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Old May 19, 2010, 2:08 AM   #1
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Default Why the exposure variance?

I know a little about cameras after ~55 years' usage. I know a little about my K20D after 2+ years' usage. But something I don't know: why is metering 'off' with some manual lenses?

Case in point: I often use Helios-44 58/2 and Nikkor 85/2 lenses, and whether in manual or auto mode, I normally have to set EV -1 on the metering. When I use a Tele-Takumar 200/5.6 it's EV -2. With a Meyer 180/5.5 it's EV -3. Whether in manual or auto, the metering is normally way off. Why?

Also of interest: I may mount the Tele-Tak or the Meyer on a flanged non-infinity-focus adapter, for closer shooting. I've scraped the finish off the adapter so it shorts all the pins, for Catch-In-Focus use. Yet the TeleTak usually reports F-- while the Meyer always says F=2.8. Why?

These are not critical questions. Not yet, anyway.
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Old May 19, 2010, 5:23 AM   #2
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Interesting you should raise that point today Rico.

Pottering around the garden today I was trying out a CZJ 135 (with and without x2 Viv TC and Raynox 250) and then later back to the Tamron 90 Di macro. Obviously the CZJ was in MF but the T90 was also used mainly in MF.

As the light started to fade the metering for some shots with the CZJ was way off - under-exposing (1/350 when it should have been 1/100) and then with the T90 (in Av) over-exposing (ca. 3 secs when it should have been 0.5 secs). Very strange - may be completely unrelated to your issue though.

As for your second question. Why with an M42 MF lens is the camera reporting any F stop ? Mine always have F-- What am I missing here ?
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Old May 19, 2010, 5:50 AM   #3
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Hi RR,

I believe that the overexposure with fully manual lenses is a metering quirk that has pretty much existed with all the models until it was improved with the new 77 segment metering sensor of the K-7. I could be wrong -- I don't have any fully manual lenses, so it's just what I've read. I also seem to recall that the original D metered ME lenses more accurately, but I could be wrong here also.

Certain pins or combinations control the max aperture of the lens reported to the camera. I don't know exactly which ones do what, but I've had to experiment with insulating different pin combinations to use Sigma lenses with the F 1.7x AFA to get a usable starting point for the aperture reading. With some, you have to insulate pins 1 and 6, on others, its 1,2 and 6, and with my 180 Macro, it's 2 and 6 to get the camera to start at f3.5 so I can get exif info that means something. It also probably has an effect on P-TTL flash exposure, since the camera has to calculate the exposure/flash duration needed from a base line of the max aperture. When Sigmas are used alone, they report correct max apertures, but the AFA automatically converts them to the correct effective aperture considering the 1.7x multiplication factor of the adapter and reports that to the camera.

My understanding is that Sigma lens chips have to be reverse engineered because they don't buy the different mount licenses from the OEM camera mfgs. Some of the values they supply in their chips don't match Pentax's protocols, so some lenses are identified strangely for exif purposes, and most max apertures aren't reported correctly when used with the AFA. I assume that this is why Sigma P-TTL flashes needed to have their chips replaced whenever Pentax changed some of the P-TTL protocols in newer bodies, while the Pentax branded models haven't needed to be updated.

I'm assuming that the lens/adapter is somehow reporting an f2.8 max aperture through some accidental pin combination that's either shorted or conductive, or both. The aperture can't be controlled by the body because there's no mechanical aperture link to the lens. -- just guessing here since I don't have a clear understanding how it works, I just know that by trial and error, insulating some pin(s) in combination with the data pin (#6) causes the AFA to report different max apertures.

Scott
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Old May 19, 2010, 8:20 AM   #4
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Quote:
Originally Posted by snostorm View Post
My understanding is that Sigma lens chips have to be reverse engineered because they don't buy the different mount licenses from the OEM camera mfgs. Some of the values they supply in their chips don't match Pentax's protocols, so some lenses are identified strangely for exif purposes, and most max apertures aren't reported correctly when used with the AFA. I assume that this is why Sigma P-TTL flashes needed to have their chips replaced whenever Pentax changed some of the P-TTL protocols in newer bodies, while the Pentax branded models haven't needed to be updated.

Scott
Wonder if that's why my Sigma 24-135mm lens is picked up as a Pentax-F 28-80mm lens? At least that's what the exif reports.
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Old May 19, 2010, 10:44 AM   #5
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Quote:
Originally Posted by MadMikeSS View Post
Wonder if that's why my Sigma 24-135mm lens is picked up as a Pentax-F 28-80mm lens? At least that's what the exif reports.
Hi Mike,

I think it is. I haven't hear of other 3rd party lens mfgs having that problem. It could cause a situation if you had both lenses and only one needed focus adjustment to shoot well. the correction would also be applied to the lens that didn't need it. Also, incorrect aberration correction could be applied to the misidentified lens if you use the feature.

On the plus side for Sigma for us -- they have been able to charge less, and supply us with some unique and useful lenses, but now that they are switching over and including OIS and HSM, the price advantage is disappearing.

Scott
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Old May 19, 2010, 5:10 PM   #6
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Rico,
I've had similar things happen when using different lenses and TC's
One time I had the camera report the aperture as F:2.5 when the lens was only capable of F:3.5-4.5 Go figure.
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Old May 19, 2010, 7:45 PM   #7
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Goldy,
I'm not really concerned about the reporting, just curious. I put a flanged M42-PK adapter in the camera, screw in the Meyer 180/5.5, get F=2.8. Then screw in the TeleTakumar 200/5.6, and get F--. Without changing the adapter. I'd expect to see various numbers depending on which pins are shorted, and that would vary with different TCs, lenses, adapters, etc. But this is curious.

No, what gets me is how the lenses require such different EV adjustments. The 180 and 200 are both slow and small, about the same size, though with rather different optics. I know that when the K20D sees a manual lens, it shifts to center-weighted metering. I'm just surprised that the metering is so inconsistent between lenses. [/shrug] But who am I to question the wisdom of the Pentax Designers?
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Old May 19, 2010, 8:41 PM   #8
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Yes, that seems to be the case with some of my lenses too, now that I think about it.
I always thought it was just me doing something wrong but since you mentioned it, it does make sense now, that is what's happening to me also.
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Old May 20, 2010, 2:07 AM   #9
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Ditto GW. I always check the Exif data to see what I'm doing right and what I'm doing wrong ... I often see variations in the EV even though I haven't adjusted it.
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Old May 20, 2010, 4:26 PM   #10
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I've never used M42 lenses, only K-mount lenses. I do know that the green button metering on the K10 and K20 was off - some of my lenses wouldn't meter correctly. They had metered all right with the DS and work well with the K7 - don't ask me what they did to get the metering to be better on the K7, but it definitely is.
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