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02-27-2008, 11:17 AM   #16
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i have found some time today to take some more shots with my smc-m 50/1.4, in _daylight_ . the behaviour is very consistent, and it confirms the previous breakdown posted here: some underexposure at 1.4, pretty much square at 2.8, allready one stop over at 5.6, by 8 it goes to almost two stops, and from there, up to 22 or so, it seems to stay "together" (about 2 stops over). note this was an informal test, and i had a hard time, as the light was changing very fast (cloud, sun, etc), a really great day for shooting, and bad day for being in the office

i am sad (it does kind of take the jam out of the cake, for using the smc-m), but i am even more intrigued. i cannot figure out what might be the cause. this happens consistently on various degrees of lighting (so the determining factor is somewhere inside, from the aperture blades going in ), but how? the way i see it, the camera will probably internally settle on a fixed "lensindicated aperture" (so as not to change the metering algorithm too much), and act as if everything is as usual, with a normal (modern) lens, reacting to the aperture closing as if it was a variation in ambient light. if this was it, the exposure should be spot on, as it allways was for me with both the k100d and now the k10d. so, either, uppon stoping down, the camera, for some reason, changes the "assumption" of indicated aperture (i could accept this might be a bug, but how can it tell how much the lens was stopped down, so it can decide how much to err? -- damn, it sounds sick ), or something else happens when stopping down. which brings me to the focusing screen: remeber those good-old split-screen mf focusing screens?, and how they become erratic (dark, but in various positions, depending on how you put your eye on the vf) when you stop down? maybe the metering cell (i will assume just one, for centerweighted) has the optics in front of it alligned such that a similar effect would occur (perhaps it is designed to be influenced less by "out of focus light", i need to test this by shooting completely oof, that might be interesting), the fact that osme people have had great luck with switching the focusing screen seems to enforce that idea, allthough only slightly, as i, for one, am not sure how the metering system is designed, and to what extent it should be influenced by the focus screen. and then comes the infor from katz eye, which state quite significant variations in exposure reading for their custom focusing screens on the k10d..

i think this must be addressed, somehow. right now, the only thing that comes to mind, is to try to research "A-converting" my smc-m, for that i would need to understand exactly how it works, though (for instance, if it is just a variable electrical resistance which the camera reads, shouldn't be so hard to do. if there's some sort of "chip", i am afraid i would be lost..), the idea being to just enable wide-open metering (no control from camera to lens, too much trouble). as things are looking now, i cannot really see how a fix in firmware would be doable, as the camera has no way of knowing what the f stop on the lens is (well, actually it does have a way, but not very reliable: on press of the green button, take a reading, stop down, take another reading, it is trivial to compute how much the lens is stopped down, but without knowing the wide-open aperture, impossible to know the actual aperture set. but than again, if the reading is wrong on stop-down, one would end up with the same problem..)

i will download my card, empty it, and quickly try to shoot out of focus, see what happens. btw: thank you all for your oppinions and relevant feedback so far, makes me proud to be here

02-27-2008, 11:55 AM   #17
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just did a quick test. can't notice any significant difference from shooting out of focus. same behaviour (1.4 to 8, iso100).

anyone with a k100d/k110d and an smc-m willing to test this?
02-27-2008, 12:27 PM   #18
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QuoteOriginally posted by nanok Quote
i have found some time today to take some more shots with my smc-m 50/1.4, in _daylight_ . the behaviour is very consistent, and it confirms the previous breakdown posted here: some underexposure at 1.4, pretty much square at 2.8, allready one stop over at 5.6, by 8 it goes to almost two stops, and from there, up to 22 or so, it seems to stay "together" (about 2 stops over). note this was an informal test, and i had a hard time, as the light was changing very fast (cloud, sun, etc), a really great day for shooting, and bad day for being in the office

i am sad (it does kind of take the jam out of the cake, for using the smc-m), but i am even more intrigued. i cannot figure out what might be the cause. this happens consistently on various degrees of lighting (so the determining factor is somewhere inside, from the aperture blades going in ), but how? the way i see it, the camera will probably internally settle on a fixed "lensindicated aperture" (so as not to change the metering algorithm too much), and act as if everything is as usual, with a normal (modern) lens, reacting to the aperture closing as if it was a variation in ambient light. if this was it, the exposure should be spot on, as it allways was for me with both the k100d and now the k10d. so, either, uppon stoping down, the camera, for some reason, changes the "assumption" of indicated aperture (i could accept this might be a bug, but how can it tell how much the lens was stopped down, so it can decide how much to err? -- damn, it sounds sick ), or something else happens when stopping down. which brings me to the focusing screen: remeber those good-old split-screen mf focusing screens?, and how they become erratic (dark, but in various positions, depending on how you put your eye on the vf) when you stop down? maybe the metering cell (i will assume just one, for centerweighted) has the optics in front of it alligned such that a similar effect would occur (perhaps it is designed to be influenced less by "out of focus light", i need to test this by shooting completely oof, that might be interesting), the fact that osme people have had great luck with switching the focusing screen seems to enforce that idea, allthough only slightly, as i, for one, am not sure how the metering system is designed, and to what extent it should be influenced by the focus screen. and then comes the infor from katz eye, which state quite significant variations in exposure reading for their custom focusing screens on the k10d..

i think this must be addressed, somehow. right now, the only thing that comes to mind, is to try to research "A-converting" my smc-m, for that i would need to understand exactly how it works, though (for instance, if it is just a variable electrical resistance which the camera reads, shouldn't be so hard to do. if there's some sort of "chip", i am afraid i would be lost..), the idea being to just enable wide-open metering (no control from camera to lens, too much trouble). as things are looking now, i cannot really see how a fix in firmware would be doable, as the camera has no way of knowing what the f stop on the lens is (well, actually it does have a way, but not very reliable: on press of the green button, take a reading, stop down, take another reading, it is trivial to compute how much the lens is stopped down, but without knowing the wide-open aperture, impossible to know the actual aperture set. but than again, if the reading is wrong on stop-down, one would end up with the same problem..)

i will download my card, empty it, and quickly try to shoot out of focus, see what happens. btw: thank you all for your oppinions and relevant feedback so far, makes me proud to be here
Glad to see I am not the only one who has this issue. what I am hoping is that all the users with different focusing screens can try this to see what makes the best working solution. I have a funny feeling that it is due to scatter from the focusing screen that results from the design to make for a bright view finder.

regarding changing the K to KA mount, Been there done that, it works sort of. I can modify a lens to give the correct maximum and minimum apature, no issue, it can even be controlled by the camera, no issue there either, the only issue is that on K mount lenses, the apature diameter varies linearly with the actuating lever, on KA lenses, it is the apature area that is linear with lever movement.

The net result, is that without reprogramming the K10D to give the correct movement for the apature being diameter and not area, the exposure is just as bad.

Until I can get someone with a focusing screen test results that seem better, I am using my *istD for manual lenses.
02-27-2008, 12:41 PM   #19
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QuoteOriginally posted by Lowell Goudge Quote
regarding changing the K to KA mount, Been there done that, it works sort of. I can modify a lens to give the correct maximum and minimum apature, no issue, it can even be controlled by the camera, no issue there either, the only issue is that on K mount lenses, the apature diameter varies linearly with the actuating lever, on KA lenses, it is the apature area that is linear with lever movement.

The net result, is that without reprogramming the K10D to give the correct movement for the apature being diameter and not area, the exposure is just as bad.
hmmm. wait a minute. what does the k10d have to do with the lever? the k10d is "crippled", it cannot read that anyway, unless you add a device to the lens to tell the k10d, electronically, what aperture it has set, what is the maximum (wide open) aperture (and the minimum? didn't know it can/will read that). afaik, all the k10d (and i think the other dslr's as well) can do is stop down to the set aperture. 0/1 style. am i wrong? maybe you were reffering to some film body you did this on, which was not crippled?

[edit]
i think i got it! the kaf2 just let's go of the aperture lever for the needed amount, this is how it controls the aperture on A lenses, and most probably on fa/da lenses also. it only needs to know maximum and minimum aperture. simple, very german solution, i would say (all one needs to do is make sure the lenses all behave corespondingly for coresponding movements of the lever). weird solution, though, seems unreliable (and, ofcourse, has proven quite reliable, for decades). is that it? if so, can you tell us how one can tell the camera what are the apertures max/min (hell, this could even be implemented in firmware, as a multiple entry custome setting, for one's few smc-m lenses; unless the camera hw needs to read this info directly for every movement, by design, that is..)
[/edit]

do tell us more, it sounds like you know more than i do on the subject. some rellevant link explaining how it works, maybe?

QuoteOriginally posted by Lowell Goudge Quote
Until I can get someone with a focusing screen test results that seem better, I am using my *istD for manual lenses.
we have several people here who have confirmed they have tested it and it works. i guess they will jump in shortly to help. however, i am a bit relluctant about changing my focusing screen, unless i cant find another way, i would rather avoid it (it is mostly a matter of "it _should_ work as is!"). there is also the temptation of actually having aperture priority fully functional, with wide-open metering (i tend to use aperture prio. almost exclusively, so this would mean any old lens capable of that is about as usefull and convenient for me as a new one)


Last edited by nanok; 02-27-2008 at 01:09 PM. Reason: sudden enlightenment
02-27-2008, 12:54 PM   #20
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2 questions that really need answering

1) DS screens w/ k10/20. Is metering much better w/ manual lenses?
MY take: Yes, manual metering (green button or DOF preview method) is much improved from numerous past posters for a few years using k10's. K20 unknown at this point.
Note: From current tests the K20 behaves much like the k10 in poor metering w/ manual lenses using the "default" screen. DS screens should fit though.

2) Does the DS screen aversly impact A, FA, ect. lenses?
My take: No definitive answers due to lack of either interest or proper tests.

As another side note: Katz Eye and assorted 3rd party screens are not helpful to manual metering issues.
Maybe I missed something though.
02-27-2008, 01:14 PM   #21
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QuoteOriginally posted by nanok Quote
hmmm. wait a minute. what does the k10d have to do with the lever? the k10d is "crippled", it cannot read that anyway, unless you add a device to the lens to tell the k10d, electronically, what aperture it has set, what is the maximum (wide open) aperture (and the minimum? didn't know it can/will read that). afaik, all the k10d (and i think the other dslr's as well) can do is stop down to the set aperture. 0/1 style. am i wrong? maybe you were reffering to some film body you did this on, which was not crippled?
there are 2 apature levers, one which reads the manual apature setting, It is usually called the apature coupling, and this is missing and is why the mount is "crippled" the other lever (the release lever) is what stops down the lens during the exposure.
QuoteQuote:
do tell us more, it sounds like you know more than i do on the subject. some rellevant link explaining how it works, maybe?
On K mount compatible bodies, the actuating lever that stops down the lens during exposure really just releases the apature and it stops down to the limit of the apature ring, therefore the manner in which the blades move relitive to the release lever is not important. that is why KA lenses can work on K mount bodies. BUT on KA bodies, where apature is controled by the body, the apature release lever is specifically controlled and moves to a precise position, holding the lens at the correct apature. In order to make this easier to control, KA mount and newer lenses have the area linear with movement of this lever, so that the stops can be more precise.

I have proposed to pentax separately that they let us, with manual apature lenses, enter maximum and minimum apature, and IF they do not sense the contact for the AE lens, to reprogram the actuating lever so that it responds to the way K mount lenses work. to me this would seem to be the best solution

QuoteQuote:

we have several people here who have confirmed they have tested it and it works. i guess they will jump in shortly to help. however, i am a bit relluctant about changing my focusing screen, unless i cant find another way, i would rather avoid it (it is mostly a matter of "it _should_ work as is!"). there is also the temptation of actually having aperture priority fully functional, with wide-open metering (i tend to use aperture prio. almost exclusively, so this would mean any old lens capable of that is about as usefull and convenient for me as a new one)
I know people have confirmed better exposure with different screnes, but the best would be the test I proposed, as opposed to just looking at the selected shutter and apature.
02-27-2008, 01:28 PM   #22
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thank you, i finally got it (by looking at the mount and the lenses i have), through reverse engineering and thanks to your hints . i agree a "software" option would be the ellegant solution, and would be great to have, but it would only work (as stated above) if the body doesn't use a hardwired circuit (independent of the firmware) to read the aperture values and drive the lever to the required positions. also, one must take into account that linear to radius (or diameter, same thing) means a shorter stroke for the same effect compared to linear to the area (for double in diameter, one would have a 4x increase in area, for example), this might mean precision which the servo-motor designed for the ka/kaf is not ready to deliver reliably (?)
02-27-2008, 01:57 PM   #23
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QuoteOriginally posted by nanok Quote
thank you, i finally got it (by looking at the mount and the lenses i have), through reverse engineering and thanks to your hints . i agree a "software" option would be the ellegant solution, and would be great to have, but it would only work (as stated above) if the body doesn't use a hardwired circuit (independent of the firmware) to read the aperture values and drive the lever to the required positions.
no it is not hard wired, the KA lenses which worke fine in AE mode on the K10D do not send actual set apature, only max and min plus AE mode selected to the body, this is all accomplished through the (up to 6) contacts on a KA lens. the 7th contact, furthest away from the lens locking pin on a KAF mount is for focal length and focusing distance data and is not needed for apature priority mode on KA lenses.
QuoteQuote:
also, one must take into account that linear to radius (or diameter, same thing) means a shorter stroke for the same effect compared to linear to the area (for double in diameter, one would have a 4x increase in area, for example), this might mean precision which the servo-motor designed for the ka/kaf is not ready to deliver reliably (?)
For me the interest would be having something that is predictable. If there is a possibility the lens might have an error, and it is smaller than the present metering problem (i.e. on an f1.4 lens -1 stop wide open, +2 stops at F11) and I can use effectively P-TTL flash, spot metering etc more reliably, I would definitely go for it.

Otherwise, pentax should invest in development of a focusing screen that fixes the issue.

Edit note:

anyone who wants to send cropped center sections, of a test series with what ever manual lenses you have send me a PM and I will give you details to send the files, or if you want post the data yourself
- just tell me lens type, focal length, and apatures used start to finish, plus camera and viewing screen and I will be willing to compile the data and post the results


Last edited by Lowell Goudge; 02-27-2008 at 03:26 PM.
02-28-2008, 05:56 AM   #24
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I still don't understand. Why is it important for the K10D to know the maximum and minimum aperture when used with M lenses and the green button? If you turn the aperture ring to f2.8 and press the greeen button, the aperture blade are released and the lens is stopped down to what it thinks is f2.8, and the camera then takes a metering reading, right? At no time does the camera actually know (or care) what the aperture ring was actually set to. Why does maximum and minimum aperture matter in this case?

State it another way: what exactly is it that is causing inaccurate metering with M lenses? Is there some sort of scaling factor involved with extrapolating how much light is hitting the meter to how much light is actually captured by CCD?

Last edited by twinda1; 02-28-2008 at 06:07 AM.
02-28-2008, 06:39 AM   #25
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QuoteOriginally posted by twinda1 Quote
I still don't understand. Why is it important for the K10D to know the maximum and minimum aperture when used with M lenses and the green button? If you turn the aperture ring to f2.8 and press the greeen button, the aperture blade are released and the lens is stopped down to what it thinks is f2.8, and the camera then takes a metering reading, right? At no time does the camera actually know (or care) what the aperture ring was actually set to. Why does maximum and minimum aperture matter in this case?
If this actually worked correctly, there is no reason, unless yoou wanted to use p-ttl flash, but again if the camera could meter correctly at all apatures, the same process could be used, because all that matters, as you correctly state, is the change in light between wide open and what ever you stop down to.
QuoteQuote:
State it another way: what exactly is it that is causing inaccurate metering with M lenses? Is there some sort of scaling factor involved with extrapolating how much light is hitting the meter to how much light is actually captured by CCD?
This is the real heart of the problem. I tried an experiment lately, as I added a new lens (tamron 28-75 F2.8 Di) and just for the fun of it, tested exposure on the K10D with the lens in AE, and by taking the lens ouyt of AE. In with the lens in AE, clearly the camera metered correctly and exposed correctly by setting the apature in manual with the rear thumb wheel, and then pressing the green button, BUT, when I took the lens out of AE and set the apature with the apature ring, the metering behaved just like an old K mount lens was attached.

While I can't confirm this, I suspect that there is a problem with respect to how light reflects off the focusing screen, to the metering and that the reflection is a function of F stop, since I find all lenses behave in the same manner. Since all lenses behave the same, a program correction would be simple in software, and I think that is what was done, but to make the correction you need to know the absolute apature.

The software proposal I gave pentax i.e. give min and max apature and the control the apature with the correct curve, fixes several "complaints" about the K10D specifically but also all Pentax cameras to some extent. It would allow for use of K mount lenses in all modes, not just manual, It would allow for P-TTL flash, since the pre flashes are done wide open, and power is set based upon the wide open performance and knowledge of Fstop, and it would fix the general exposure non linearity of the K10D specifically with K moutn lenses

Last edited by Lowell Goudge; 02-28-2008 at 07:00 AM.
02-28-2008, 08:13 AM   #26
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QuoteOriginally posted by Lowell Goudge Quote
While I can't confirm this, I suspect that there is a problem with respect to how light reflects off the focusing screen, to the metering and that the reflection is a function of F stop,
Yes, I think so, too.

Your other suggestions are good, but it appears that Pentax is not interested in these old lenses anymore, because no improvement is happening. Neither new firmware nore in the new K20D.

I haven't given up hope though for a K1D featuring all those good things.
03-04-2008, 05:06 PM   #27
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i have done some more testing this weekend:

i set the green button function in the menu to each of the settings. results are identical, it seems the k10d is smart enough to ignore that setting when it has an m lens mounted on it (thus cannot set the aperture, but only the shutter speed).

just to get this out of the way

ps: here is some helping hand:
https://www.pentaxforums.com/forums/pentax-dslr-discussion/22089-exposure-tes...0-4-macro.html
03-04-2008, 05:15 PM   #28
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QuoteOriginally posted by nanok Quote
i have done some more testing this weekend:

i set the green button function in the menu to each of the settings. results are identical, it seems the k10d is smart enough to ignore that setting when it has an m lens mounted on it (thus cannot set the aperture, but only the shutter speed).

just to get this out of the way

ps: here is some helping hand:
https://www.pentaxforums.com/forums/pentax-dslr-discussion/22089-exposure-tes...0-4-macro.html
that is correct, the K10D can't control the apature of a manual lens, it only allows it to open to the mechanical limit of the lens as set by the apature ring.
03-04-2008, 05:23 PM   #29
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QuoteOriginally posted by nanok Quote
i have done some more testing this weekend:

i set the green button function in the menu to each of the settings. results are identical, it seems the k10d is smart enough to ignore that setting when it has an m lens mounted on it (thus cannot set the aperture, but only the shutter speed).

just to get this out of the way

ps: here is some helping hand:
https://www.pentaxforums.com/forums/pentax-dslr-discussion/22089-exposure-tes...0-4-macro.html
Thanks for the link to my thread - I was about to put it in myself. I did find that with my K10D, the green button system was useless. Only the dof preview in manual exposure was consistent. I am pleased that consistent exposure can be obtained, but disappointed that I am stuck with three apertures only.
03-04-2008, 05:50 PM   #30
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The LL60 *istD screen was the ticket for me, I have a manual 400mm I used to use alot and the screen worked great for me. I use it for all my lenses and seems good to me. No guarantees ,but if you have the money it's worth a shot. here's a link where I got mine -I like the grid on it better tooPentax | LL-60 AF Divided Matte Focusing Screen | 38572 | B&H
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