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05-31-2011, 03:52 AM   #16
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"Meter Coupling Range"
K-x : 1 EV to 21.5 EV

The K-5 and K-7 go down to "0 EV"

In the Film cameras instruction books there were tables to show the range of shutter speeds that could be metered for each ISO sensitivity

05-31-2011, 05:02 AM   #17
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QuoteOriginally posted by geekette Quote
metering when stopped down
center weighted metering gives better and consistent exposure in these circumstances then spot metering (just my experience)
05-31-2011, 05:21 AM   #18
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I have powted this link before. It shows the lack of linearity of the camera metering as a function of aperture.

while each lens and camera is a little different, the same pattern holds relitively close for any manual aperture lens

IMO the K10D and K20D are the worst metering cameras for manual aperture.



I have tested the *istD, K10D, K10D with *istD screen, K10D with Split image screen and K7D.

I wanted to get a project started where people all did sample shots so characterize each lens and camera combination but no luck
05-31-2011, 10:51 AM   #19
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Would be nice if someone wrote a program that has a database of lenses, cameras and focus screens: you pick which lens, camera, focus screen, f stop setting, iso you are using and it comes back with a shutter speed based on the metering the camera gives you when the lens is wide open--and possibly add the option for it to give you the shutter speed based on the camera, lens, focus screen, f stop, iso and lightcandles (or whatever unit of light measurement is most popular these days) a spot/matrix light meter gives you. Both an Android and iPhone app would be handy


Last edited by geekette; 05-31-2011 at 10:58 AM.
05-31-2011, 01:19 PM   #20
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QuoteOriginally posted by Lowell Goudge Quote
I wanted to get a project started where people all did sample shots so characterize each lens and camera combination but no luck
Hi Lowell!

Well, I'm game for it, if you want...
To my disposal is :
- K10 (stock screen), K20 (EE-S), Kx (stock screen) and K5 (EE-S)
- 18-55(II and WR), FA50/1.4, F50/1.7, tamron 28-75 & 70-200, Samyang 85/1.4 and some others (DA18-250, sigma 18-200, some m42...)

Maybe each post could be one lens/body combination, with a simple speed/aperture table for a given ISO (with speed averaged on 3-5 takes, with 1/3Ev steps enabled). Then we could format all this into a nice graphic...
05-31-2011, 02:49 PM   #21
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The chart I produced was made using a uniformly lit surface, paved road, block wall etc, to meter from

I then plotted the measured greyscale median value in my photo editor PSP X3. In PSP every 45 grey scale is 1 stop when the histogram is about 110. Things go non linear below about 40 or above 210
06-01-2011, 01:31 AM   #22
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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?
06-01-2011, 06:50 AM   #23
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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?
the issue is not so much stopped down metering, but the non linearity of the camera's metering as a whole, roughly as a function of aperture.

Look at my chart for the F1.4 50mm lens.

normal exposure should put the camera in the range of 110-120 greyscale for the hystogram average. By using a uniform colored surface (block wall, paved road, tree trunk etc) this generates a very narrow histogram with a noticible peak. That is how I checked the metering, buy the value of the peak.

wide open, the 50/1.4 is underexposing seriously on the K20 with stock screen.

what I think (Note this is supposition on my part, since I doubt pentax will ever reveal what they do) is that the metering is set to be accurate at between F4 and F5.6, because that is what most kit lenses are, and pentax have programmed correction factors for lenses with larger (smaller F number) apertures, so that open aperture metering works.

As a result of the measured performance, metering wide open will only work if the lens is relitively slow.

the best way would be to meter at something between F4 and F5.6, and correct both ways from that, or shoot all the time at F5.6 and forget about it

06-01-2011, 09:54 PM   #24
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QuoteOriginally posted by geekette Quote
I am testing manual aperture lenses. e.g. Pentax-M, Pentax K, and Sears. I am using manual mode and pressing green button for stop down metering. It meters fine wide open, but is totally unpredictable as you adjust the f stop up, especially after f8 on upwards to f22 +
I wouldn't worry too much about what happens beyond f11 anyway; any lens worth having on your camera will be wilting by f11, and worse than that when you take it to f16, f22.

It's the inevitable result of diffraction - stopping down too much just results in soft images.
06-01-2011, 10:20 PM   #25
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QuoteOriginally posted by m42man Quote
any lens worth having on your camera will be wilting by f11, and worse than that when you take it to f16, f22.
I didn't know that

Try taking photo of a group of 200 students and keep the image of each of them reasonably sharp (or the parents won't buy the photo).
06-01-2011, 11:59 PM   #26
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QuoteOriginally posted by SOldBear Quote
I didn't know that

Try taking photo of a group of 200 students and keep the image of each of them reasonably sharp (or the parents won't buy the photo).
Yes, if you really need the DOF, you'll use f16. F22 is certainly getting a bit rough, though.
06-03-2011, 06:52 AM   #27
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I found that green button set to "optical preview" in K-x did not work for me. The metering worked very well when GB was set to "Tv Shift" in manual mode.

Last edited by violini; 06-03-2011 at 10:12 PM.
06-03-2011, 08:14 AM   #28
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QuoteOriginally posted by violini Quote
I found that green button set to "optical preview" did not work for me. The metering worked very well when GB was set to "Tv Shift" in manual mode.
Where did you get green button for optical preview?

I am assuming you are using a K7 like your signature? K7 green button functions are Av Shift, Tv Shift, Program line. and off (unless off reverts to optical preview)

With non automatic (i.e. non A lenses) regardless of selection of green button settings other than off, it always sets shutter speed to match aperture.
06-03-2011, 10:05 AM   #29
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QuoteOriginally posted by violini Quote
I found that green button set to "optical preview" did not work for me. The metering worked very well when GB was set to "Tv Shift" in manual mode.
I have a Pentax K-X and I use the "+/-" button--up top next to the green button--to do stop down metering through K & M lenses. I don't see any reason to set the green button for "green button" when the "+/-" button does the exact same thing. Unless there is some difference I don't know about. From my understanding the "+/-" button--with those lenses in manual mode--is effectively green button with Tv Shift.
06-03-2011, 10:08 PM   #30
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>>I have the green button assigned to DOF preview.>>

When the green button is assigned to "Tv Shift", then GB and +/- work the same and no metering problem in K-x.

Since the light sensor is located in the viewfinder, I keep my eye close to the eyepiece while pressing the green button because light entering from the eyepiece may affect the metering particularly when the aperture is small. I got this habit from using MZ-5N film camera (the camera came with an eyepiece cover when using self-timer).

Last edited by violini; 06-04-2011 at 07:31 AM.
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