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11-07-2020, 04:04 AM   #1
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Strange behavior of A 50mm 1.2 during Av mode?

Hey guys. As some of you may know, I recently acquired a Pentax-A 50mm 1.2 lens. I attached it to my MZ-3 and fiddled around with the aperture ring.

The camera correctly showed apertures when the lens was set at A, but showed "F --" when I clicked the lens outside of the A position. The metering seems to work, but I was wondering if this is typical behavior for A series lenses.

11-07-2020, 05:38 AM   #2
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Yes

Yes that is normal.
11-07-2020, 06:18 AM   #3
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quite normal

the "F---" coding shows that the camera has no idea what the aperture value is, since you've moved the aperture ring from the 'A' position....
11-07-2020, 06:18 AM - 1 Like   #4
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The user manual states (in manual mode an other than A setting):
"When a lens other than an F or FA lens is used, no approximate aperture indication will appear in the viewfinder."

It also says:
"When using a Pentax A f/1.2 lens with the aperture ring set other than the A position, the center weighted metering mode will be set instead of the multi-segment metering mode. As the exposure will come out 1 stop over, set the lens aperture ring to [ A ], or adjust the exposure deliberately 1 stop under."

-pg 61 of manual - available here: Pentax Manuals

DA and D-AF lenses should show the approximate aperture but the manual was written before these lenses existed

11-07-2020, 08:09 AM   #5
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QuoteOriginally posted by Not a Number Quote
The user manual states (in manual mode an other than A setting):
"When a lens other than an F or FA lens is used, no approximate aperture indication will appear in the viewfinder."

It also says:
"When using a Pentax A f/1.2 lens with the aperture ring set other than the A position, the center weighted metering mode will be set instead of the multi-segment metering mode. As the exposure will come out 1 stop over, set the lens aperture ring to [ A ], or adjust the exposure deliberately 1 stop under."

-pg 61 of manual - available here: Pentax Manuals

DA and D-AF lenses should show the approximate aperture but the manual was written before these lenses existed
So basically if I want to shoot in Av mode I should set the exposure compensation dial down to -1 for all apertures or only at 1.2?

Also thanks for linking the manual, I'll have a read through when I can
11-07-2020, 11:12 AM - 1 Like   #6
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QuoteOriginally posted by someasiancameraguy Quote
The metering seems to work, but I was wondering if this is typical behavior for A series lenses.
The F--- display behavior is expected when off the A setting.* The exposure comp requirement for the f/1.2 is not typical and may be due to how the metering works on that and similar bodies.

QuoteOriginally posted by someasiancameraguy Quote
So basically if I want to shoot in Av mode I should set the exposure compensation dial down to -1 for all apertures or only at 1.2?
My reading would be for all apertures on the the f/1.2 lens. You can easily compare the Av or M meter readings to the EV measured in P or Tv modes if you need to know for sure.


Steve

* Some bodies simply go to a blank display.

Last edited by stevebrot; 11-07-2020 at 11:18 AM.
11-07-2020, 12:42 PM   #7
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QuoteOriginally posted by someasiancameraguy Quote
So basically if I want to shoot in Av mode I should set the exposure compensation dial down to -1 for all apertures or only at 1.2?
If you want to shoot in Av mode, you should use the 'A' setting. If you do not use the 'A' setting, the camera will not stop down to your selected aperture--you'll be shooting wide open, and with center-weighted metering.

If you use other than 'A' setting, the you should shoot in manual, and you'll have to use the compensation.





11-07-2020, 02:30 PM   #8
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QuoteOriginally posted by johnyates Quote
If you want to shoot in Av mode, you should use the 'A' setting. If you do not use the 'A' setting, the camera will not stop down to your selected aperture--you'll be shooting wide open, and with center-weighted metering.

If you use other than 'A' setting, the you should shoot in manual, and you'll have to use the compensation.



This is incorrect for the MZ-3. The manual p. 56 states you should set the aperture to the desired setting and the shutter speed will be set automatically as one would expect for Av mode.
11-07-2020, 02:33 PM   #9
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QuoteOriginally posted by johnyates Quote
If you want to shoot in Av mode, you should use the 'A' setting.
The MZ-3 has modal controls similar to that on the Super Program and in line with those on the MZ-5(n) and MZ-S. I am not sure, but I believe the need to compensate with the f/1.2 lens is related to compromises in meter tuning to the focus screen, a situation similar to the stop-down metering issues with wider/narrower apertures on Pentax dSLRs.


Steve

Last edited by stevebrot; 11-07-2020 at 02:40 PM.
11-07-2020, 03:41 PM   #10
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QuoteOriginally posted by stevebrot Quote
The F--- display behavior is expected when off the A setting.* The exposure comp requirement for the f/1.2 is not typical and may be due to how the metering works on that and similar bodies.
Yep the A50.1,2 also works differently on the MZ-S. The camera will not meter in matrix and defaults to centre weighted for this lens.

Phil.
11-07-2020, 05:28 PM   #11
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On a related aperture ring note .. Pentax killed off the old rheostat aperture detector and forced us to use stop down metering years ago on DSLRs.
Well remember those aperture readout windows on the prisms like the KX, MX and others?
So how about a modern version with OCR so that the body can know exactly what aperture K and M series lenses (and 3rd party) are set at!?
The info is always in that same spot on those lenses, should be a relatively simple optical and software implement, no mechanics needed.
I would welcome such a feature!
11-07-2020, 06:43 PM   #12
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QuoteOriginally posted by gofour3 Quote
The camera will not meter in matrix and defaults to centre weighted for this lens.
I think every cameras metering system from the MZ-3 on down reverts to CWA with A series lenses.
11-07-2020, 07:12 PM   #13
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QuoteOriginally posted by Digitalis Quote
I think every cameras metering system from the MZ-3 on down reverts to CWA with A series lenses.
A-series lenses support matrix metering just fine; however, it only works when the camera is controlling the aperture setting. As a result, the MZ-3 and similar models having modal controls (aperture ring controls aperture in Av and M modes) are not able to offer matrix metering in other than P and Tv modes. The same rule exists for Pentax dSLRs.


Steve

(...the K-3 behind me is in matrix meter mode with A 50/1.7 attached and aperture ring in the "A" position...)
11-07-2020, 08:15 PM   #14
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The MZ-S can't control a lenses aperture if the A/F/FA lens aperture is in the "A" position. (The Z-1P can) Any lens must be off the "A" position for Aperture Priority.

The MZ-S supports matrix metering with all A/F/FA lenses and only supports centre weighted with K/M lenses. The exception is the A50/1.2 which does not support matrix metering for some reason. (Page 127 of the MZ-S manual has a chart with the supported functions each lens series supports.

Phil.
11-07-2020, 10:37 PM   #15
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On DSLRs matrix metering needs to know the min and max apertures of the lens. On MF "A" lenses this is set by the passive contact pattern on the lens mount when the aperture is set to "A". It's probably the same on MZ-3/S and other models with matrix metering..

With all the pins in contact the min and max are f1.2 and f22 which is the range of the A-50mm f/1.2 lens. So if you shorted the "A" any K or M lens with a conductive base would read a range of f.12 to f22. It is possible the MZ-3/Z check the passive contact setting even without the "A" pin being shorted. If that is the case the body wouldn't be able to distinguish between a K or M lens (with conductive base) and an A-50mm f/1.2 lens.

With F, FA and later lenses the min and max apertures is signaled over the "data" pin as well as the passive contacts.

*Passive because the only information is whether it is "on" (connected to ground) or "off" (insulated or no connection to ground).
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