a reading with a light meter:

1/40 @ F1.8 ISO 400

I hooked up the spotmatic with a 55mm 1.8 SMC Tak

Open aperture metering:
1/60 @ F1.8 ISO 400 (close)

Stopdown metering
1/60 @ F16 ISO 400 Erm????


It seems to say that at F16 no matter what the shutter speed is, that that is the correct exposure. The meter also swings around wildly in stopdown mode so I guess there must be some connection issue.

Anyone got any ideas?


PostPosted: Tue Nov 25, 2008 8:56 pm    Post subject: Reply with quote
Martin, are you taking the stop-down readings with the SMC Tak? If so you will get a false reading. You must use a non-SMC lens, any M42 lens will do but not SMC or S-M-C. Please take new readings with another lens and come back with the results.


PostPosted: Tue Nov 25, 2008 10:20 pm    Post subject: Reply with quote
The SP-F yes... full aperture metering with the SMC. The meter should respond in a linear fashion as you close the aperture ring without stopping down the lens. The meter's always on - so you have to keep the lens cap on to avoid battery drain.

Peter's right re. non SMC - the stop down test will work with these.


PostPosted: Wed Nov 26, 2008 7:13 am    Post subject: Reply with quote
Ok, so let me understand this, I cannot use my SMC tak in stopdown mode? i've downloaded the manual, but it doesn't seem to mention this. A quick google shows that a couple of others have found the same, which either mean we all have the same fault or it's designed to work this way. Which seems strange.

I thought I could always use stopdown mode and it was only compatible lenses that would allow open aperture metering.


PostPosted: Wed Nov 26, 2008 11:32 am    Post subject: Reply with quote
martinsmith99 wrote:
Ok, so let me understand this, I cannot use my SMC tak in stopdown mode? i've downloaded the manual, but it doesn't seem to mention this. A quick google shows that a couple of others have found the same, which either mean we all have the same fault or it's designed to work this way. Which seems strange.

I thought I could always use stopdown mode and it was only compatible lenses that would allow open aperture metering.

Martin, it is not a fault, this is the way it is intended. The older Spotmatics were designed to use any M42 lens and the stop-down lever doubles as a meter switch. In other words the meter is only switched on when the lens is stopped down to the aperture setting.

The SPF operates differently. It was supplied with an open-aperture metering lens as standard and the manual assumes this is the type of lens you are going to use. The meter is always on as long there is enough light to measure, and the manual says that the stop-down switch should normally be in the down position and should only be pushed up when:
1 - Checking the depth of field
2 - Using close-up accessories such as bellows and extension tubes
3 - Using standard M42 lenses without a diaphragm coupling lever.

The implied point is that when you are using an SMC lens, you must meter with the switch down. Pushing the switch up activates the second metering circuit for stop-down lenses and, as there is only one meter, if you have an SMC lens fitted the two values are added together and the meter needle gives a wrong indication.

I know this may seem a bit "Heath-Robinson" today, looking backwards from our fully auto experience, but in the early 70s open-aperture metering was a truly ground-breaking development, but not one, I should add, that I particularly welcomed. I much prefer the old stop-down metering.


PostPosted: Wed Nov 26, 2008 11:49 am    Post subject: Reply with quote
Thanks Peter. It's good to know it's not a fault with the camera.....just the user.
Sad


PostPosted: Wed Nov 26, 2008 12:00 pm    Post subject: Reply with quote
martinsmith99 wrote:
Thanks Peter. It's good to know it's not a fault with the camera.....just the user.
Sad

It's OK, we all have to learn things. Smile

Now go and check the two metering modes with different lenses and come back and tell us how they compare.


PostPosted: Wed Nov 26, 2008 6:08 pm    Post subject: Reply with quote
Ok, quick test when I got in. Done using a dask lamp shining onto a plain painted wall. All readings were ISO 400 @ 1/15


Light meter:
Sekonic L358 F2.8

SPF Open Aperture:
with SMC Tak 55mm F4

SPF Stopdown
with Helios 58mm F4.8
with CZJ Flek 35mm F4.8

Canon 40D
with Helios 58mm F4.8
SMC Tak 55mm F4.8
off topic but with Sigma AF 17-70 F7.1 WHAT?????

I've no idea what happened in the 1st and last readings but I've done them 3 times and they're consistant.

Conclusion:
The SPF in stopdown mode agrees with my 40D. In OA metering it's a half stop out. I will continue to distrust light meters. Laughing


PostPosted: Wed Nov 26, 2008 10:34 pm    Post subject: Reply with quote
martinsmith99 wrote:
Ok, quick test when I got in. Done using a dask lamp shining onto a plain painted wall. All readings were ISO 400 @ 1/15


Light meter:
Sekonic L358 F2.8

SPF Open Aperture:
with SMC Tak 55mm F4

SPF Stopdown
with Helios 58mm F4.8
with CZJ Flek 35mm F4.8

Canon 40D
with Helios 58mm F4.8
SMC Tak 55mm F4.8
off topic but with Sigma AF 17-70 F7.1 WHAT?????

I've no idea what happened in the 1st and last readings but I've done them 3 times and they're consistant.

Conclusion:
The SPF in stopdown mode agrees with my 40D. In OA metering it's a half stop out. I will continue to distrust light meters. Laughing

I think this looks very positive for a first trial of a well-used old camera.

How did you set the aperture at f4.8? I'm wondering if that should be 2.8, which would agree with the Sekonic meter. The Sigma reading looks suspect, I think I'd discount this altogether as it just complicates things.

The SPF open-aperture metering system uses two CdS cells, one for low level and the other for high level light values, and each cell has its own calibrating resistor. The service manual describes how to calibrate these using a bench-mounted tester with several screens of different light values. The test you did is quite a low level and I think you should also try with a bright subject. You may well get different results.

Finally, the actual aperture size of a lens can be fairly unreliable compared to the marked settings, so best practice is to use lenses wide open and adjust the shutter speed.


PostPosted: Thu Nov 27, 2008 7:14 am    Post subject: Reply with quote
Yes, I did mean 4.8. OA gave F4 and stopdown needed an extra half stop. If I can match them it'll make life easier

Ok, I will try again in daylight, otherwise known as the weekend.

I'm wondering if the Sigma, being a modern lens, makes better use of the light. This is off topic so I'll talk about this later in another thread, comparing new/old lenses.
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