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Comment by buris posted on Anyone bought from these Guys (Ebay)?? at 18/10/2008 - 20:28
Thanks for the replies to my posting, there are however a few points Iwould like to point out.
As I understand the point of having an A setting on the A-type lens is that it is possible, provided there is some kind of predictability in the relationship between the aperture actuator and the actual aperture, to let the processor in the camera decide the aperture when shooting in Auto or Shutter priority.
The way it usually is realised is by having the aperture set at the smallest setting of the lens.
The "A" setting on pentax A lenses, is (as far as I know) an example of this, therefore the actuator from the body has to stop down differently, given different chosen apertures by the camera program, if it didn't, the aperture would have to be controlled by some other means (electric contacts?) in order to change the aperture.
That the function is as described is how I understand the explanation on the k-mount homepage (see link in my previous posting).
Another aspect is that the lack of the coding contacts in the mount also gives problems for the camera body - in order to function corectly the camera needs 3 informations from the lens: the minimum and maximum apertures and whether or not the aperture ring is set to the minimum
For non A type lenses none of these informations are given, or rather the information given are:
minimum aperture: 22 maximum
aperture: 1.2
since all of the lens contacts are "on".
If my guess is correct, then it should be possible to see apertures suggested by the camera meter, which are larger than the max aperture of the lens in some instances.
(Slow shutter time, abundant light)
Another aspect is the question of how an(old) A type lens will work in Non A mode I assume You mean if the aperture control is set to a value other than A - in that case I would not expect similar results, because even if the aperture lever works the new cameras do not have the aperture indexing lever so what the actual chosen aperture is will not be known of the camera.
I must say that I do not(yet) own a pentax k10d, so I am not able to test the hypotheses myself.
As I understand the point of having an A setting on the A-type lens is that it is possible, provided there is some kind of predictability in the relationship between the aperture actuator and the actual aperture, to let the processor in the camera decide the aperture when shooting in Auto or Shutter priority.
The way it usually is realised is by having the aperture set at the smallest setting of the lens.
The "A" setting on pentax A lenses, is (as far as I know) an example of this, therefore the actuator from the body has to stop down differently, given different chosen apertures by the camera program, if it didn't, the aperture would have to be controlled by some other means (electric contacts?) in order to change the aperture.
That the function is as described is how I understand the explanation on the k-mount homepage (see link in my previous posting).
Another aspect is that the lack of the coding contacts in the mount also gives problems for the camera body - in order to function corectly the camera needs 3 informations from the lens: the minimum and maximum apertures and whether or not the aperture ring is set to the minimum
For non A type lenses none of these informations are given, or rather the information given are:
minimum aperture: 22 maximum
aperture: 1.2
since all of the lens contacts are "on".
If my guess is correct, then it should be possible to see apertures suggested by the camera meter, which are larger than the max aperture of the lens in some instances.
(Slow shutter time, abundant light)
Another aspect is the question of how an(old) A type lens will work in Non A mode I assume You mean if the aperture control is set to a value other than A - in that case I would not expect similar results, because even if the aperture lever works the new cameras do not have the aperture indexing lever so what the actual chosen aperture is will not be known of the camera.
I must say that I do not(yet) own a pentax k10d, so I am not able to test the hypotheses myself.
Comment by buris posted on K10D Stop down metering exposure problem at 18/03/2007 - 20:03
Hi I have been following this stop down metering discussion for a while because I am very tempted to buy a pentax k10d.
I wonder if the inconsistencies concerning stop down measurement and actual exposure for non-A pentax k mounts could be due to the function of the aperture lever.
According to the k-mount website http://www.bdimitrov.de/kmp/technology/K-mount/Ka.htm
the old pre-A k mounts had a linear response of the aperture actuator with respect to the diameter of the aperture, while the kA-mount lenses have a linear response with respect to the area of the aperture.
Since the digital cameras use the "crippled" KA mount (no metering index lever)- and therefore don't receive any infomation as to what the actua chosenl aperture is, AND stop down using the apereture actuator AS IF it is an A type mount this must lead to, in some instances, quite substantial differences in exposure.
My guess is that the actuator of the camera only moves the way it should if an A-type lens was attatched whereas the actual pre-A mount will work if the actuator moves to or beyond the required aperture, which won' be at the same position in those two instances.
I also guess that the inconsistencies experienced with different k-mount lenses (pentax or others) are due to the fact that prior to the A-mount the only factors required for correct operation were:
- A correct mechanical coupling between lens and camera body (the index lever at the right side of the camera body when you look at the mount) telling the camera body the chosen aperture.
- A coupling between aperture actuator and body which insured that the lens was wide open when mounted, and that it could close all down, when the picture was taken.
This would explain the differences between correct metering and progressive overexposure some have reported.
It would also, assuming that Pentax (or others) produced non-A lenses with apertures having a linear response concerning aperture area, explain why some don't have these troubles at all, but it wold be due to the lenses and not to the camera house.
I wonder if the inconsistencies concerning stop down measurement and actual exposure for non-A pentax k mounts could be due to the function of the aperture lever.
According to the k-mount website http://www.bdimitrov.de/kmp/technology/K-mount/Ka.htm
the old pre-A k mounts had a linear response of the aperture actuator with respect to the diameter of the aperture, while the kA-mount lenses have a linear response with respect to the area of the aperture.
Since the digital cameras use the "crippled" KA mount (no metering index lever)- and therefore don't receive any infomation as to what the actua chosenl aperture is, AND stop down using the apereture actuator AS IF it is an A type mount this must lead to, in some instances, quite substantial differences in exposure.
My guess is that the actuator of the camera only moves the way it should if an A-type lens was attatched whereas the actual pre-A mount will work if the actuator moves to or beyond the required aperture, which won' be at the same position in those two instances.
I also guess that the inconsistencies experienced with different k-mount lenses (pentax or others) are due to the fact that prior to the A-mount the only factors required for correct operation were:
- A correct mechanical coupling between lens and camera body (the index lever at the right side of the camera body when you look at the mount) telling the camera body the chosen aperture.
- A coupling between aperture actuator and body which insured that the lens was wide open when mounted, and that it could close all down, when the picture was taken.
This would explain the differences between correct metering and progressive overexposure some have reported.
It would also, assuming that Pentax (or others) produced non-A lenses with apertures having a linear response concerning aperture area, explain why some don't have these troubles at all, but it wold be due to the lenses and not to the camera house.
Comment by buris posted on K10D Stop down metering exposure problem at 17/03/2007 - 08:53
In both instances i must say i have every reason to be satisfied with the company, the price was very reasonable and the shipping was quick - furthermore track and trace number from the shipping company (ups) was supplied as soon as the items were processed for shipping. All in all i have been very satisfied.
cheers
Buris (Copenhagen, Denmark)
(k10d, 10-17, 12-24, 16-40, 21 + 40 da, 35-105 + 100 A, voigtlander 58 nokton + several tamron and a few sigma)