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03-02-2012, 01:54 AM   #1
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K7, ISO and middle gray

Hi, first time here!

I was wondering, could anyone do a simple test with K-7? Shoot some white paper with CW or spot metering, adjust the WB and look what kind of RGB values do you get. RAW with no adjustments or some neutral JPG (I get about similar results with both).

Reason for this is that I'm curious whether my K-7 is wrongly adjusted (sensor gain) or is it just Pentax mapping "middle gray" quite dark. I did some reading about the ISO standards, seems that manufacturers can rate the ISO values pretty much how they want.

In sRGB gamma 2.2 colorspace, middle gray should be about 0,18^(1/2.2) x 255 = 117 (so RGB 117, 117, 117). Tested with my Canon G11, it is pretty much spot on (shooting that white paper).

With the K-7 however, I get about 87, 87, 87. Shooting with A-priority and same ISO as with the G11, the shutters speeds are within 1/3 EV same.. so some tolerance in the exp meter/ISO of the cameras. But the 87, 87, 87 is about 0,5 - 1 stop away from the Canon's middle gray, no wonder the results have been a bit dark


Last edited by jaeaetee; 03-03-2012 at 04:04 AM. Reason: grammar
03-02-2012, 12:39 PM   #2
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Your question is quite deep. From dpreview, it seems different manufacture post their ISO differently. Some conservative and some quite aggressive. If you think your G11's result is right on then why not tune your K7 to the same value and see if you like the output or not. I think all K7 should be tuned within the same spec of pentax.
03-02-2012, 04:45 PM   #3
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The answer is simple the camera doesn't meter for middle grey, non does and non will because of the international standard.

Middle grey comes from the printing world, it's 50% black and 50% white, Ansel Adams thought it was a good idea to use those and asked Kodak to make grey card for him.
Thats why we use those grey cards.

Sorry got my math wrong...
87 is indeed to dark, the number should be around to 100 (12,5% grey) the ANSI standard for lightmeters says 12,3% if we do the math but there is a range they mus fall in so it can be lower and higer.
We all know that Pentax often is quite perserve when it comes to highlights the K5 fairs better though so it seems they are improving.
I had my K10D constantly on a slight overexpose but not the K5.

Last edited by Anvh; 03-02-2012 at 05:03 PM.
03-03-2012, 04:03 AM   #4
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Yeah, if the dark middle gray is correct, Pentax has done it probably to protect highlights etc. But, if someone could confirm that their K-7 is putting out similar middle gray RGB values, chances are that my camera isn't faulty Somewhere in the region of 80..90 then.. The metering is most likely OK, comparing shutter speeds and apertures to my other cameras.

03-21-2012, 12:41 PM   #5
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To answer myself, the low "middle gray" was correct, my local service replaced the front dial and I asked them to check metering and sensor calibration too. All OK
03-21-2012, 01:08 PM   #6
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I dug up an old post of my where i tested the RGB values for the exposure, i got with the K10D

used "centre weighted" and shot a white paper.

QuoteQuote:
Just took a shot.
numbers are with a 101 by 101 average:
R 96
G 95
B 94

this the math Luminosity = 0.3 R + 0.59 G + 0.11 B so...
0,3 x 96 + 0,59 x 95 + 0,11 x 94 = 95,19
We agreed that 118 is 18% grey and I said 12% was 97
Seems the K10D meters for 12% grey (or close to it) and not 18% grey.
http://www.pentaxuser.co.uk/forum/topic/sunny-16--21892/p-1

Last edited by Anvh; 03-21-2012 at 01:14 PM.
03-21-2012, 01:51 PM   #7
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When I do exposure tests I shoot anything from a cement side walk to block walls to asphalt paved roads, the camera does not care. These surfaces all come out at a greyscale value (shooting neutral jpeg) at 120. With neutral settings the camera should meter to this level.
03-21-2012, 03:28 PM   #8
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Lowell, would love to see some proof off that.

I'll redo the paper test to see how the K10D and the K5 behave.

03-21-2012, 04:05 PM   #9
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QuoteOriginally posted by Anvh Quote
Lowell, would love to see some proof off that.

I'll redo the paper test to see how the K10D and the K5 behave.
Check this thread,

https://www.pentaxforums.com/forums/general-technical-troubleshooting/179542-...pentax-k7.html

there is a post in it with a chart, showing the issues with DSLR metering with non A lenses, but also included is an A 50/1.2 in A mode shooting a plane target. The line is flat except for F22 where the aperture has a slight error
You want proof, you get it
03-21-2012, 05:13 PM   #10
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QuoteOriginally posted by jaeaetee Quote
I was wondering, could anyone do a simple test with K-7? Shoot some white paper with CW or spot metering, adjust the WB and look what kind of RGB values do you get. RAW with no adjustments or some neutral JPG (I get about similar results with both).
Liveview or viewfinder metering ?

If the latter, then are you using the stock focus screen ?

I suspect that the K-7 metering is tweaked internally to correct the non-linear brightness vs fstop response of that screen. With the cut-down Canon Ee-S screen I have to use about +1EV correction to get good exposures. I'll have a go once it gets daylight over here.
03-21-2012, 09:25 PM   #11
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QuoteOriginally posted by Anvh Quote
I dug up an old post of my where i tested the RGB values for the exposure, i got with the K10D

used "centre weighted" and shot a white paper.


Sunny 16.

Wow, that was a good link, I didn't find the thread when I did some searching.. thanks. This Headroom in Highlights : Where is Zone V in The Digital World? | LibRaw seems like a good read too. Might help in getting good exposures, must read when I have time

QuoteOriginally posted by kh1234567890:

Originally posted by jaeaetee
I was wondering, could anyone do a simple test with K-7? Shoot some white paper with CW or spot metering, adjust the WB and look what kind of RGB values do you get. RAW with no adjustments or some neutral JPG (I get about similar results with both).



Liveview or viewfinder metering ?

If the latter, then are you using the stock focus screen ?

I suspect that the K-7 metering is tweaked internally to correct the non-linear brightness vs fstop response of that screen. With the cut-down Canon Ee-S screen I have to use about +1EV correction to get good exposures. I'll have a go once it gets daylight over here.
Stock screen and vf metering. I try to re-do the test today.
03-22-2012, 05:22 AM   #12
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QuoteOriginally posted by kh1234567890 Quote
Liveview or viewfinder metering ?

If the latter, then are you using the stock focus screen ?

I suspect that the K-7 metering is tweaked internally to correct the non-linear brightness vs fstop response of that screen. With the cut-down Canon Ee-S screen I have to use about +1EV correction to get good exposures. I'll have a go once it gets daylight over here.
All the cameras have the exposure calibrated for the non linearity of the focusing screen and its impact on metering. WHat seems consistent, is that the range of F4-5.6 seems to be correct, however, if you think about it this makes sense, because kit lenses and consumer lenses are F4-5.6.

What you get when you change focusing screens, or if, for example you add a teleconverter that does not modify the aperture to reflect the change from adding the TC, is a shift or offset, which follows the non linearity of the screen.

The K10/20 cameras were really bad for this, because of the change between F2.8 and F4 for example, but the K7 is a little better at the low end than the K10/20 and the K5 is very flat in response so the shift is hardly noticable
03-22-2012, 09:54 AM   #13
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QuoteOriginally posted by jaeaetee Quote

Stock screen and vf metering. I try to re-do the test today.
Yep, it's 87,87,87.. So about 10% "reflectance" is the output of the K-7, metering is more normal (12 or 18% gray?). I've been reading about this subject quite a lot now, it has been very educating.. maybe I have more chances now to get the exposures/results I want.. or better put, get the most out of the camera

Protecting highlights has clearly been the point for Pentax to map middle gray low. Looking at DxO mark results, the DR of K-7 isn't that great, actually it's 0,5 EV worse than Canon G11's (to which I have been comparing it to here).
03-22-2012, 10:32 AM   #14
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What kind of scale did you use there?
And very strange it's different then what i got with the K10D...
03-22-2012, 10:52 AM   #15
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QuoteOriginally posted by Anvh Quote
What kind of scale did you use there?
And very strange it's different then what i got with the K10D...
I am somewhat suprised you get different resuilts, however, lets go through step by step, because blende8 was able to reproduce exactly the same behavior with his camera, lenses and measurement of greyscale. the lines with his name are using data he posted in a thread discussing this issue, so I added them to my graph. Note also, I have similar flat line results using y *istD and Sigma APO 70-200 F2.8 EX, and have also measured a slightly increasing over exposure but also a streight line, using my tamron 28-75F2.8 so I know th enethod works.


So for the test, I use either a paved road, sidewalk, or cement block wall. THe surface is uniform shading and illumination for 100% of the frame.

I use spot metering if there is no installed split image finder, but average metering if there is a split image finder, regardless the results seem the same.

I use .5 EV steps because manual lenses have detents in either .5 or 1 stop increments

I set the aperture and let the green button pick shutter speed

I measure the greyscale in the center 10% of the frame, so that lens vignetting does not impact the results (same reason I use spot metering when possible.

I shoot JPEG with neutral contrast and saturation, with highlight and shadow protection off (as these all influence things) Shooting RAW can be misleading unless your raw editor is programmed to use the image jpeg defaults (perhaps a point of error in your test)

the photo editor gives the greyscale in 8 bit resolution, and i did linearity tests by going from a completely black (greyscale 0) to completely white (greyscale 255) in 1/3 stop increments, and from this I know that from about 25-225 greyscale each stop is about a change in greyscale value of about 45. the last 25 or so greyscale at each end of the exposure contains about 3 stops of exposure, but it is very non linear, with the last full stop only having a change of 3, the next 7 and the next 15, before the linear portion of the exposure curve.

I plot greyscale on the vertical, and aperture on the hotizontal, but let the horizontal axis be a log scale, so that there is equal space between full stops.

can you post some data, and make sure that ev comp is zeroed.

also watch out for back light entering the viewfinder and influencing the data
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