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06-11-2013, 10:55 PM   #1
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Spotmatic F metering - old topic, new twist

Hello,

I've owned Spotmatics for years but until recently they've all been early models. I recently picked up an SP-F and at first I thought for sure the light meter was whacked, but then I read the manual and was really happy to find out that unlike my early models, it meters wide open knowing what the aperture ring is set at. Wow, no more stop down metering, this is great I thought. Well, at least for the one SMC 50mm lens I have anyway.

But this is where the fun and excitement kind of ended. I tested the meter with a fresh zinc air battery and the meter seemed fairly accurate so put a roll of film in it, packed a couple of old lenses in my bag and went out to have some fun. I shot the first 5-7 shots with the nifty 50 and was really excited about my new purchase, then I screwed in my long zoom lens and went back to "old school" stop down metering, as if any Spotmatic isn't old school. It was kind of a June Gloom morning and so I was shooting wide open at f4.5. My first thought was "I don't have to stop down to meter if I'm shooting wide open", but I couldn't resist the temptation to push up the switch just to see if it gave the same reading with the switch on as off. The needle sank into the underexposed region and my heart sank with it, I didn't know what to think. I started wondering if maybe this model can't open meter with early lenses, even if they are wide open, but it makes no real sense to me why that would be. So I put it away and waited until I could do a little research. I read a few threads here at Pentax Forums and all I really found (so far) were discussions about people discovering their SP-F is open metering and learning how to use it, but not really anyone experiencing the weirdness I just experienced.

I was so dumbfounded by the whole thing that I decided to pull out a light meter and see if I could figure out what's going on, and as it tuns out, the meter is pretty accurate with the 50mm on it with the switch off of course, and surprisingly the the meter is somewhat accurate with my early lenses as long as don't push the switch up, I have to flip the switch on the lens to manual mode and meter that way to set my aperture and shutter speed, and then I can flip it back to auto so I focus and compose.

Has anyone seen this behavior before? Where the meter works fine as long as the switch is off, but when it's on it reads as if there's way less light than is actually available?

Many thanks for your thoughts, experiences and suggestions.

Rick

06-12-2013, 06:26 AM   #2
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Perhaps it is just a way to tell you No NO No about ever trying to meter with the switch up. Likely the switch shuts off the light meter.
06-12-2013, 06:57 AM   #3
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The SPF meter is always on, so battery drain is minimized by putting on a lens cap (which increases the CdS cell resistance). The normal Spotmatic metering switch stops the lens down and switches the aperture-follower pot out of the circuit.
Testing mine with an SMC lens (aperture follower) and a Super Takumar (no aperture follower - so stop down with the switch), I get meter readings about 1/2 stop difference; which is as close as different lenses and methods can expect to match. (Tried with both a CRIS adapter and genuine PX625 mercury cell.)
HOWEVER, metering a Super Takumar wide open will not give the same reading switch off to switch on. With the switch off the aperture follower pot is still in the circuit in an "impossible" position, and will give an erroneous reading (with the lens I had some 5 stops off). Super Takumar lenses must meter with the switch ON.
06-17-2013, 06:49 AM   #4
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Hey, thanks for the excellent information TomB and sorry for the slow reply, I was hoping to have the roll of film developed before replying so I would have the results of metering a Super Takumar wide open with the switch down, which I foolishly did before posting to ask about it.

What you describe is exactly what I suspected how it works with exception of the "impossible" situation. It seems so unlikely to me that the engineers would design the circuitry so that despite the fact that the follower isn't able to detect the aperture setting, the TTL metering wouldn't be accurately metering the light that's coming in.

If you are right and that is in fact how it works, then my shots taken that way are going to exactly how you described, some five stops underexposed, and it also means that there is something wrong in the circuitry when the switch is on because I compared the metering with the switch up to a known working light meter and it was telling me to use settings that were about the same five stops off, meaning that occording to what my handheld light meter was telling me, if I used what the camera was telling me, I would be five stops over exposed. I never took any shots that way, I didn't want to waste the film, I just went back to the 50mm so I could finish the roll without using the switch.

I'll drop the roll off tonight and have more information soon, I'll let you know the results. I'll also do more comparisons with the light meter and even try putting the same lens on my other known working Spottie and copare the readings from the two cameras.

The saga continues . Thanks again, Rick

06-17-2013, 11:23 AM   #5
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QuoteOriginally posted by TomB_tx Quote
The SPF meter is always on, so battery drain is minimized by putting on a lens cap (which increases the CdS cell resistance). The normal Spotmatic metering switch stops the lens down and switches the aperture-follower pot out of the circuit.
Testing mine with an SMC lens (aperture follower) and a Super Takumar (no aperture follower - so stop down with the switch), I get meter readings about 1/2 stop difference; which is as close as different lenses and methods can expect to match. (Tried with both a CRIS adapter and genuine PX625 mercury cell.)
HOWEVER, metering a Super Takumar wide open will not give the same reading switch off to switch on. With the switch off the aperture follower pot is still in the circuit in an "impossible" position, and will give an erroneous reading (with the lens I had some 5 stops off). Super Takumar lenses must meter with the switch ON.

The reasoning behind this design is fairly simple. With the switch down, the camera is set up for full aperture metering with SMC lenses. With the switch up, the camera emulates older Spotmatics that don't have the full aperture mechanism and gives correct readings with lenses that are not built for full aperture metering.
06-18-2013, 07:11 AM   #6
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Well, apparently you guys know what you're talking about (not that I ever had any doubts) because I picked up my film last night and all the shots taken with the non SMC lens wide open with the switch down were severely underexposed. I'm guessing around four stops or so. What is strange is that when I compared the camera's metering with the switch up to a hand held light meter it seemed to be off in the other direction, so I guess I still need to do more testing. I either have problem in the circuitry in the switch up position or I didn't perform a good comparison when I did my first test. I admit that I did it indoors at night using room lighting as the lighting source.

The irony is I didn't trust the camera's meter with the switch up because of my comparison test but I would have been much better using it because at most I would have been a little over exposed instead of so severely over exposed I don't think I can save any of those shots.

For the sake of others down the road who run into this situation I will conclude the post with some final results testing. I will do a much more exhaustive comparison test using lenses that are better for the comparison than the long lens I used, I will do it in sufficient natural light, and I will break out my best hand held light meter.

To be continued, Rick
06-19-2013, 09:38 AM   #7
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You definitely want to be making your test in adequate daylight. I have found that my Spotmatic F's meter performs very slowly -- and potentially inaccurately -- in low light, expeically artificial light.

I too only have a single SMC lens (50mm) and the rest are Super Taks. As a result of never remembering which way to push the meter switch, I only shoot with a handheld incident meter. I never regret shooting this way -- my results are bang-on and consistent from frame to frame, which I never manage 100% with a TTL meter.
06-20-2013, 06:45 AM   #8
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Thanks for the input Filmamigo, very helpful to know that the poor reading in low light is typical of these cameras and not just mine. I have a light meter and I don't mind using it when taking photos in the certain situations, but to me, the beauty of a 35mm camera is it's portability, compactness, and the fact that it's self contained. I usually know when to not trust a camera meter, like back lit situations etcetera, to compensate with an added stop or two to get detail in the shadows.

I haven't had a chance to do a better test yet and I'm hoping that when I do I find the meter to be accurate enough to use without dragging a light meter around. Otherwise this camera may become my backup and I go back to using my SP II because at least I know the meter in that camera is "spot" on. Pun intended. ~Rick

06-20-2013, 08:56 PM   #9
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QuoteOriginally posted by Seerixpix Quote
poor reading in low light is typical of these cameras and not just mine
Pretty typical for CdS meters in general. The lower the light the slower the needle.


Steve
06-21-2013, 09:06 PM   #10
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The results are in!

OK, I ran a barrage of tests. I have compared the camera's metering abilities against two 35mm film cameras known to be in good working order (Canon AE-1 and Olympus OM-1) and a hand-held light meter.

When comparing it to the other cameras I used two lenses, a standard 50mm lens and a 135mm lens so I could see if the longer lens was a factor. I tested in bright light and also in the shadows to see if it was more accurate in one condition versus the other.

The Conclusion: The metering system is not broken but it's definitely not as accurate as my other cameras which were spot on with each other. The Spotmatic was consistently off by 1 stop, occasionally more, always metering less light than there actually was. It didn't seem to make much difference between bright light or in the shadows as long as the shutter speed didn't drop below 60 with an f-stop of at least 2.8. If the light got below that it did get more inaccurate than 1 stop.

In all test conditions, putting on the 135mm exaggerated the inaccuracy, but especially when combining low light and the medium telephoto lens where it was nearly 2 stops off, which makes perfect sense and explains why it was it was at least 3 stops off when I compared the camera's meter to my hand-held light meter on a gloomy morning with a 200mm telephoto on it.

So now that I know exactly how it performs, I'm not afraid to take it out and use it without a hand-held meter. I know it's exactly one stop off under most conditions so I can just let the needle rest at the lower end of the acceptable zone and maybe even a half stop more if the conditions are not good or if I'm using a medium tele. I'll probably bring a meter if I plan to screw on anything longer than a 135mm.

Coincidentally I also learned that my vintage Lunasix isn't as accurate as I thought it was, it's good when the light is good but also looses accuracy in low light conditions.

I hope my tests and this thread of posts turns out to be useful for others and I want thank everyone who contributed their thoughts, knowledge and experience. They were a big help to me!

Best, Rick
06-22-2013, 06:06 AM   #11
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CdS cells when new have very high resistance in the dark, much lower resistance in bright light. With age (and moisture seeping into the cell) the "dark" resistance drops. This makes readings underexpose - especially in dim light - since the cell reacts as if the light were brighter. The readings in bright light can be close (or can be adjusted to read correctly in daylight), but the dim light readings will be farther off. I've had repairmen re-calibrate meters so they read correctly - but not down to the low light levels they would originally. The cure to restore the full range is to replace the CdS cells. (Spotmatics have two - either side of the eyepiece.) Chances are one of the cells in your camera is bad.
I've sometimes bought parts bodies off eBay and found good cells to replace in other Spotmatics. Of course, Eric tests and replaces cells as needed when he services Pentax cameras - so you can get this restored to spec if you want.
06-22-2013, 09:18 AM   #12
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TomB, once again you are a wealth of information. This not only completely explains what's going on with my camera, it explains what's going on with my light meter and thousands of vintage cameras using CdS cells for their meters.

I probably won't do anything to repair this camera, as strange as it may sound I actually don't mind it being inaccurate, it adds to the challenge of being a knowledgable photographer who can get good exposures in any condition. Now that I know exactly what the problem is and how much to compensate for it under all conditions, I feel confident I can use the camera's meter effectively as-is. Film has more leverage, or room for error so to speak than digital does so it's not all that critical to use exacting aperture and shutter speed settings. To me this situation is not a lot different than using exposure compensation for back lit situations or any situation where I know the meter is probably picking up more or less light than there is on the actual subject being photographed. Like filmamigo said, an incident meter reading will always be better than a reflective reading, but the spontaneity that comes with along with using a TTL meter is the joy of a 35mm camera for me.
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