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Author Topic: Shooting to the right and raw conversion  (Read 33300 times)
KAP
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« Reply #80 on: July 26, 2007, 09:56:57 AM »
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Conventional wisdom is that for digital cameras it is best to expose so that the right side of the histogram is as far to the right as possible without blowing the highlights.  
However;  the raw converter I use allows exposure compensation of +/- 2 stops.
If I under expose does this mean I can bring out detail in the shadow areas or is this a false move.  Having preached the conventional wisdom I was asked this question during a talk and I could not answer.  Can anyone help.
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Within limits I think expose for the shadows and dev for the highlights works best for me.

Kevin.
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Ray
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« Reply #81 on: July 26, 2007, 06:16:54 PM »
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Conventional wisdom is that for digital cameras it is best to expose so that the right side of the histogram is as far to the right as possible without blowing the highlights.  
However;  the raw converter I use allows exposure compensation of +/- 2 stops.
If I under expose does this mean I can bring out detail in the shadow areas or is this a false move.  Having preached the conventional wisdom I was asked this question during a talk and I could not answer.  Can anyone help.
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Back to the beginning   .

I think you're asking whether shadow detail can be brought out, in an underexposed image, by using the +/- exposure sliders in ACR.

I think the answer is both yes and no. The appearance of the shadows can be lightened and more detail made visible by using the Exposure Compensation slider during conversion, just as it can with many other techniques in Photoshop, such as use of curves.

But as far as I know, there is no recovery of shadow detail in the way that a minus adjustment of the EC slider can bring out highlight detail.

In order to get the most shadow detail in the RAW conversion it's necessary to have the 'shadows' and 'contrast' sliders at zero in order to minimise shadow clipping, and of course convert into 16 bit. Having done that, it doesn't make much difference if the EC slider is at +1 or -4. All the shadow detail that's there will be converted in both cases, but the conversion with a -4 setting will need to be lightened by other methods, such as use of curves or the shadow/highlight tool.

(Notice I wrote it doesn't make much difference. It probably makes some difference as a result of quantization issues, but nothing outside the realm of extreme pixel peeping, that I can see.)

Exposing to the right is basically just a technique of making the most of the dynamic range of your camera. If the scene you are shooting is of low contrast, it's not such a big deal. If the scene has a greater dynamic range than that of your camera, then it's very important to correctly expose to the right in order to minimise shadow noise. The alternatives would be exposure bracketing on a tripod and digitally blending the different exposures.
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digitaldog
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« Reply #82 on: July 26, 2007, 06:28:43 PM »
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Exposing to the right is basically just a technique of making the most of the dynamic range of your camera. If the scene you are shooting is of low contrast, it's not such a big deal. If the scene has a greater dynamic range than that of your camera, then it's very important to correctly expose to the right in order to minimise shadow noise. The alternatives would be exposure bracketing on a tripod and digitally blending the different exposures.
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My take is this. Expose to the right places the most actual data in the last stop as the camera can record instead of noise. I'm not sure how that equates to dynamic range which I would have to believe is fixed.

If you have a camera that can record 6 stops from shadow to highlight and you have a scene that's less than 6 stops, this is pretty easy to capture, however, you still want to place as many levels in the last stop as possible. So with a 12 bit file, that means 64 levels can possibly be utilized for data. If you under expose, you'll get less than 64 levels and if you go way too far, you'll lose shadow detail of course (and get muddy highlights you'll have to adjust).

IF you have a 6 and a half stop range, well you better decide what half a stop you're not going to capture on one end or the other. Or play with fill or other lighting techniques to adjust the scene dynamic range. If shadow detail is key, you're going to lose ˝ stop of highlight detail; you can't fit the entire range, the sensor can't handle it. If you go for the highlights, you lose half a stop of shadow detail. But in either case, you want to expose to get as much data into that last stop, which means not under exposing or all the levels get shifted into the wrong direction.

Does this sound reasonable?
« Last Edit: July 26, 2007, 06:30:09 PM by digitaldog » Logged

Andrew Rodney
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Ray
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« Reply #83 on: July 26, 2007, 06:56:32 PM »
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I'm not sure how that equates to dynamic range which I would have to believe is fixed.........


.......Does this sound reasonable?
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Sure does. I was just trying to simplify matters. If your camera has a dynamic range of 7 stops and you give 2 stops less exposure than you could have given (without blowing highlights), then it's equivalent to using a lesser camera with a lower dynamic range of 5 stops in which the shot has been correctly exposed to the right.

Dynamic range limitations are often a major concern with digital cameras. If you don't expose to the right, you are not making full use of the dynamic range capability of your camera.
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bjanes
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« Reply #84 on: July 28, 2007, 08:40:50 AM »
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Do you know what a sigma of 7 or 8 electrons means?  It doesn't mean that most counts are right, and occasionally one will be 3.5 - 4 electrons off.  If the average signal is 15 electrons, you will have many pixels clipping at zero, and going as far off as 40 or 50 electrons worth of analog signal.  It's a big mess.

Yes, I do know what a standard deviation is, even though you seem to think you are the only one who understands anything. First of all, your method of determining the read noise is non-standard. The standard method used by Roger and also illustrated at RIT is to subtract the dark frame from a bias frame, determine the SD, and divide the result by 1.414. This eliminates any clipping at black and your crude corrections for clipping are not needed.

For example, with the D200, here is a dark frame of the green channel at ISO 1600 as produced by Iris:

[attachment=2897:attachment]

And here is the result of adding 200 to one dark frame (the bias) and then subtracting a second dark frame. Note that one gets a smooth symmetrical bell shaped curve as one should for a normal distribution.

[attachment=2898:attachment]

Note that in both cases the standard deviation is 22.63. In the second case, the standard deviation of the subtraction is 32.304 and dividing by 1.414 gives 22.84.

In the first case, the mean is 20.469 and the standard deviation is 23.881; the usual 95 confidence interval is the mean ± 2 SD or 20.369 ± 22.84 * 2. This means that 95% of all determinations will be within this interval. I did not determine the camera gain, but using Roger's data, it is 0.5 electrons/ADU. Therefore, the SD expressed in electrons would be 11 electrons, which is close to the value Roger obtained in his analysis. 20.369 ± 22.84 * 2 gives negative counts, which is an impossibility, and results in "clipping". However with the bias frame subtraction frame one has all positive numbers. Your fudge factor does not seem necessary.
« Last Edit: July 31, 2007, 06:56:40 AM by bjanes » Logged
bjanes
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« Reply #85 on: July 28, 2007, 08:46:08 AM »
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What is the origin of this number?
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See my [a href=\"http://luminous-landscape.com/forum/index.php?showtopic=17706&view=findpost&p=130247]Post[/url] for comments on this fudge factor.
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