Originally posted by newarts Steve, I'm sorry to tell you that what you say is simply not true in the case of the K100D (and K200D according to rpriedhorsky - I don't think s/he's making it up either.) when using an m42 lens with a flangeless adapter.
In Av mode, with electrical contact, an m42 lens will meter correctly open or closed down; without electrical contact it will not meter properly in Av mode.
Pentax K and M mount lens automatically short the contacts when mounted because of the chrome plating on their base. If you cover the chrome plating on a K or M lens with a non-conductor they'll work exactly the same as a m42 with no contact (they won't meter correctly in Av mode.)
I don't think I can say it more clearly.
Dave
What do you mean, by "not meter correctly"? I am not trying to be demeaning, sarcastic, or pedantic. There are issues with stop-down metering with many Pentax dSLRs, but those issues exist for all Pentax lenses capable of stop-down metering (M-42 and all K-mount lenses with an aperture ring) regardless of whether the base is conductive.
Are you saying that shorting the "A" contact will correct the meter inconsistencies present when using non-A lenses? If so, ignore the rest of this message and PM me to tell me more. If not, read on...
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In regards to my personal experience...
I don't have a K100D or K200D. All I have is a K10D and a couple of M-42 lenses and several non-A K-mount lenses that I use for the majority of my serious shooting. Neither of my M-42 lenses have a conductive base and both operate just fine in AV mode. (At least as fine as my non-A K-mount lenses, but that is another story...I will be posting some exposure bias plots sometime in the near future for all of my manual lenses.)
In regards to the Pentax-A mount, a little historic reference might be helpful here. Please don't be insulted if you already know this stuff. The important thing to remember is that the Pentax-K and Pentax-M lenses were designed before the "A" mount was developed and that both, along with the M-42 lenses supported AV exposure automation by design. Subsequent variations of the K-mount, with the exception of the "crippled" versions allowed for full forward and backward compatibility between cameras and lenses for all variants.
The "A" variant of the K-mount was designed originally to allow for full program exposure automation. This required that the mount signal both minimum and maximum aperture to the camera body. The body also had to know whether the mounted lens supported aperture control from the body (not true for Pentax-K or Pentax-M). That last piece was done via the "A" contact.
The "A" contact is recessed into the mount on the body. The original Pentax-A lenses had a short pin that protruded to make contact ONLY when the aperture ring is at the "A" setting. A circuit through the "A" contact signaled the camera to control the aperture from the camera body. No contact meant that body control and thus program automation was not possible. This allowed full backward compatibility to M-42 and non-A, K-mount lenses while enabling program exposure for the newer lenses. Pentax-K and Pentax-M DON'T make contact even though many (not all) have conductive bases. That is the whole purpose of the design. Conductive bases ARE required for the "A" setting to work, however.
Information regarding maximum/minimum aperture was communicated by a pattern of insulated points on the lens that corresponded to electrical contacts on the camera. The base would conduct at all points except for where it was insulated. Unless contact is made through the "A" contact, all the others would be ignored. This scheme has been modified somewhat on the FA and DA lenses, but is still pertinent for this discussion.
On the original A and FA(non-crippled) bodies, K-mount lenses with aperture set through the lens ring communicated aperture information (not the actual f/stop, but the offset from maximum aperture) via a mechanical linkage that worked a variable resister in the camera body to bias the meter. This mechanical linkage is missing on the FA(crippled) mounts found on current Pentax dSLRs. As a result, all non-A lenses must meter in stop-down mode when used with the crippled mount on these cameras.
There is one other piece to the puzzle that is important. FA and DA lenses always short the "A" contact. Information regarding the aperture ring position, max/min aperture, and other data is transmitted through a dedicated data pin.
So what does this all mean?
- When you insulate your FA lens from the body with a sheet of paper, the camera thinks it has a non-A lens attached. Usually this means a significant underexposure at large apertures with fast lenses and other metering issues.
- When you mount a non-A lens with a conductive base, the camera thinks that it has a non-A lens attached (no "A" contact means that the body cannot control the aperture). Metering may be flaky...
- Ditto for non-A lenses with non-conductive bases
- Ditto for M-42 lenses with non-conductive bases
- Ditto for M-42 lenses with conductive bases
When you manage to "short" the "A" contact with a lump of foil or some-such when using a non-A lens, the body behaves (near as I can tell from my experimentation) as follows:
- All exposure modes are enabled as if an A-mount lens is mounted
- Since no data pin or insulating points are present, the body assumes the min/max aperture hard-coded into its firmware. The maximum is f/1.2 and this is the value that is displayed on the top LCD on my K10D. I don't know what the minimum is.
- The body assumes an auto-aperture lens with the aperture full-open and meters accordingly. If in AV mode, changing the aperture with the thumb wheel will result in a corresponding change in the shutter speed (duh...).
- Actual taking aperture at exposure time with K-mount lenses may be somewhat variable since the actuation lever movement is calibrated to the A-mount standard. X amount of movement of the actuation lever translates to Y aperture on the lens. The Pentax-K and Pentax-M do not comply with that standard and may be all-on or all-off or somewhere in between depending on the aperture ring position and the aperture chosen on the body.
- M-42 lenses may meter accurately (I have not tested this) in AV mode if the body thumb wheel is set to the lens maximum aperture and the M-42 lens is set for manual aperture operation.
The last point relates to my original comment. This is essentially the same as operating the same M-42 lens in AV mode without shorting the "A" contact. As such, I can't see why the bother. Unless, of course, it might magically correct the lack of meter linearity that we all see with stop-down metering. In which case, you have my attention.
Steve
(Studied the original concept of the "A" mount with considerable interest back in the day when it was first introduced...)