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06-04-2011, 03:26 AM   #31
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with the green button set to something other than green button (e.g. optical preview), in manual mode, with a manual lens, the +/- button does stop down metering and adjusts the shutter speed (i.e. Tv Shift). I don't see why I would have to set the green button to green button as well.

06-04-2011, 10:51 AM   #32
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QuoteOriginally posted by simico Quote
Why don't you just meter with the lens wide open and then do you calculation (in an exposure calculator app or in your mind) for the desired f-stop?
Mostly because the meter is at its worst behavior wide open.

Like Lowell and dlacouture, I have done a fair amount of research and experimentation on this subject. I have also done exposure series similar to Lowell's, though with a little different methodology. (Used a A-contact lens meter readings as the gold standard and plotted the variance from those readings for each f/stop. No need for Photoshop.) I did all of the lenses in my collection that have an aperture ring and came to the following conclusions with the K10D:
  • Severe underexposure at apertures wider than about f/4
  • Fairly good linearity down to about f/11
  • Moderate to severe underexposure narrower than f/11
  • Actual curves varied by lens
  • Behavior was identical with the stock screen, Katz Eye w/ Optibrite, and Katz Eye w/o Optibrite
From what I can tell, metering is closely matched to the viewfinder light environment in wide-open mode and is NOT based on a simple meter measurement. Correction factors are applied based on the lens maximum aperture as communicated through the A-contacts to account for the non-linearity of light passed by the focus screen. If the maximum aperture is unknown (M42 and non-A-contact lens), a default correction is applied. That default factor is simply wrong when shoot at other than moderate apertures.

The issue is compounded somewhat when the light is dim and the aperture narrow. Meter sensitivity is at fault here. EV 0 is easily attained at the sensor when ambient light is dim and the aperture is small. This is normally not a problem when metering wide open, but is historically an issue for stop-down metering in general. Cameras used to come with a chart that showed the range of acceptable shutter speed/aperture combinations at ASA 100 (ISO now days).

As expected, my tests is relatively low light showed a flat meter response at narrower apertures. The sobering thing was that the meter is not linear to EV0, but begins to lose sensitivity at about EV 2.

I would post the test plots, but I recently did some house cleaning and purged them from the hard drive.

All this being said:

The above issue is typical of the K10D, K20D, and other Pentax dSLRs of similar vintage. The other models have a pentamirror system so the behavior is a little different, but the general principles are the same.

On recent Pentax dSLRs the situation is much better. Starting with the K-7, Pentax implemented a linear meter program for M-mode when aperture information is not present. I have yet to upgrade from my K10D, but a quick test with the K-7 when it was first introduced indicated linear meter response in M-mode using stop-down metering. This is the procedure recommended in the camera manual. If you use other modes (e.g. Av) YMMV.


Steve


(BTW...I shoot mostly with vintage and non-A glass and generally either test/chimp/shoot or use a hand-held meter with those lenses.)

(BTW, BTW...Pentax is not the only camera brand with stop-down metering issues.)
06-04-2011, 12:52 PM   #33
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QuoteOriginally posted by stevebrot Quote
On recent Pentax dSLRs the situation is much better. Starting with the K-7, Pentax implemented a linear meter program for M-mode when aperture information is not present. I have yet to upgrade from my K10D, but a quick test with the K-7 when it was first introduced indicated linear meter response in M-mode using stop-down metering. This is the procedure recommended in the camera manual. If you use other modes (e.g. Av) YMMV.
Well, I have to disagree here... Yes, the situation is better with K7 & K5, but they still don't have a linear response in stop down metering (at least with none of the 10 lenses or so I've tried)...
The only thing that made my K5 meter linearly is the Canon EE-S focus screen I cut down myself. And it also works wonders in a K20 (made one for my brother)...
06-04-2011, 01:22 PM   #34
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QuoteOriginally posted by dlacouture Quote
Well, I have to disagree here... Yes, the situation is better with K7 & K5, but they still don't have a linear response in stop down metering (at least with none of the 10 lenses or so I've tried)...
The only thing that made my K5 meter linearly is the Canon EE-S focus screen I cut down myself. And it also works wonders in a K20 (made one for my brother)...
M-mode or Av?

I am sorry to read this. My quick test was limited to a single series with a Pentax-M 50/1.7 done when the K-7 road show was in town. I shot in M-mode against a well-lit, blank interior wall and got identical frames from f/1.7 to f/22. Apparently, I was lucky.

I have been considering an upgrade to a K-5, but will not do so unless the stop-down metering is much improved from my K10D.


Steve

06-04-2011, 06:46 PM   #35
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I don't have stop-down metering problem with my K-x and K-7. I use my right index finger to press on the green button on K-x (right thumb on K-7) WITHOUT MOVING MY EYE AWAY FROM THE EYE-PIECE.

I've opened MZ-5N and MZ-6 and know where the light sensors are located. In these cameras, light entering through the eye-piece would affect the metering. I don't know any thing about DSLRs, maybe they are constructed differently. I have a collection of old Nikon lenses and enjoy more using my D7000 because of the uncrippled lens mount.

Last edited by violini; 06-04-2011 at 07:11 PM.
06-04-2011, 07:13 PM   #36
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QuoteOriginally posted by dlacouture Quote
Well, I have to disagree here... Yes, the situation is better with K7 & K5, but they still don't have a linear response in stop down metering (at least with none of the 10 lenses or so I've tried)...
The only thing that made my K5 meter linearly is the Canon EE-S focus screen I cut down myself. And it also works wonders in a K20 (made one for my brother)...
I would agree here. See my chart. While the K7 is better than the k10 for large apertures it still suffers badly for small aperturers
06-04-2011, 07:44 PM   #37
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For lenses from A to DA, the metering is done with aperture wide open. Do we have metering problem with these lenses? Why the Pentax engineers wouldn't implement the correct calibration of their M-lenses into the software? I almost never use aperture smaller than F/8 (only in those few occasions when I point the camera to the sun to get star-burst effect).

Last edited by violini; 06-04-2011 at 08:30 PM.
06-04-2011, 07:49 PM   #38
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You know I'm really glad I use pdf formated docs rather than actually printing out every thread on this forum that has info I think is useful in it. At the rate I am going I would have a 6" binder half filled by now!



06-05-2011, 05:04 AM   #39
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QuoteOriginally posted by violini Quote
For lenses from A to DA, the metering is done with aperture wide open. Do we have metering problem with these lenses? Why the Pentax engineers wouldn't implement the correct calibration of their M-lenses into the software? I almost never use aperture smaller than F/8 (only in those few occasions when I point the camera to the sun to get star-burst effect).
They can't put correction in when they don't know the maximum and metering apertures. I proposed one time that they offer a max aperture input like the one for focal length. In that way they could put correction in by metering twice with the green button. Once wide open once stopped down
06-05-2011, 08:44 AM   #40
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QuoteOriginally posted by Lowell Goudge Quote
They can't put correction in when they don't know the maximum and metering apertures. I proposed one time that they offer a max aperture input like the one for focal length. In that way they could put correction in by metering twice with the green button. Once wide open once stopped down
...too easy for them I guess, eh?


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06-05-2011, 11:39 AM   #41
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As I said before, all these problems disappear when using a Canon EE-S screen...
The stop-down metering response is flawed by the brightness optimization done by Pentax on their screens. They are optimized to boost perceived brightness for apertures between f/1.4 and f/5.6, using their Fresnel lens (located on the underside, and the same culprit as for the limited DoF preview).

From my tests, I've not seen any indication that Pentax uses some internal tweaking between A-position and a given aperture, except for the fact that Matrix metering cannot work out of A-position. Using CW give me similar results between A and the widest aperture in stop-down.

What they should do is simply offer one linear focus screen for us legacy lenses users... But as Canon already does, why bother?
06-05-2011, 12:37 PM   #42
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In The M lens Club, every one shows off the fantastic pictures and no one ever mentions the problem of non-linearity of stop-down metering w.r.t. aperture. Therefore, I would guess it is not a problem at all under good lighting condition.
06-05-2011, 01:02 PM   #43
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It's a problem under all conditions, even harsh sunlight... But as only K10/K20 models were heavily affected, you don't hear much about it...

All other models were mildly affected, with sometimes only a -0.5/+1 Ev variation, so nothing really bothering in everyday shooting.

And even those using the K10/20 often changed their focus screens for the LL-60 (made for the *ist), as it's more linear (aside note : if you don't have a K10/K20, and want a brighter viewfinder for your A-lenses, you can order a LL-80 screen, it will fit - except for K5/K7 models!).
06-05-2011, 10:55 PM   #44
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QuoteOriginally posted by violini Quote
In The M lens Club, every one shows off the fantastic pictures and no one ever mentions the problem of non-linearity of stop-down metering w.r.t. aperture. Therefore, I would guess it is not a problem at all under good lighting condition.
The solution is to test/chimp/shoot. Once you have a working exposure setting for a given subject and lighting there is no need to change unless the light or subject changes. Much attention is given to intelligent metering systems, but in truth, correct exposure usually does not change from second to second or even from minute to minute in many cases.

I regularly shoot with non-A lenses and find the above solution the easiest to live with. Strangely enough, I also often use a similar technique with my A-contact lenses as well. I recently was shooting a stage venue where the subject was a brightly spotlit presenter with a dark background and two large rear projections screens that were often behind her. The speaker was moving around a lot and the built-in meter was constantly being biased by either the dark background or the bright screens. After wrestling with this during the first of several presentations, I finally went to the stage itself before the program started and manually metered a gray card in M-mode to the stage lighting. From there on out I had perfect exposures.


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06-06-2011, 05:12 AM   #45
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QuoteOriginally posted by violini Quote
In The M lens Club, every one shows off the fantastic pictures and no one ever mentions the problem of non-linearity of stop-down metering w.r.t. aperture. Therefore, I would guess it is not a problem at all under good lighting condition.
Not at all, it is a problem under all lighting conditions, but there are some things to note if you look at the chart I posted.

The *istD for example, has a very predictable error, moving linearly from -1/2 stop at F1.4 to +1/2 stop at F22, it's metering is very very good, with manual lenses, as are all the &*istDX series cameras. The K7 is quite good for apertures up to about F8 , and even the K10/20 is OK at F4-5.6 so if you shoot at these apertures, then there is no problem. The big issues are at wide open on very fast glass, and at F11-16 (i.e. stopped down quite a lot)

Also, there are legacy users who, for example, may be shooting with M42 lenses that can be quite slow. and they only see 50% of the problem, i.e. over exposure at F11-16, by up to 2 stops.

But if they set the EV comp to -1, then they cut the problem a long way.

The bottom line is KNOW your equipment. There is no substitute for knowing how the metering behaves using all lenses
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