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05-30-2008, 12:23 PM   #46
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QuoteOriginally posted by dopeytree Quote
I wouldn't dream of buying a pro spec camera to use on holiday unless i knew how to use it Please do your self a favour and learn first! then go book an expensive holiday...

Actually, that's what I did. Bought the K20D, set it on green, and went to Venice to take pictures. My pictures turned out great! Nothing like beginner's luck!!

05-30-2008, 12:32 PM   #47
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I think it's not camera problem
05-30-2008, 02:58 PM   #48
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QuoteOriginally posted by Jaleel Quote
I think it's not camera problem
Maybe it is me, and it probably is but then could someone tell me that while I have the EV compensation set to +1 or +1.5 the pictures that I went outside to take with my dog were all underexposed still? Camera set to Tv mode because the dog is running around so I want to set the fast shutter speed. Maybe I need to put the ISO higher or something?

I just thought that these modes like shutter priority are supposed to figure out the EV by themselves. I tried multiple metering modes.
05-30-2008, 04:43 PM   #49
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QuoteOriginally posted by TangentReq Quote
Maybe it is me, and it probably is but then could someone tell me that while I have the EV compensation set to +1 or +1.5 the pictures that I went outside to take with my dog were all underexposed still? Camera set to Tv mode because the dog is running around so I want to set the fast shutter speed. Maybe I need to put the ISO higher or something?

I just thought that these modes like shutter priority are supposed to figure out the EV by themselves. I tried multiple metering modes.
maxtix metering would be your best bet but first:
White dogs or black?
Post an example.....
Also, no your camera does not use AI to figure out what you want...

05-30-2008, 09:06 PM   #50
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QuoteQuote:

Forgive me for saying so but if you want good pictures straight out of the camera, use a point and shoot because if you are using the P mode, you are just using a point and shoot (an expensive one at that) and not getting the full potential out of the K20. If you want great pictures, learn how to use your camera and learn your PP software.
The "mode" thing is frequently mis-represented.

The mode used - whether auto or manual or anything in between - makes no difference whatsover to the "quality" of the image.
There is nothing different if the camera "automatically" sets the shutter and aperture to 1/500 and f4, or the photographer does.

What is really meant - though not explained - is that the light meter's suggested exposure must be interpreted (very light/dark subject? backlit?) and in this regard, the camera may choose an incorrect setting - regardless of "mode".

However, in consistent light, fully manual M will provide consistent exposures (right or wrong) regardless of changing reflectance values of the subject (such as white T-shirt vs black T-shirt).

I shoot P mode 90+% of the time, and multi segment, and have very few exposure problems - though I have a K100D and not a K20D.

Almost all of the OP's images should have had +EV compensation as the metered areas do not average to middle gray.

.
05-30-2008, 10:40 PM   #51
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What SpecialK says is what i had in mind on all shots i saw, sun behind subject, mostly white wall around a kid, bright sunlit sand behind a kid, all things that are going to confuse the camera that always tries to expose to "achieve gray". (Shoot a mostly white/bright scene and you get gray (underexposure), shoot a mostly black/dark scene and you get gray (overexposure), shoot a gray scene and you get gray (perfect exposure))

The K20D's exposures are pretty much what i would expect them to be, at least i got something confirmed in this thread that i had only suspected, EDR being a jpeg only option. The raw side effect is also interesting.

To mrechte: get used to how any modern camera meters and learn to compensate for the actual scene, stay away from spot metering until you are getting more comfortable shooting with the K20D.
05-31-2008, 03:25 AM   #52
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QuoteOriginally posted by TangentReq Quote
I have the EV compensation set to +1 or +1.5 the pictures that I went outside to take with my dog were all underexposed still? Camera set to Tv mode
Just an idea... In Tv mode, the camera cannot go to slower speeds, only to the widest aperture. If it doesn't suffice: underexposure. EV compensation then has no effect. You may want to check your underexposed images for widest aperture settings.

There is a firmware setting (in this long list) to tell the camera to override chosen setting in Tv or Av if light doesn't allow it.
05-31-2008, 03:45 AM   #53
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QuoteOriginally posted by mrechte Quote
thanks my VISA # was not there
[...]
I am not very clear with the Effective LV and Segment LV. [...] why in spot measure the Effective LV is not the centre segment LV ?
[...]
->"Still, the histogram shows that the image is underexposed by a full 1EV. Like the original light values all ranged from 11.5 to 12.5." Where do you read this information ?
Your VISA was there, it reads 1634 6354 8465 2956

Segment LV: The measurements
Effective LV: LV computed from speed, aperture, ISO; formula->Light value - Wikipedia, the free encyclopedia BTW, EV is LV at ISO100.

So, the effective LV reflects the decision taken by the camera and/or the user. It isn't stored in the EXIF but computed from the other info.

In spot metering, the effective LV must not match the metered value. I believe that you, like most people including myself, are easily fooled by what spot metering is: Spot metering must assume that the measured value is a defined level of grey, and adjust exposure to properly catch black to white.

Therefore, your K20D exposed with center spot LV +1.1EV. This is correct I guess, like assuming a 40% gray card.

Spot metering really is only useful with a gray card held into the spot, or with extensive test shots validating the proper EV compensation then to stick at.

All your spot metered shots are destined to be ill-exposed as you never dialed in a corresponding exposure compensation -- or used a gray card.

Some camera just work fine because they have a huge center spot. Not so the Kx0Ds... Good so.

->Last image 11.5 to 12.5...
The image was taken at 13.5. The histogram ranged from 0 to 127, so 1 full EV stop (the upper half of the histogram) is missing. So, the real LV values ranged up to 12.5 only (13.5 - 1).

Maybe, take an image of a white wall (with whatever metering). It must come out underexposed. Look at the image, note meter readings (should all be the same) and effective LV. Look at the histogram, where the single peak is. Compute the real LV from the peak location (255: LV = effective LV; 127: LV = effective LV -1; 63: LV = effective LV -2; ... Log formula in general). Compare real LV to meter readings. If different, you may have a defecctive camera. However, other users still have to do this test to validate that a K20D normally passes the test (I haven't yet!).

UPDATE: The above formula is too simple. One has to take gamma correction into account as well... So, better try to expose to bring the peak near 240.


Last edited by falconeye; 05-31-2008 at 03:58 AM.
05-31-2008, 05:15 AM   #54
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QuoteOriginally posted by SpecialK Quote
Almost all of the OP's images should have had +EV compensation as the metered areas do not average to middle gray.
.
Disagree. Please check 1110 vs. 1109: it shows the spot measure is doing the opposite of what it should do !
05-31-2008, 05:26 AM   #55
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QuoteOriginally posted by morfic Quote
What SpecialK says is what i had in mind on all shots i saw, sun behind subject, mostly white wall around a kid, bright sunlit sand behind a kid, all things that are going to confuse the camera that always tries to expose to "achieve gray". (Shoot a mostly white/bright scene and you get gray (underexposure), shoot a mostly black/dark scene and you get gray (overexposure), shoot a gray scene and you get gray (perfect exposure))
Please let us talk on precise facts, rather than general considerations. In that case please explain why 1109 is more exposed than 1110. The white wall should, as you say, fool the camera in multi-seg measure, but I end up with a less exposed picture using the spot measure on the face ! I have been using a spot meter camera (with a tiny central spot) for a while and never got that abnormal behaviour.

If this is normal for a K20D, I wonder who uses the spot meter !
05-31-2008, 07:20 AM   #56
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QuoteOriginally posted by mrechte Quote
Hello to all,

I recently purchased a K20D with the starter kit (18-55 + 50-200).

I am back from a 4WD trek in Morocco with a load of pictures.

I am amazed to realize that most of the pictures are underexposed. Actually for most of the picture I must use the Photo laboratory software to increase the light (and not slightly !)

Even some pictures taken in low contrast conditions are underexposed. My son's cheap camera does far better than my K20D !

I am terribly disapointed with this camera.

I cannot imagine somenone is satisfied with the K20D light measuring.

I hope Pentax releases a new firmware to correct this problem very soon.

Regards

Marc
I have been through all your pics and I would say you have a faulty unit or lens that is consistently underexposing by 0.5 stops just going from my cameras typical behaviour in similar scenes. I very seldom require EV comp unless there is backlight or a very light coloured subject.

However you could have checked the LCD and compensated at the time - it would have saved your holiday. In the meantime get Pentax to have a look at it.
05-31-2008, 07:23 AM   #57
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QuoteOriginally posted by mrechte Quote
Please let us talk on precise facts, rather than general considerations. In that case please explain why 1109 is more exposed than 1110. The white wall should, as you say, fool the camera in multi-seg measure, but I end up with a less exposed picture using the spot measure on the face ! I have been using a spot meter camera (with a tiny central spot) for a while and never got that abnormal behaviour.

If this is normal for a K20D, I wonder who uses the spot meter !
@mrechte, I'm addicted to your thread because I learn a lot from it

so, let me quickly revisit 1109 vs. 1110 with what I've learned from PhotoME...

In the following, I give four number: LV_spot/LV_min/LV_max->LV_eff

1109: 14.5/14.0/16.4 -> 14.9 (matrix, average in middle 8 is 14.9, outer 8 is 16.1)
1110: 14.6/14.0/16.4 -> 15.8 (spot: +1.2EV)

So, spot meter works as advertized (cf. my other post).

Matrix metering (1109) looks overexposed (from what I am used to see normally). Also, why did it completely ignore the outer zone, using an exposure correct for the inner zone (14.9) only. Maybe, it wasn't matrix but multi-zone mode? That would explain it, but the EXIF says otherwise. Does multi-zone show up in EXIF differently from matrix at all? Did you use multi-zone?

And then: Why isn't 1109 clipped at all? The white background in the lower right shows up at rgb 205 (80%). With 2.2 Gamma correction (sRGB), this is 61%. Or -0.71EV from max. Shot taken at 14.9LV makes the wall 14.2LV. No EDR used.

But the matrix meter readout for this part of the wall is 16.4LV.

So, the meter readout is +2.2EV too high. How can this be? In the other example (last image), it was +0.5EV too high.

So, if the meter is +2.2EV too high for 1109, and +0.5EV too high for 0698, then there either is alien light in the viewfinder and/or something even stranger is going on here.

Anyway, and I repeat my verdict here, with matrix readings of 14.5/14.0/16.4, the camera shoudn't have used 14.9 (more appropriate for multi-zone). For matrix metering, it overexposed by 1 stop for unknown reasons -- even if the result is pleaseant because the metering values were too high.

I just saw *isteve's comment now.
Ok, now you have two votes that your camera may be metering too high.

Last edited by falconeye; 05-31-2008 at 07:39 AM.
05-31-2008, 11:35 AM   #58
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I have now carried out some own, more detailed tests.

First, some of the info I had provided before is wrong. I apologize for this.

(1) EffectiveLV
The first correction is for the EXIF value reported by PhotoME as "EffectiveLV".

It is defined in Exiv2 - Image metadata library and tools as "Exif.Pentax.EffectiveLV: Camera calculated light value, includes exposure compensation"

I carried out some exposure brackets and used EV compensation, too. This value doesn't change and is different from the LV used when taking the image!

For instance, study the fourth attachment to this post (I hope that EXIFs stayed intact...):

Taken at ISO100, 1/500, f/8 -> 15.0 LV
(Sometimes, the maker notes would say 1/751.9 rather than 1/750. It seems the shutter speed is measured and deviations are recorded!)

However, the EXIF EffectiveLV is 16.8 LV. Why?

Now, look at AE-Shutter Speed = 1/442.7
This is what the AE recommend and the next rounded available speed was 1/500.

So, sometimes, you must just fall in between speeds 1/2 stop apart and two consecutive images are exposed differently because of this...

AE incl. compensations says at ISO100, 1/442.7, f/8 -> 14.82 LV

Now take +2EV exposure compensation into account, +1EV from EV compensation and +1EV from the bracketing series this is a part of.

AE without compensations -> 16.8 LV
So, now we know what EffectiveLV means:
It is the LV the camera electronics thinks it would be best to take the image at, ignoring EV compensation advice if given
EffectiveLV doesn't change with EV compensation or during a bracketing.
(2) Calibration of metering
I have done a test shot of a sheet of paper (fourth attachment).

As we already know, LV taken with is 15.0 LV.

The white background is at 90% (79% gamma-corrected), or -0.34EV from full white. So, it should be 14.7 LV.

But big question and my second mistake:


Should a subject with LV value N taken at LV N be recorded ar RGB=255?

From Light meter - Wikipedia, the free encyclopedia I note the sentence: "ISO 2720:1974 recommends a range for K of 10.6 to 13.4 with luminance in cd/mē. Two values for K are in common use: 12.5 (Canon, Nikon, and Sekonic) and 14 (Kenko and Pentax); the difference between the two values is approximately 1/6 EV."

So, the issue is more complicated and my take at it was pretty naive.

Anyway, the matrix field readings are:
15.6LV (middle) and ranging between 15.1 and 16.1LV. The EffectiveLV of 16.8LV reflects the +1.1EV rule for spot metering still seen to be valid.

The average is 15.6 which is +0.9EV compared to the scene luminance computed above (14.7).

So what it all does mean?

Well, first a short departure into gray cards here: Meters Don't See 18% Gray by Thom Hogan

It suggests that a 12% gray card, rather than a 18% gray card, should show up in the middle of a histogram. We don't need the argument here.

But the camera meters cannot know the difference between black, gray or white. So, whatever subject they meter, they should always try to take the picture such that it ends up in the middle of the histogram.

So, a subject with light meter reading of X, when taken at x -1EV exposure, should end up near the right edge of the histogram.

So, if we take this into account, our histogram measure of 14.7 should be corrected by 1EV and read 15.7 LV -- pretty close to the actualy meter readings.

Having said all this, we need a break and I provide three more photos for your enjoyment, taken at
- matrix
- mutlifield
- spot (no EV correction)
- the sample discussed above
- a 100% crop from it (because it is so f**g sharp

Last edited by falconeye; 06-15-2011 at 05:28 AM.
05-31-2008, 01:09 PM   #59
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Gotta LOVE these forums! Amazing the expertise & brainpower brought to bear on the task of helping somebody that most have not met.

Most of this discussion is way over my head, but there is enough info for a general grip on what is going on.
06-01-2008, 11:22 PM   #60
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@falconeye: thanks for all your very interesting posts, they are very instructive. It will take me some time to review and understand them ! I'll post my comments afterwards.

QuoteOriginally posted by falconeye Quote
Spot metering must assume that the measured value is a defined level of grey, and adjust exposure to properly catch black to white.
You are saying that the spot measure would give a different value than the mult-seg on a same uniform frame. If it is so, this spot meter measure is pretty awkward. One will use spot meter to adjust the exposure of his/her subject on a given interest and does not expect to have to compensate, unless this zone of interest is abnormally dark or very bright.

On Olympus (SLR) camera (0M2 spot, OM4), brand which has pioneered the multi-spot measure, this was working perfectly (that is the picture was exposed accordingly to the spot zone). The documentation was complete on the subject (an not 2 tiny pages out of the 280 pages user manual). To cover the above two extremes cases (dark, or bright subjects) there were 2 buttons highlights / shadows that one would presses after taking the spot measure which over / under expose accordingly.

Furthermore the K20D does not indicates anything in the viewfinder (hardly readable in sunlight), nor on the LCD display about which measure mode is set ! That is why I forgot to reset the spot mode to multi-seg mode after use. This makes me say that Pentax has not designed this feature to be used in the field (may be in a lab with a piece of 30% grey cardboard handy...), especially if on top of that, one has to systematically add 1,5 LV on a average subject... Very disappointing !

To my taste K20D comes with too many modes (green mode, Sv, TAv), functions that are good on paper but unusable in the field (Liveview, spot-measure), confusing menu / Fn with again too many functions (especially photo-lab like functions) , too many buttons (AF is even duplicated).

So why did I buy this camera ? I was attracted by: 3/2 format, old PENTAX lens still usable, wireless flash system, spot measure, shake reduction on the sensor itself, sensor dust detection, immediate start-up time.

My next input on this thread will be to investigate whether my camera is faulty or not. Fortunately this forum exists, and again I thank all you, because for the time being PENTAX support has still not replied to my initial query (the guy was absent, then busy, be sure you'll get soon an answer) ...
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