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12-26-2013, 09:36 AM   #1
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K3 new metering system -tend to underexpose?

Hi, I am a New K3 user and really love and enjoy the upgrade from K3.

However, with the new 86000 pixels metering sensor, I found that My K3 tends to underexpose. Does any owner feel the same way as I do?

Thanks...

12-26-2013, 09:54 AM   #2
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Not sure if it's the new metering system or just Pentax preference to avoid clipping highlights. My k-x, k-5 and k-5IIs all under-exposed by 1/3 to 2/3 stops. I almost always had EV comp on +2/3 on the k-5.

K-3, if anything seems to be closer to 'correct' than the k-5 series. But that's what EV comp is for. Different lenses, different scenes, different preferences will need different settings. The meter is just an average, use the EV comp button to set it where you want.
12-26-2013, 10:48 AM   #3
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I shot quite a bit at the zoo the other day when it was completely overcast and everything was considerably under-exposed. I'm used to under-exposure, though this was more extreme. I didn't have any EV Comp dialed in as I shoot RAW and it's so easy to correct in post. Still, I would say that the K-3 tends to under-expose a bit more than previous bodies, at least in my experience.
12-26-2013, 11:10 AM   #4
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QuoteOriginally posted by Sleepy Quote
...I found that My K3 tends to underexpose. Does any owner feel the same way as I do?
Wow. I was going to post the opposite inquiry. I think my K3 chronically over exposes, and over flashes. It is significant enough that I fear it is an chimping illusion until I get the photos to the computer, and then yes indeed, the uncorrected shots are blown out, and the ones stopped down 2/3 or more tend to be right on the money. Do take that with a grain of salt, though. I'm just learning the camera and am shooting JPGs, not raw.

12-26-2013, 11:51 AM   #5
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QuoteOriginally posted by Sleepy Quote
Hi, I am a New K3 user and really love and enjoy the upgrade from K3.

However, with the new 86000 pixels metering sensor, I found that My K3 tends to underexpose. Does any owner feel the same way as I do?

Thanks...
When there are lots of highlights the k3 does its best to avoid clipping them. That's not a bad strategy imo, as shadows are a lot easier to recover.

Adam
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12-26-2013, 12:26 PM   #6
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I generally set my exposure compensation to +0.3, except when snow is in the scene, thn I tend toward +1 to+1.7
12-26-2013, 03:20 PM   #7
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What Adam said. The K-3 is very careful not to overexpose highlights, even if very small ones. I have often got pictures that appear underexposed but when I look carefully I always find some detail in the photo that is almost saturated, or perhaps even saturated. The camera would have dialed down the exposure precisely to protect those details.

In most cases I think this is a very good approach but there are some cases where it can be counterproductive, and that is where there are bright details that you don't actually care about being overexposed. For instance if there is the sun reflecting from some window in the background the camera will try to keep that under control and prevent it from saturating - so you get a perfectly exposed shiny window and the rest of the photo is almost black. It will be some more years until cameras become clever enough to know that you don;t care about that window being over exposed. Actually it might never be possible for the camera to know for sure as two different photographers might want a different result from the same situation.
12-26-2013, 03:27 PM   #8
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QuoteOriginally posted by BretW Quote
Wow. I was going to post the opposite inquiry. I think my K3 chronically over exposes, and over flashes. It is significant enough that I fear it is an chimping illusion until I get the photos to the computer, and then yes indeed, the uncorrected shots are blown out, and the ones stopped down 2/3 or more tend to be right on the money. Do take that with a grain of salt, though. I'm just learning the camera and am shooting JPGs, not raw.
Are you using an A lens? I found that with any F or D lenses it does really well, bounce or slave. With my A 50 1.4, it is overexposed about two stops.

12-26-2013, 04:05 PM   #9
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Remember that you can spot meter and hit AE lock.
12-26-2013, 04:08 PM   #10
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If the k-3 underexposes slightly, I believe it is merely continuing a design philosophy of Pentax. My k10 does it, my father's k20 does it, my k5 also does. The default position seems to be highlight protection. As long as its consistent and predictable, that's the key. And if you want the camera to "divine" your metering intentions (ala the bright window story) then I think the only way that will ever happen is by shooting in "m", which will require us to know our intentions. Which is where I struggle sometimes, lol.

One of the best things about Pentax Dslr ownership is that you HAVE to learn the camera. They aren't an over-bright, over-saturated, green-mode-only glorified p&s like some other brands consumer models can be.
12-26-2013, 05:02 PM   #11
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I'm using the 18-135 kit lens, and seeing overexposure. I can and do compensate, but it is the wrong direction to protect the highlights.

Perhaps I've fiddled a setting in WB space or something.
12-26-2013, 05:31 PM - 1 Like   #12
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QuoteOriginally posted by Adam Quote
When there are lots of highlights the k3 does its best to avoid clipping them. That's not a bad strategy imo, as shadows are a lot easier to recover.
That's an important comment.

The k-3's high-res AE array will detect specular reflections more successfully than the 77-segment array in the K-5. I've had issues, when photographing high jump, where the strong reflection of sunlight off the shiny, narrow horizontal jump bar is blown irrecoverably (I always shoot raw), even with -1 EV Comp. With over 1,100x more segments (33x more in the vert. & horiz. directions), the chances of detecting a small, highly reflective area are much better.

So, the AE metering has better resolution (important also for the WB and AF tracking). So any possible tendency in the K-3 to "under-expose" may just be the better detection of extreme highlights.

As regards AE, what does the camera do next? The designers will probably have a percentage limit of pixels that can be acceptably clipped in a shot (e.g. 0.2%). A very small clipped region is unlikely to be noticed, but if it's on a person's cheek, forehead or nose, even this may be noticeable. The 86K AE sensor can better detect these hotspots, but what can the camera do about it? As far as I'm aware there are only 4 means available:

  1. Reduce the overall brightness level by lowering either the exposure (shutter-speed or aperture) or gain (ISO), depending on which AE mode you're using. This could end up making the rest of the image too dim, just for the sake of a tiny, visually-insignificant region. I believe part of the propriety/secret voodoo that each manufacturer designs in their matrix/evaluative metering system is scene recognition or how much weighting to apply to different AE metering segments in the calculation of the overall scene luminance level. I presume segments on the periphery, in general, and in the upper half (sky) in a landscape shot, are allowed to clip more, then for example, the centre.
  2. The user selects different Tone Response Curves e.g. Natural, Bright, Vibrant. The scene brightness distribution is no longer being linearly converted. With a "knee" & "toe" included in the TRC, and perhaps the gamma (slope of the TRC) too, the original DR in the scene is reduced, so that the overall image DR is more pleasing when reproduced within the limited DR space of a computer monitor or printer.
  3. Highlight/Shadow Correction. Over/under-expose/boost a region of the TRC and apply some compensating correction afterwards.
  4. HDR. Combine different brightness regions from multiple images taken with different exposures, and then map the tome of the image with a special TRC, either in-camera or in PP. The range encoded without clipping can be very large, but problems often occur in the tone mapping process in successfully portraying this DR, within the limited display DR, without looking unnatural.

What I don't think any camera currently offers in the TRC, but which might work, is the photographic equivalent of a single-ended audio dynamic compressor/limiter. This would dynamically vary both the location of the knee and its bend, in response to the percentage of extreme highlights metered in the scene, so as to reduce the final amount of clipped pixels.





If shooting raw, you can try to get even the brightest parts unclipped, and then fiddle with the shape of a suitable TRC in PP. The image out of the camera from a high-contrast scene will look predominately dim, but Sony Exmor sensors (as used in recent Pentax DSLRs) have very low-noise ADCs, and thus good Total Read Noise performance, which means that the shadows can be boosted a lot before noise becomes a problem.

One thing a lot of people don't realise is that, in the vast majority of DSLRs, the sensor has a fixed sensitivity (the exception being the Aptiva switchable sensitivity sensor used in the Nikon V1/J1). The sensels in the sensor should only saturate (reach Full-Well Capacity) at the true base ISO. The reason you apply ISO gain is not to increase the sensitivity of the the sensor to light. It's to boost the weaker signal from a sensor which, due to the combination of the scene luminance level and your choice of shutter speed and f-stop, is not "well filled". This boosted signal is then presented to the ADC and results in larger DNs (Digital Numbers) in the resultant digital image data. And higher DNs are rendered more brightly. So the image captured by the sensor in this situation is under-exposed, but is rendered with a good brightness level by the application of either analogue or digital gain (ISO increasing).

So, if you accept that the exposure process is the capturing of the photons (i.e. all about the light), rather than the rendering of the converted signal, then only a shot taken at base ISO has the possibility of being well-exposed. Once the combination of scene luminance, shutter speed and f-stop is no longer sufficient for the vast majority or for any of the sensels to reach FWC, then we're dealing with an under-exposed image. The signal boosting can occur either in-camera (typically analogue at low to mid ISOs; digital at high ISOs) or in PP (digital).

As the amount of under-exposure increases, the sensor output signal is weaker and weaker, and the max. levels in sensels in the brightest bits of the scene are failing further below their FWC. This is necessitating the application of more analogue gain between the sensor and the ADC. So when highlight clipping occurs in a properly designed digital imaging system, when operating above base ISO, it's not the sensor saturating that's causing it. Instead, it's the DN maxing out. (Of course, a specular highlight/reflection may be sufficiently big enough so that some sensels in the sensor reach FWC, even when the rest of the sensels are at quite low levels, and thus you'd normally use 1-2 stops of ISO boost to increase the overall rendered image brightness. But the likelihood of any sensels having reached FWC decreases as you apply higher ISOs - that's the reason why you're using higher & higher ISOs.)

The under-filling of the sensels at higher ISOs is shown in the table in this message: https://www.pentaxforums.com/forums/115-pentax-k-5/162940-k5-vs-k7-dynamic-ra...ml#post1689551

Another reason why analogue gain is used, is to boost the weakest parts of an under-exposed image above the noise floor of an ADC. However since Exmor sensors have very low-noise ADCs, the need to due this is reduced. This introduces the possibility of shooting high-contrast scenes in raw at either base ISO or lower ISOs than you normally would, and then boosting the image brightness afterwards in PP. While the review image will be quite dim in the back LCD screen, the advantage is that the extreme highlights are much less likely to have been clipped, when taking the shot. Afterwards, If you were just to boost them directly up to same rendering level as normal ISO boosting would, you would still face the same likelihood of DN max clipping occurring, as before. But since you are free to fiddle with the TRC during PP, you can adjust it to accommodate the rendition of the highlights which have not yet been clipped. Looked at this way, an "ISOless" (exposure-centric rather than ISO-centric) approach to shooting with a suitable camera offers a superior form of highlight recovery.

Dan.

Last edited by dosdan; 12-26-2013 at 05:56 PM.
12-27-2013, 08:58 PM   #13
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Thank you very much for all your replies. I guess it should be normal that my K3 tends to underexpose a bit, and I'll take more shots during New Year holidays. As long as it's consistent, I am fine with that. Thanks again and Happy New Year!!!
08-06-2014, 04:52 PM   #14
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Sandy

Hi everyone!
I agree that all K3's tend to underexpose. All Digital Cameras Over/Under expose. It's really no different than buying a new Big Screen TV. The new owner must take the time to calibrate/setup the new device.
It even speaks about this in the owners manual. It also talks about white balance.
Pentax would rather have us start out with underexposed pictures knowing we are going to post process. When the highlights are blown......There is not much left. Underexposed images are usually salvageable.
Get used to using and interpreting the histogram (I use this to expose Film in my Hasselblad). It's so easy to just push one button and crank in exposure compensation. I strongly recommend going to the menu and setting EV (exposure Value settings to 1/3. Use a gray card, a white shirt or anything that will show you on your histogram where you are "CLIPPING" the bright whites in your shots.
Underexposure is no big deal once you have the WHITES adjusted.
08-06-2014, 06:21 PM   #15
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It is worth noting that the K-3 has 3 specific metering modes (Multi-segment, centre-weighted, and spot), as well as an option to adjust the exposure according to the position of the AF point used. Each mode will likely produce a different exposure result, depending on the scene. It's a good idea to get to know all 4 exposure modes.

If you aren't shooting with a fixed ISO, the auto-ISO adjustment parameters you have in place [slow/standard/fast] can also make a difference sometimes to how your scene exposure turns out, particularly highlights.

I find the K-3 exposure metering system is more accurate and 'intelligent' than in any previous Pentax I have used. No complaints here of under or over exposure, even under mixed lighting. I stick to multi-segment metering 95% of the time now with the K-3 - even when shooting telephoto - whereas with the K-5 I swap a lot between centre-weighted and multi-segment, depending on the lens used and the scene.

Last edited by rawr; 08-06-2014 at 06:28 PM.
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