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12-26-2010, 07:13 PM   #1
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Pentax 645D: Chronic underexposure?

I've owned this camera (with the new 55mm) only 3 days, during which time the sun has showed itself maybe an hour. The hour I couldn't get outside, of course! I've managed some dreary test shots anyway, both inside and outdoors.

I'm finding I have to add at least 0.7 stop or more of "exposure" in post (in Lightroom) to just about every image. It's more pronounced in the outdoors shots, due to the challenging contrast conditions produced by gray skies and darker foregrounds. But I'm seeing this with the interior shots as well.

I guess maybe I'm expecting more "thinking" from the matrix-metering mode than it can deliver. In such outdoor scenes, regardless of the camera I'm using, I'd normally dial in +0.7-1.0 exposure comp; or else meter from a more "18%-looking" part of the scene, click on AE Lock, and recompose to shoot. I expected a bit more help from matrix metering, though.

The degree of underexposure I'm getting when I have no exposure comp set is quite striking. it's been suggested that this is the camera's "bias", in order to capture the longest possible DR. But it can't help shadow-noise levels to have to jack exposure up in post a stop or more for every frame just to get the scene brightness right and the histogram looking good.

I really want to like this camera; I can see hints of what it can do, and those hints have me salivating. The files look creamy and luscious, sharp but not cutting, smooth and rich, when everything comes together. This thing blows the doors off my now-departed D300 (as it damn well should at 6x the price), and it's visibly better than the Kodak ProBack/Contax 645 combo I used to shoot. Don't know how it would look compared to a D3x, or to the D4x or whatever Nikon's got coming up. (Doesn't matter to me how it compares to the equivalent Phase One, 'Blad, or Leica camera or back, since I can't afford those anyway.)

I just hope it gets to be less and less work, as I get to know this beast, to get things to that coming-together stage!

Thoughts welcomed. Even if it's just to tell me to STFU and be patient!

12-26-2010, 09:03 PM   #2
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When I had a 645D for a couple of hours, I think I left it in center weighted and it did fine for the most part.
Have you given that a try?
12-26-2010, 11:45 PM   #3
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STFU and send me the camera!

I don't know if this relates, but when I went from a Nikon D70 to the Pentax K20d I had a similar reaction to you - the Pentax severely underexposed shots that I took outside. Nikon and Pentax appear to have very different approaches to metering, Pentax apparently preserving highlights at all costs, while Nikon appears to strike a different balance. It's just something that you get used to over time.

Best of luck with the new toy, and welcome to the forum!
12-27-2010, 08:28 AM   #4
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The short time I have had the 645D, I have used Aperature priority, allow camera to set shutter speed. After each shot I looked at the histogram, and concluded that, at least on grey days, I needed +.7 dialed in. With my Alpha 900, +.7 has been permantly dialed in for a year and a half. Good luck.
Dave

12-27-2010, 12:27 PM   #5
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Thanks all. I've tried metering in center-weighted mode, and it doesn't appear significantly different. In spot metering mode, of course, it works fine as long as there's something about 18%-gray-equivalent in the scene: like grass, or a neutral mid-tone wall, or something.

It seems, then, that any fault lies between my ears and not with the camera.
12-27-2010, 03:27 PM   #6
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I don't have this issue at all. In fact I found that my 645d nails exposure much better than any of my canon bodies.

Have you tried spot metering at all? I use this whenever the scene I am photographing has very high constrast.

On another note I found that the Pentax 645D is a little underexposed when matching it up with a Sekonic meter. If I use my Sekonic I have to dial in a + .3 to the Pentax 645D. My Canons I dial in a +.5 to match.
12-27-2010, 07:25 PM   #7
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@Shuttershane, I've tried all three modes. As long as there's something about "18% gray" in the scene, spot-metering produces the best result, as expected. Problem is, this time of the year in Kentucky, with snow on the ground and gray sky above, every outdoor scene is a high-contrast scene, as are indoor scenes with artificial light. Looking through the few images I've actually made (less than 100 in 4 days of owning this thing), in the few with a more "normal" range of tones, the meter is much closer, though I still have to add up to +0.5 stop of exposure comp.

It seems unlikely this is a flaw in the camera; but rather, one of those quirks possessed by all cameras that I simply have to learn to deal with. Anyone think otherwise?

I appreciate everyone's thoughts here.

Consensus I've been able to suss out online is that the camera may have some inherent "bias" towards underexposure, in order to preserve highlight detail; and that even with matrix metering, it still requires quite a bit of help to get things right in challenging situations.
12-27-2010, 08:24 PM   #8
rlj
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MikeSeb,

I'd have to agree with the other posters, I think Pentax has a slight bias to underexposure. I too have seen it with Sony A900 and now with my 645D images. But personally, I would rather be on the under exposed side than over.

Of course, some scenes with great contrast can fool any metering system, and would probably require bracketing anyhow to get the whole thing properly recorded (making a under exposure moot as you'll have to make many exposures anyway).

I look at the peculiarities of any camera as simply one we, as you say, 'have to learn to deal with'. (And no matter what, much better than the film days when we'd only figure out a mistake days later).

12-28-2010, 08:07 AM   #9
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It seems like none of you guys is using expose-to-the-right methodology, which surprises me. With ETTR, the term 'underexposure' means (as much as it can mean anything) highlights that fall short of the right edge of the histogram. Exposure-wise, nothing else matters.

To my knowledge, there are no camera metering systems that can do ETTR effectively, in a variety of contrast situations, without a fair amount of user intervention. The technique I've adopted is to use a spot meter on a (zone XIII-ish) textured highlight, placing it whatever number of stops up from the middle of the meter's EV scale I've determined via testing to be optimal for that camera/ISO combo (taking into account LR's highlight recovery capacity). I also try to learn how to read that camera's histogram and hi-lo warnings relative to this highlight placement, to use for verification.

You can't really work this way without 'calibrating' things, no matter what camera. With the 645D, I'm hoping to get into the studio today to get this all figured out.

Tim
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12-28-2010, 08:57 AM   #10
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I have observed the same underexposure habit. With critical work, though, since no camera maker has invented an "expose to the right" mode yet, you'll just end up tweaking for optimum exposure anyway. Putting the exposure compensation on one of the dials in your most used modes is an easy way to add a little more exposure, if you're shooting in automatic.

- N.
12-28-2010, 11:08 AM   #11
rlj
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Nick,

It would be nice if the 645D would allow a "UniWB" mode. I've not been able to easily do this with Pentax (as can be done with Canon or Sony). If Pentax could provide a UniWB mode through software, it would be a unique tool for photographers.

-Robert

_____
Uni-White Balance (UniWB) background:

UniWB displays a histogram of the RAW data, not the usual histogram of the processed JPG. Since there are 2 green pixels for each red and blue in a Bayer sensor, the UniWB mode on a Canon/Sony camera appears green, but allows one to truly ETTR, better utilizing the sensor to maximize the S/N.

If one does an ETTR now with the JPG histogram, one is not actually ETTR for the sensor, but simply ETTR for only the red channel (and sometimes the blue). This leaves the green underexposed and increases noise. Thus, ETTR on virtually all digital cameras is misleading.

Last edited by rlj; 12-28-2010 at 11:20 AM.
12-28-2010, 11:22 AM   #12
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Robert, I do what you're describing with my Canons, using a custom white balance created from a blow-out frame. And like you, I haven't figured out how to do this for the 645D. (Never heard it called "UniWB", though. I'll have to google that.)

Tim
Chicago

Last edited by Tim Wilson; 12-28-2010 at 11:30 AM.
12-28-2010, 03:36 PM   #13
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Correct. Leica just introduced a RAW histogram with the latest firmware update to the S2, so it may be technically feasible to introduce this down the line. Would be great! Till then, we just have to know our cameras.

- N.
12-29-2010, 07:27 PM   #14
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@Tim, ETTR is exactly what I'm trying to do. I guess I overestimated the camera's ability to help me do that . (For $10k I expect some love from this thing... )

Now, if only I had bought up a few lenses a month or so ago when I put in my order for the camera. They've skyrocketed in price, and plummeted in supply, since then.
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