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08-25-2011, 04:06 PM   #1
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Noise when underexposing, maybe

A comment I made in the no-more-tests-just-pictures thread has provoked several responses. That thread is probably the wrong place for the topic, so I'll resurrect it here.

My comment was "Avoid underexposure!!! as it raises noise significantly, even in the K5", and the first response was "That's not true. I always underexpose by 1 EV. You can gain 5 EV without noise problems".

Firstly, I responded to a member's question about high ISO IQ, so my comment pertained to noise when shooting at high ISO. Secondly, I should have been more specific and included "by several stops". I'm sure that a one stop underexposure would not contribute noise, but try underexposing by more than 2.5 stops and then have a look at the shadow areas. My own observations are that significant underexposure results in significant noise, even at ISO 800. More specifically, bringing up an underexposed image to acceptable levels in PP will result in more noise than would have been present in a properly exposed image. This, of course, becomes more apparent when sharpening is applied.

Having found that quite a few of my images show noise in dark areas, I did a bit of investigation, which turned up a fair bit of information on the relationship between underexposure and noise. Hence my original comment. An example can be seen here: K5_S2691.DNG - File Shared from Box.net - Free Online File Storage). Open the image in ACR, turn off noise reduction, raise ACR's Default Exposure setting by one stop, and have a look at the neck and chin area. Note that this was taken at ISO 800 - not exactly "high ISO".

Having said all that, I find myself questioning my own conclusions. On the one hand, the image in the above link shows noise even in non-dark areas, e.g. the orange vest, and its owner's arms. On the other, I certainly have higher ISO images that show less noise than this particular 800 ISO example. Can anyone here can explain this inconsistency? If the noise is not a result of underexposure, what is causing it?

Thanks for any input.

08-25-2011, 05:28 PM   #2
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At what magnification am I supposed to be looking at this photo? Because at anything less than 200%, I'm not seeing anything resembling noise. At That point, yes, it can do with a little NR, Sharpening, as well as a little fill light.

08-25-2011, 05:58 PM   #3
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The pull-up from under exposure applies mainly to low ISO (after all, this is an APS-C-sized sensor, not FF or MF). At ISO800, the camera has applied 3 stops of analogue amplification. By the time you apply 1 stop of exposure correction on top of that, you are looking at the read noise (in the shadows) and photonic noise (in the high-lights) in a image equiv. to shooting at ISO1600, or ISO4500 if you apply 2.5 stops of EV boost (although without any non-defeatable on-sensor NR applied since you have not exceeded an ISO1600 camera setting). The K-5 sensor is good (due mainly to the very low read noise), but not that good.

Dan.

Last edited by dosdan; 08-25-2011 at 06:39 PM.
08-25-2011, 06:38 PM   #4
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QuoteOriginally posted by JeffJS Quote
At what magnification am I supposed to be looking at this photo? Because at anything less than 200%, I'm not seeing anything resembling noise. At That point, yes, it can do with a little NR, Sharpening, as well as a little fill light.

Hi Jeff,

I assume you're not just viewing the embedded JPEG.

I agree that it takes 200% magnification to make the noise very obvious, but set ACR to Default, and raise Fill to, say, 30. The noise in the chin and neck area is visible even at 100%. Raising the Sharpening to 35 makes it clearer still. I'd say that I can even see the noise on Orange Man, who has no dark areas, and this I find rather puzzling.

It is not my intention to be argumentative. If what we're seeing in this image is par for the course, so be it. But I do have ISO 1600 images that appear cleaner than this 800 ISO example.

Bill

08-25-2011, 07:23 PM   #5
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QuoteOriginally posted by bxf Quote
Hi Jeff,

I assume you're not just viewing the embedded JPEG.

I agree that it takes 200% magnification to make the noise very obvious, but set ACR to Default, and raise Fill to, say, 30. The noise in the chin and neck area is visible even at 100%. Raising the Sharpening to 35 makes it clearer still. I'd say that I can even see the noise on Orange Man, who has no dark areas, and this I find rather puzzling.

It is not my intention to be argumentative. If what we're seeing in this image is par for the course, so be it. But I do have ISO 1600 images that appear cleaner than this 800 ISO example.

Bill
No, I didn't look at an embedded JPG. CS5 ACR which loads it without adjustments since I don't have your adjustment xmp file for ACR. All I'm saying is I had to go beyond 100% to see anything resembling noise. I'm not denying something is there, in the orange guy as well as the people behind him, just telling you what I see, on my computer, on my screen, in my CS5 ACR.

08-25-2011, 07:43 PM   #6
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QuoteOriginally posted by dosdan Quote
The pull-up from under exposure applies mainly to low ISO (after all, this is an APS-C-sized sensor, not FF or MF). At ISO800, the camera has applied 3 stops of analogue amplification. By the time you apply 1 stop of exposure correction on top of that, you are looking at the read noise (in the shadows) and photonic noise (in the high-lights) in a image equiv. to shooting at ISO1600, or ISO4500 if you apply 2.5 stops of EV boost (although without any non-defeatable on-sensor NR applied since you have not exceeded an ISO1600 camera setting). The K-5 sensor is good (due mainly to the very low read noise), but not that good.

Dan.
I understand the math, etc, though not sure about the first sentence. However, if I understand your point correctly, boosting in PP is equivalent to shooting at higher ISO. I believe someone has previously done a comparison of higher ISO vs pushing in PP, but I can't recall the results.
08-25-2011, 08:22 PM   #7
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QuoteOriginally posted by bxf Quote
if I understand your point correctly, boosting in PP is equivalent to shooting at higher ISO.
Yes, except the noise overall is a little lower if you shoot at say ISO1600, then if you shoot 4 stops under-exposed at ISO100 and then boost 4EV in PP. What you gain by shooting under-exposed at base ISO is much extra clipping headroom, which can be a big help in difficult metering situations e.g. dark scenes with bright lights in the frame.

See: https://www.pentaxforums.com/forums/pentax-k-5-forum/135603-isoless-sensor.html

Dan.

Last edited by dosdan; 08-25-2011 at 10:01 PM.
08-25-2011, 11:24 PM   #8
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QuoteOriginally posted by bxf Quote
More specifically, bringing up an underexposed image to acceptable levels in PP will result in more noise than would have been present in a properly exposed image. This, of course, becomes more apparent when sharpening is applied.
That has been true since the very first digital camera so it shouldn't really be a surprise for anyone.

08-25-2011, 11:40 PM - 1 Like   #9
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QuoteOriginally posted by Gimbal Quote
That has been true since the very first digital camera so it shouldn't really be a surprise for anyone.
With a camera that has very low read noise, there is little difference between exposing 1 stop underexposed at base ISO, say ISO100, and fully exposed at ISO200. With the K-5, there isn't much of a noise penalty at all.

Talk the same dimly lit scene and shot it the following two ways with the same shutter speed & aperture, varying only the ISO sensitivity:

1. ISO100 (4 stops under-exposed)

2. ISO1600 (fully-exposed)

The noise improvement from the 2nd/traditional method with the K-5 in this situation is only about 0.3 stops (from Guillermo Luijk's measurements) or 0.5-0.6 stops (Falk Lumo's measurements). So the 4-stops analogue gain before the ADC, when using ISO1600 in-camera, is not giving a very significant improvement in SNR, due to the inherently low read noise of this Sony sensor with its integrated-on-to-the-sensor-chip low-noise ADC, whereas in other cameras analogue boost before the ADC input is needed at low light levels to reduce the impact of high read noise.

But while analogue boost (i.e. increasing the ISO in-camera up to ISO1600 - above that it's just digital multiplication anyway) is not having much of an effect on SNR in the K-5, it is significantly reducing the clipping headroom at high ISOs.

Dan.

Last edited by dosdan; 08-26-2011 at 02:53 AM.
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