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03-23-2012, 10:24 PM   #16
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QuoteOriginally posted by stevebrot Quote
I have not tested the K-5, K-r, or K-x, but I have tested the K-7 and conductive base does not change meter performance in Av mode. M mode works equally well (meaning very good) with both conductive and non-conductive bases.
Interesting. Are you talking about M and newer lenses, or m42? Are you saying that the K-7 is one of the models that does not need to have shorted contacts/metal lens mount for stop down metering to work in M mode? With the K-r, if the contacts are not shorted, it doesn't stop down, but instead meters wide open.

QuoteOriginally posted by stevebrot Quote
These are interesting observations regarding the K-r. Legacy behavior traceable back to the Kx00D series, perhaps? I am not sure what you mean by the aperture control sliding and the mirror flopping. On the K10D, the aperture actuation lever is triggered regardless of what lens is mounted at exposure time and with green button and with DOF preview. For non-A lenses, it simply goes full-travel. My mirror never "flops".
I mean that when the contacts short with an m42 lens on the K-r, you can hear the "snik snik" of the aperture mechanism moving, and the mirror flops, every time you press the green button to meter in M mode. With the contacts insulated, it's silent, and no flop and it meters correctly (enough).

03-23-2012, 10:47 PM - 1 Like   #17
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QuoteOriginally posted by joe.penn Quote
I don't think this is the case - I am sure that in "AV" mode if the pentax body does not get an aperture reading or the aperture data from the lens that metering is activated in real time, no matter what lens is attached - meaning that live "Zone" metering is happening as long as the body has not went to idle.

'Correct Me If I Am Wrong"
You are wrong...partially. It depends on the camera model. Here are the bullet points:
  • All Pentax dSLR bodies have the "crippled" K-af mount meaning that there is no mechanical coupling between the aperture ring and the body.
  • With the "crippled" mount the lens NEVER communicates its set aperture to the body. As a result, non-A contact lenses can only meter in stop-down mode.
  • A vs. non-A glass is detected by presence or absence of the continuity through the "A" contact
  • Lenses that lack the contact always fail detection
  • Lenses that have the contact will fail detection if the aperture ring is off the "A" position (the pin on the lens side either retracts or an internal circuit is broken)
  • On the earlier models (pre K-7), Av mode with a manual aperture M42 lens delivers identical meter behavior as green button or DOF preview metering
  • The exposure settings are not based on the raw output from the meter on those earlier cameras. Instead, mathematical "splines" are applied based on the known maximum aperture of the lens (as detected from the other mount contacts). Exposure errors happen when the maximum lens aperture is not known. (Exposure logic defaults to a generic profile that often as not is wrong.) The *istD is supposed to be fairly immune to this problem, however.
  • On later models (K-7 and newer), Pentax modified the exposure logic to use a different algorithm for stop-down metering in M mode only. On those cameras M mode will meter differently than Av mode...even though the same amount of light is hitting the sensors. It is not fool-proof, but a huge improvement over the previous system. This feature is documented (though rather obliquely) in the user manual.
Well, there you go...the world according to Steve...


Steve
03-23-2012, 10:56 PM   #18
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QuoteOriginally posted by Philoslothical Quote
I mean that when the contacts short with an m42 lens on the K-r, you can hear the "snik snik" of the aperture mechanism moving, and the mirror flops, every time you press the green button to meter in M mode. With the contacts insulated, it's silent, and no flop and it meters correctly (enough).
Not on my camera. The aperture actuator always moves with green button press unless an A-contact lens is mounted and the aperture ring is in the A position. In that case, the camera simply takes reading with the aperture full open. The key is detection of the A contact which has nothing to do with base connectivity, per se. The mirror never "flops" (maybe due to lack of live view?) except, perhaps for the digital mode DOF preview which I don't use.

Then there is the matter of my demon-possessed Jupiter-9...why the body thinks it can control that lens is beyond me. I have other M42 lenses with conductive bases, but the J-9 is the only one that does that. (See my comment a few notes above.) For that reason, I am unwilling to be totally dogmatic about what does/doesn't work with these cameras and what is/is not used for various camera functions. What I would suggest to people is that they not spend a lot of time attempting to "hack" the system. The best you might do is manage a somewhat workable solution. The worst you might do is ruin a body or a lens. (I remember one forum member who had fragments of aluminum foil wandering around the innards of his camera after a try at shorting the pins...)


Steve

Last edited by stevebrot; 03-23-2012 at 11:05 PM.
03-23-2012, 11:06 PM   #19
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I'm really enjoying this discussion. I was absolutely positive that my k200d was metering correctly in Av mode on my manual glass (I'm not distinguishing between non A and shorted M42), but now i'm going to have to do some fun experimentation to see just what really is going on. None of you would know this, but I have a history of poor recollection... I doubt that the K200d would be any different than the other models referenced in the discussions above.

I will say this for sure... when I'm converting my DNG files, I'm actually surprised when my images aren't at least 1/3 stop under exposed, and usually more (regardless of mode). Then again, I just got my camera last july and haven't been using anything but manual glass almost exclusively since, so that would explain that.

03-23-2012, 11:08 PM   #20
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QuoteOriginally posted by pezmaker Quote
I'm actually surprised when my images aren't at least 1/3 stop under exposed
For most of us, 1/3 stop would be a blessing! At f/1.8 the bias on the K10D is closer to 2 full stops.


Steve
03-24-2012, 02:11 AM   #21
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QuoteOriginally posted by Philoslothical Quote
You shouldn't be using m42 lenses in Av mode. Period. They will expose much more accurately in M mode, with the green button to take a reading.

Av is designed to meter wide open, and calculate the exposure for the aperture you set in camera based on that. Since you can't set the aperture in camera, it will only work wide open, and even then it won't be as accurate because the camera hasn't a clue what the actual aperture size is.
Surely the big advantage of M42 lenses, when compared to K-mount manual lenses, is that the aperture can be stopped down and they can be used in Av mode when not wide open? This use is limited, because beyond f/4 it becomes difficult to focus as you're not focusing wide open therefore light can be very limited. I have a Katzeye screen installed and beyond f/4 the prisms start to darken. But if you want to take a 50mm f/1.4 Tak and use it in Av mode at f/2.8, that's very feasible, not so with a K or M fast 50. However, at least on a K200D, that requires paint removal or film to short the contacts, otherwise the full +2EV comp will be required, and you lose positive EV comp when shooting because it can't go any higher.

To be honest, the easiest and best solution would be for Pentax to allow an EV comp of -4 to +4, and the user would be able to do as he pleases according to his knowledge of the lenses he's using.
03-24-2012, 05:30 AM   #22
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QuoteOriginally posted by stevebrot Quote
Not on my camera. The aperture actuator always moves with green button press unless an A-contact lens is mounted and the aperture ring is in the A position. In that case, the camera simply takes reading with the aperture full open.
On the K-x, with an "A" lens setting and "aperture ring enabled" in the menu,
the green button press does close down the aperture to give a depth of field preview
(to the limited extent possible with viewfinder or LCD screens).

The mirror does not "flop" in this mode.

Last edited by lytrytyr; 03-25-2012 at 12:10 PM.
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