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12-21-2007, 12:09 PM   #1
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K10D stopped-down metering weirdness

I normally use my Vivitar Series 1 70-210mm lens on my K10D in fairly low light, so I'm normally shooting near wide-open. This is a "non-A" lens (no A setting on aperture ring). Today I was playing around with it in the afternoon sunlight, and noticed something odd. If I set the aperture to f/2.8 press the green button in "M" mode, I get a decent exposure (center-weighted, on tripod). As I stop down in aperture, meter and shoot, the shots are progressively more and more over-exposed as I approach f/22. Thinking there might be oil on the aperture blades, I checked, and sure enough there is the slightest bit on one of them.

However, now comes the weird part; I set the K10D f/16 @ 1/ISO (sunny 16 rule) and took a sequence of shots at the same EV (i.e. open one stop, increase shutter speed one stop, repeat, rinse). No metering, strictly manual. They all exposed fine. So the oil is not preventing the blades from closing during exposure...so why the discrepancies in metering? Why would the diaphragm response be slower when metering (as it appears to be) than during the actual exposure?

12-21-2007, 12:38 PM   #2
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QuoteOriginally posted by Duck Dodgers Quote
I normally use my Vivitar Series 1 70-210mm lens on my K10D in fairly low light, so I'm normally shooting near wide-open. This is a "non-A" lens (no A setting on aperture ring). Today I was playing around with it in the afternoon sunlight, and noticed something odd. If I set the aperture to f/2.8 press the green button in "M" mode, I get a decent exposure (center-weighted, on tripod). As I stop down in aperture, meter and shoot, the shots are progressively more and more over-exposed as I approach f/22. Thinking there might be oil on the aperture blades, I checked, and sure enough there is the slightest bit on one of them.

However, now comes the weird part; I set the K10D f/16 @ 1/ISO (sunny 16 rule) and took a sequence of shots at the same EV (i.e. open one stop, increase shutter speed one stop, repeat, rinse). No metering, strictly manual. They all exposed fine. So the oil is not preventing the blades from closing during exposure...so why the discrepancies in metering? Why would the diaphragm response be slower when metering (as it appears to be) than during the actual exposure?
Just the way it is w/ the k10 (and the only Pentax to do this)... Meter becomes progressivly erratic (well there is some linearity to it I guess) as you use aperatures smaller than f4...
Best part is the "hack" is to buy a DS focusing screen and use that. The NEW, IMPROVED Pentax bright matte screen screws up the meter w/ manual lenses.......
BTW: This does occur w/ other brands as well so it's not Pentax specific. More like focusing screen specific.
and in case you wonder about other screens (ie Katz-eye) they don't help either...
12-21-2007, 12:51 PM   #3
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Huh. Well, that's lame. I guess a work-around would be to meter at f/4 then maintain the EV for the desired aperture. Pooh. Or use a hand-held meter.

Thanks for the response, even if it wasn't really what I wanted to hear. *grin*


Thinking out loud...you would think that a firmware algorithm change might help in this instance.
12-21-2007, 01:06 PM   #4
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welcome to the wild and wonderful world of K10D metering.

Initially I as of the opinion that people like you who reported this were nuts, but I went out and tried some tests.

The procedure is as follows:

pick a uniformly illuminated light grey solid surface. Personally I like paved roads, sidewalks, or block walls.

Take a set of shots from wide open to minimum apature using the green button each time.

Take a second set of shots, with metering done at F32 and make sure that your shutter speed is below 1/50th adjust your ISO to do this. Then take shots at that shutter speed and ISO over the full range of apatures.

Take a 3rd set of shots by setting the lens wide open and adjust ISO to get 1/2000, then take a set of shots successively stopping the lens down to F32.

What you get is the following.

The first set of shots will give you something where you can check the linearity of your exposure, use the histogram value on grey scale and plot vs f stop. You will see the histogram rise up from around 125 to perhaps 180 and then begin to drop again as you move from F2.8 to F32. At normal contrast and picture mode eacg change in grey scale of 45 is about 1 stop between the range of 25-230.

The second & third set of shots will, or should plot a streight line from the middle to either the high end or low end of grey scale, with 2-3 stops perfectly linear between 125 and either 25 or 230, and then it should taper off as you approach either 0 or 255.

This gives yoou a good check to see that your apature is linear. I have yet to find a lens that isn't. all of the error is in the camera.

I have checked all of my lenses this way for both my K10D and *istD, and to my surprise, the *istD is far better in metering with K mount lesnses than the K10D.

I am just waiting for some time to put charts on the lens data base for pentax lenses I own, and for a new database for other manufacturer's to be created to add the data I have on them

12-21-2007, 01:23 PM   #5
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Hey mike,
the LL-60 screen for the *ist worked fine for me, I like the grid better too.
There are some threads on this if you do a search
12-21-2007, 01:34 PM   #6
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I wonder if this is something a future firmware upgrade could fix....
12-21-2007, 01:45 PM   #7
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Lowell,

Here's what I found the meter was reading, in 1/3 and 1/2 stop increments:

Code:
F        1/3        1/2
----------------------------
2.8 1/2000 1/1500
4 1/640 1/750
5.6 1/200 1/180
8 1/80 1/60
11 1/30 1/30
16 1/15 1/8
22 1/8 1/4
Even a cursory glance shows it's just all over the place.
12-21-2007, 01:46 PM   #8
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Thanks, Tom. I'll have to look into that.

12-21-2007, 02:00 PM   #9
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It's the sensitivity of the meter. That's why with auto lens, it meters full open and calculates the stepped down exposure, rather than metering stepped-down.

I reported a similar problem:
https://www.pentaxforums.com/forums/pentax-dslr-discussion/14418-k100d-stop-d...g-problem.html
12-21-2007, 02:06 PM   #10
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Mike

Looked at your data and it seems pretty consistent with some of the tests I have done, although grey scale is much better way to check.

As others have suggested, there are some different screnes that reportidly work better than the standard screen.

Most of the time, however, I just use the histogram aafter a test shot.

What buggs me is the *istD is spot on and linear.
12-21-2007, 03:26 PM   #11
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Not exactly relevent but:

The "interesting twist" on metering...: News Discussion Forum: Digital Photography Review
Some interesting "talk" on screens and trade-off's ect. Doesn't exactly explain anything re: Pentax though, but a bit of history and some insight like:
For instance, when using the 50-300mm Nikkor
> ED on the E-300, I generally set the aperture to around f/6.3 and
> seldom touch it. I dial in my known fudge-factor and I'm good to go
> in aperture priority mode. At least I'm almost as good as I am with
> a ZD lens that talks to the camera. All auto-exposure schemes have
> their problems anyway.


Sometimes, Jay, you have a talent for understatement.
Sorry, found something to go with this:
BTW this is not a net-myth. In Aug 2005 Chuck Westfall (Director of Media & Customer Relationship at Canon USA) confirmed the following:

The EOS 20D focusing screen is optimized for superior brightness at moderate apertures from about ƒ3.5 and smaller, compared to conventional ground glass designs. This makes the viewfinder image brighter and easier to focus at those moderate apertures, but the trade-off is that it passes disproportionately more light to the metering system.

When a Canon EF lens is mounted to an EOS camera, a variable exposure compensation factor (a program curve, not just a fixed compensation factor) for this phenomenon is fed through the system in order to provide correct metering for all apertures.

However, when using a non-coupled manual diaphragm lens [such as a Leica R lens - AZN], no such communication takes place, so the responsibility for exposure compensation reverts to you. It's unnecessary to use an external meter. Instead, you can take a series of test shots at the working aperture(s) you plan to use, then analyse the test photos to determine the most desirable exposure compensation factor for each aperture.

The 20D's auto exposure bracketing (AEB) function speeds up the process of taking the test photos, and you can use the Info palette in Photoshop to determine the most accurate exposure. If you can standardize on one particular aperture you plan to use (for maximum sharpness, desired depth of field, etc.), that will simplify the calibration process by eliminating the need for tests at other apertures.

Other work-arounds?…

Leica FAQ - Leica R lenses on Canon DSLRs
NOTHING w/ these cameras is simple it seems.........

Last edited by jeffkrol; 12-21-2007 at 03:32 PM.
12-21-2007, 04:22 PM   #12
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Metering when using aperture ring.

This is pretty well hidden in the manual. Do not use the green button, use the stop down on the on/off switch. k10d manual, p. 210, bottom. The green button method only works when using the edials to set the aperture.
12-21-2007, 09:00 PM   #13
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QuoteOriginally posted by Canada_Rockies Quote
This is pretty well hidden in the manual. Do not use the green button, use the stop down on the on/off switch. k10d manual, p. 210, bottom. The green button method only works when using the edials to set the aperture.
Ah, indeed. They never quite come right out and say why not to use the green button, do they? "...an exposure error may occur." Harumph!

Thanks. Now I don't have to send either the body or the lens to the shop.
12-21-2007, 09:19 PM   #14
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QuoteOriginally posted by Duck Dodgers Quote
Ah, indeed. They never quite come right out and say why not to use the green button, do they? "...an exposure error may occur." Harumph!

Thanks. Now I don't have to send either the body or the lens to the shop.
I got this from Pentax Service in colorado, but following the revision to 1.3 the time the lens is stopped down with the green button is double, not documented any where, but they slowed it down to insure that the metering was accurate. There is no difference between green button and stopping the lens down
12-21-2007, 10:33 PM   #15
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QuoteOriginally posted by Lowell Goudge Quote
I got this from Pentax Service in colorado, but following the revision to 1.3 the time the lens is stopped down with the green button is double, not documented any where, but they slowed it down to insure that the metering was accurate. There is no difference between green button and stopping the lens down
I'll give that a test tomorrow, but I've already got 1.3.
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