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Old Apr 1, 2006, 1:29 PM   #1
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I have just bought an A200, and been trying it out. I'm quite pleased with it so far, but there is one occasional phenomenon that puzzles me.

I take a photo inside a building without flash. The light is poor, but I reckon just adequate for a feasible photo. I set exposure comp. to -1.0, so I have a better chance of avoiding camera shake. The image I see in the viewfinder is rather dark, but I judge from experience that I can probably lighten it up into an acceptable picture in Photoshop afterwards.

But the image that comes out of the camera into my computer seems to be just a patch of pure black, or very nearly.

Nevertheless I try to lighten up the image by applying an enormous percentage in Photoshop. Then one of two things happens:

In some cases, to my amazement, I get a fine image that I'm very happy to keep: nicely exposed, nicely contrasted, nicely focussed.

In other cases, the result is still utter garbage.

When I check the EXIF data of the good images, I find in one case (eg) exposure time 1/20s, f3.2, ISO50. In another exposure time 1/50s, f3.2, ISO50!

Can anyone explain what is going on here?


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Old Apr 2, 2006, 7:12 AM   #2
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what is your LCD setting? do you have (LCD) exposure priority on?If not, your LCD gains up in low light *so you can frame the picture better but giving misleading information about exposure. That is why the picture appears lighter on LCD than on your PC. I always set LCD to exposure priority mode so it doesn't gain up and under low light conditions I shoot in manual* so I can see exactly what I am getting. No hassle with exposure compensation... or too slow and ever changing shutter speeds. No need to choose the right metering mode for difficult scene...
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Old Apr 2, 2006, 7:19 AM   #3
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one more thing... ISO 50 seems to me a bit low for badly lit scenes. keep it between 100 - 200 to get higher shutter speeds... do not underexpose much, use tripod instead. there will be too much noise in your photos despite lighting them up in Photoshop...With antishake you can take sharp pictures even at 1/15 if you do not zoom much .*
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Old Apr 2, 2006, 7:51 AM   #4
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I'm not worried about the LCD. It simply showed what I expected to get, and indeed have got in other shots under similar conditions.

The ISO and the shutter speed are the amazing things. I can't understand why the camera should have chosen those automatically (and thus produced well focussed near-darkness) in the relatively dark conditions.


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Old Apr 2, 2006, 11:29 AM   #5
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Well, since you used a -1.0 EV setting via Exposure Compensation, the images are supposed to be darker (you're deliberately underexposing them by a stop compared to what the camera would have used for exposure settings).

As for the reason there is a difference between them, something probably changed in the image. If some have more light or dark areas compared to others, that will influence metering. Were you using Matrix?

Why use ISO 50 and deliberately underexpose when you've got higher ISO speeds available? You'll still have just as much or more noise as if you would have used a higher ISO speed, after brightening an underexposed shot anyway.

Underexposing and pushing the image back up during post processing can also hurt dynamic range.

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Old Apr 2, 2006, 1:20 PM   #6
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I see now that I failed to explain the problem clearly enough. I'll have to ask anyone who has the patience to ignore all the above and read the following.



For years I've been taking photos inside buildings, with a deliberately underexposed setting to minimise camera shake.



I've just bought an A200. I spent a whole week doing nothing but taking photos of buildings, of which at least 1000 (literally) were taken indoors - all deliberately underexposed by 1.0, with the dial set to P, so that the camera worked everything else out.



About 980 shots were OK, just as I'd expect: a bit dark of course, but I could use Photoshop to turn them into acceptable images.



About 20 shots were not like that. They came out of the camera virtually black. This surprised me because when I took them (looking through the viewfinder, not the screen) I had no reason to think they would turn out darker than any of the others.



When, out of curiosity, I tried to lighten up these near-black images by applying an enormous percentage in Photoshop I found to my great surprise that in about 5 cases I got a fine image that I'm very happy to keep: nicely exposed, nicely contrasted, nicely focussed. In the other 15 cases, the result was still utter garbage.



When I examined the EXIF data of the good images, I found in one case exposure time 1/20s, f3.2, ISO50. In another case exposure time 1/50s, f3.2, ISO50!



So the essence of my problem is: why should the camera just occasionally choose such crazy settings in a low-light situation?
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Old Apr 2, 2006, 2:31 PM   #7
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Something probably changed in the images (something brighter in the images with faster shutter speeds and exposed to protect the highlights).

What metering mode are you using? If you're using something like spot, you're going to need to be careful what you're metering on.

In any even, I would set ISO speeds higher versus deliberately underexposing, since underexposing can lead to loss of dynamic range.

http://www.luminous-landscape.com/tu...se-right.shtml

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Old Apr 2, 2006, 2:47 PM   #8
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JimC wrote:
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Underexposing and pushing the image back up during post processing can also hurt dynamic range.
It does that when using JPEG... underexposing 1 EV effectively halves amount of light, and in JPEG every color has 256 possible brightness values so it shouldn't be hard to think what happens when amount of brightness values used is halved.
(RAW is other thing)

Underexposing and push processing would be rational if you can't increase sensitivity or aperture anymore and need to keep certain shutter speed.


But in this case there should be no reason to do that, just increase ISO to 200 which should give really good results after slight noise removing.

Even higher can work well:
http://forums.dpreview.com/forums/re...ssage=16708739
http://forums.dpreview.com/forums/re...ssage=17516952


While not anything like DSLRs Sony's 2/3" 8MP sensor used in A2 and A200 is capable to much better than most compacts and just released claimed "high ISO capable" small sensor ultrazooms.

I did little test some time ago and run Steve's ISO400 and ISO800 noise test shots quickly through Noise Ninja:
http://rapidshare.de/files/14515406/...essed.exe.html (self extracting RAR-archive)
I would say even that ISO800 shot is good for near A4 size prints... and for small prints there should be still some reserve for underexposing&push processing.

You can find originals from here
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Old Apr 2, 2006, 3:13 PM   #9
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Most Dynamic Range tests I've seen shooting in raw show leaning towards overexposure is better in raw, too.

Dave Etchells has started including some Dynamic Range tests using Imatest in his reviews (ditto for Phil Askey's reviews).

You can see DR results for a Konica Minolta Maxxum 5D here (it was better overexposing using ACR):

http://www.imaging-resource.com/PROD...M5DIMATEST.HTM

I don't overexpose with a KM 5D. But, I certainly wouldn't underexpose at ISO 50. I'd increase ISO speed instead, only underexposing if I ran out of ISO speed choices.

Edit/PS:

The Konica Minolta Maxxum 5D outperformed a Canon EOS-1Ds Mark II on DR tests shooting in raw the same way (overexposing with both cameras shooting raw and using ACR to convert).

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Old Apr 2, 2006, 3:21 PM   #10
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The question of increased-ISO v under-exposure wasn't my main concern in starting this thread. But I'm grateful to you both for raising it, and I'm going to do some experiments to study it.

The thing that really concerns me is that when I had the dial set to P and was using all the main default settings (except for underexposure) the camera seemed to do crazy things (eg choosing an ISO of 50 even though there wasn't much light) a few times for no apparent reason.

Two possibilities occur to me:

1 I accidentally set and unset some advanced options (perhaps involving metering) without ever knowing about it (eg by brushing against the control buttons), so that for a few shots the camera was set to a crazy combination of settings.

or

2 There is a bug in the logic of the camera which unpredictably affects a small proportion of shots in poor lighting conditions.

The first seems to me pretty unlikely, so I'm tending towards the second.
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