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08-27-2015, 08:15 AM   #16
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My imagined solution was to have the shutter button half-press - not needed for AF actuation with K and M lenses - linked to the stop-down aspect of the green button, so that metering would become a seamless part of taking the picture. The problem then is how you arrange your focus confirmation. Split-prism focusing screen seems the most likely answer there.

08-27-2015, 08:24 AM   #17
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QuoteOriginally posted by MadMathMind Quote
This is not possible because the meter is located in the pentaprism. Once the mirror goes up, no light can reach the meter.
The Pentax ist ds does not put the mirror up when the stop down metering button is pressed to set the shutter speed.
What about the later Pentx dslrs? I only have the ist ds plus K-01
08-27-2015, 12:01 PM   #18
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QuoteOriginally posted by wombat2go Quote
As I understand it:
For the M lenses, the stop down lever is a simple on/off motion that only releases the M lens to spring down to the setting of the ap ring.

For A lenses in A mode, the f/- is sent in binary via 2 ( or 3) gold contacts.
For DA lenses, f/- is sent as data

As mentioned, I think Pentax users benefit from the new cameras still having all the above functionality, duplicated so the old K lenses still work.
it mimics a simple on/off switch with M lenses because of how the firmware is set up. there's no reason it couldn't move the aperture lever half way, etc.. the pins on A lenses only tell the camera the maximum and minimum aperture, allowing it to calculate how far to move the lever for a given f-stop setting. entering the maximum aperture manually and having the camera map out the non-linear movement of the aperture on M lenses should allow them to be used like A lenses.

---------- Post added 08-27-15 at 02:07 PM ----------

QuoteOriginally posted by wombat2go Quote
The Pentax ist ds does not put the mirror up when the stop down metering button is pressed to set the shutter speed.
What about the later Pentx dslrs? I only have the ist ds plus K-01
stop down metering uses the same mechanism as dof preview. it would be pretty pointless to have the mirror flip up and block the viewfinder. it does flip up after the fact though on the k-30, which has a single simplified mechanism for both the aperture lever and mirror.

Last edited by maltfalc; 08-27-2015 at 12:10 PM.
08-27-2015, 12:21 PM   #19
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It is my understanding that M lenses and K lenses aren't random; supposedly the movement of the aperture is tied to the diameter of the opening rather than the area. Meaning moving it a certain distance cuts the diameter of the opening in half vs. cutting the area in half which is how the A lenses supposedly work. All of this is based on old web research and I would have to look again to find sources.

08-27-2015, 12:58 PM   #20
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QuoteOriginally posted by fwcetus Quote
I'd agree with all of that ~except~ "and you can even meter properly using the Green Button" -- I think the metering is actually quite poor with K and M lenses (and, I assume M42 lenses, too).

Having to use the Green Button is not a huge inconvenience, all things considered, but the non-linearity of metering is very frustrating to me (YMMV).
I dunno. I've never had a problem. Been using old glass with Green Button metering on every digital body I've had: K100DSuper, K10D, K7, K5, and now K3. I've even had replacement focussing screens in most of those. Always gives me a solid starting point. You occasionally have to chimp a bit but, yeah, I've taken literally hundreds and hundreds of photos that way without it ever seriously bothering. So yeah, YMMV.
08-27-2015, 03:13 PM   #21
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QuoteOriginally posted by chickentender Quote
I dunno. I've never had a problem. Been using old glass with Green Button metering on every digital body I've had: K100DSuper, K10D, K7, K5, and now K3. I've even had replacement focussing screens in most of those. Always gives me a solid starting point. You occasionally have to chimp a bit but, yeah, I've taken literally hundreds and hundreds of photos that way without it ever seriously bothering. So yeah, YMMV.
Yes, I see. I guess I'd have to say that I do generally get "a solid starting point", too. And then, I usually count aperture ring clicks (of course, having to know what each click indicates for each particular lens), and change the shutter speed (from what the wide open aperture metering chose) to match the "counted" aperture setting that I chose. But, it still would be nice if the metering were more linear/consistent.
08-27-2015, 04:55 PM   #22
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QuoteOriginally posted by fwcetus Quote
I think the metering is actually quite poor with K and M lenses (and, I assume M42 lenses, too).
Yep, though not any worse than stop-down with other brand dSLRs. The problem is very endemic on PDAF dSLRs. The focus screens on those cameras have been optimized for the unavoidably dim view finder image traceable to splitting light off the main mirror for the AF system.


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08-27-2015, 04:59 PM   #23
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QuoteOriginally posted by wombat2go Quote
As mentioned, I think Pentax users benefit from the new cameras still having all the above functionality, duplicated so the old K lenses still work.
Unfortunately there is little return on investment (ROI) to Ricoh for improving compatibility with lenses that were made more than 30 years ago. Credit should be given, however, for their willingness to retain the current degree of backward compatibility at all levels of the product line.


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08-27-2015, 05:13 PM   #24
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QuoteOriginally posted by maltfalc Quote
there's no reason it couldn't move the aperture lever half way, etc.. the pins on A lenses only tell the camera the maximum and minimum aperture, allowing it to calculate how far to move the lever for a given f-stop setting. entering the maximum aperture manually and having the camera map out the non-linear movement of the aperture on M lenses should allow them to be used like A lenses.
Assumptions are always the binding point for good ideas. In this case, it is contained in the statement immediately preceding the above.

QuoteOriginally posted by maltfalc Quote
it mimics a simple on/off switch with M lenses because of how the firmware is set up.
This is not strictly true nor is there any reason to believe it is true. The body actuator acts like a guillotine because there is no guarantee of what the lens is going to do. It was only with the KA mount that the aperture actuator behavior was specified and even then it was proprietary.

The assumption of your idea is that the gamut of non-linear actuator designs in the wild allow for small increments that might be mapped with adequate precision* to at least 1/2 EV steps at the iris opening. I own a number of K-mount lenses (20+) without "A" contacts and the aperture actuation behavior is quite variable overall. In general, I believe a safe generalization is that makers were aiming to open/close the iris in as short a period as possible with little interest in pausing along the way (so to speak).

Yours is not a bad idea. I just suspect that the majority of vintage K and M series lenses would not be able to successfully calibrate onto a supporting body.


Steve

* The current KA2 design, along with similar Nikon product, are a continuing challenge to lens makers due to the precision requirements. Literally, that is making sure that n-degrees of lever deflection translate to n-change of iris opening in a consistent manner regardless of temperature, humidity, etc. (getting the same aperture twice in a row). That is a problem despite having full control over the mechanism and its behavior. Sigma has said as much and cites the actuator control design as one reason why they provide only a limited selection of lenses in K mount.

Last edited by stevebrot; 08-27-2015 at 05:33 PM.
08-27-2015, 06:47 PM   #25
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I'm surprised that the A and newer lenses act as everyone is saying,
That the actuator arm in the body is calibrated, and only lets part of the aperture open. It would seem simpler to have the body control electronically set a "stop" in the lens, much like a pre-set lens, and have the actuator arm just get out of the way so the aperture snaps closed to that set point the same as the K & M lenses.

As much precision and reliability as you can get out of an electronic system, to have this mechanical actuator be the final link seems fiddly and prone to error.
08-27-2015, 07:45 PM   #26
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QuoteOriginally posted by K-Three Quote
I'm surprised that the A and newer lenses act as everyone is saying,
That the actuator arm in the body is calibrated, and only lets part of the aperture open. It would seem simpler to have the body control electronically set a "stop" in the lens, much like a pre-set lens, and have the actuator arm just get out of the way so the aperture snaps closed to that set point the same as the K & M lenses.

As much precision and reliability as you can get out of an electronic system, to have this mechanical actuator be the final link seems fiddly and prone to error.
That's exactly how some other manufacturers do it. But they also abandoned their old lenses being compatible with some of their bodies and a few even changed mounts. This is how it works because that's how it always worked on A series lenses. It's a system that may have a few warts but it lets us use very old glass effectively.
08-27-2015, 08:21 PM   #27
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QuoteOriginally posted by K-Three Quote
It would seem simpler to have the body control electronically set a "stop" in the lens
Yes, though the technology to do so did not exist at the time and the current system actually works as well as it ever did (i.e. very well) and provides excellent backward compatibility.

QuoteOriginally posted by UncleVanya Quote
That's exactly how some other manufacturers do it.
Yep. A good example would be the EOS system where even manual focus lenses are dependent on the body to power the motor-driven aperture diaphragm. Most Nikon cameras and lenses use a system very similar to Pentax with the exception of E and PC-E lenses which use a motor-driven diaphragm. Nikon E and PC-E has seen implementation limited to lenses where a mechanical linkage is impractical.


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08-27-2015, 08:43 PM   #28
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QuoteOriginally posted by stevebrot Quote
Unfortunately there is little return on investment (ROI) to Ricoh for improving compatibility with lenses that were made more than 30 years ago. Credit should be given, however, for their willingness to retain the current degree of backward compatibility at all levels of the product line.


Steve
It must be said, however, that the backward compatibility of all their lenses has long been one of the selling points used in advertising for Pentax bodies, including the DSLRs, but any RoI arising from the improved compatibility of postM42/pre-A series lenses would be rather hard to measure, and confined to the realm of good will, and not in the accountancy sense of the term.
08-27-2015, 09:21 PM   #29
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Even if it would work, setting up each lens in an in-camera database ( I have something like 40 at this time and have 10% or so annual turnover), remembering to call each lens each time it is mounted and whatever other user-work this would require seems a big pain and overly prone to user error.

Using the Green Button or Optical Preview lever works well enough and is simple to implement at the user level.
08-27-2015, 09:43 PM   #30
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QuoteOriginally posted by monochrome Quote
Even if it would work, setting up each lens in an in-camera database ( I have something like 40 at this time and have 10% or so annual turnover), remembering to call each lens each time it is mounted and whatever other user-work this would require seems a big pain and overly prone to user error.

Using the Green Button or Optical Preview lever works well enough and is simple to implement at the user level.
no database needed. click one button when switching lenses, input the max aperture, the camera takes a quick series of meter readings and you've got an M lens that acts like an A.
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