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05-13-2011, 02:41 PM   #1
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Highight correction

Can anyone explain how highlight correction and/or shadow compensation works?
Why would you have to use iso of 400 or higher (unless you overide it)?
Thanks in advance
Jake

05-13-2011, 02:55 PM   #2
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Well I don't know about highlight correction *per say, but I believe the general idea behind DR on Pentax sensors revolves entirely around the shadow recovery side of things. In fact, aside from a very specific sensor made by Fuji, I don't think I've ever seen any other camera make use of highlight recovery in part of DR recovery.

Anyways, I'm really not sure about your ISO400 or higher question. But for the most part, in shadow recovery, the lower the ISO the higher the dynamic range overhead will be(I think). Though I really haven't studied this in depth, and so I can only speak based on personal observations.

Anyways, as for how shadow compensation works, I'd have to ask if you're asking about how to make use of shadow compensation or... if you mean how such things work from a sensor design perspective?
05-13-2011, 02:59 PM   #3
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The way I understand it for highlight correction, the camera looks at any data that is at the top of the dynamic range and digitally expands the dynamic range of the top of the range such that highlights are more dynamic or you can recover some of the detail from overexposed images. Now, as you increase the sensitivity of the film/sensor beyond ISO 400, you lose some dynamic range, so the highlight correction is supposed to make up for that.

I suppose that shadow correction would work the same way, except at the bottom end of the range.

I got most of my information from reading dpreview on the K-x and other cameras when looking to buy a DSLR. For example, Pentax K-x Review: 17. Photographic tests (DR): Digital Photography Review
05-13-2011, 03:00 PM   #4
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thanks for the reply. I found that shots made with highlight correction will make the darker areas a little lighter (unless that was shadow correction). But I did see a difference. I was wondering if this"correction' degrades the actual image quality (as noise reduction can).
And the minimum iso with this feature on is 400 and higher (default), but you can overide that. i was wondering why?
Jake

05-13-2011, 03:04 PM   #5
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So far as I can tell in practice, it makes noise stand out more. I use it on my K-x, but it's very noticeable when I use highlight correction at ISO 200 on my K20D. I don't have lightroom, so I like to capture as to do as little noise reduction PP as possible.
05-13-2011, 03:26 PM   #6
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It generally reduces contrast, which is as a result of a reduction in the dynamic range of the sensor to make the compensation possible. This can be considered a reduction in quality, but not necessarily so (depends on the contrast of the original scene being photographed).

The sensor uses a one stop bias in order to do this and therefore has to lose one stop of the low ISO range for it to work as designed. I'm assuming you're using a K-x not in expanded ISO range mode, and thus the minimum ISO using highlight correction will be 400.
05-13-2011, 03:48 PM   #7
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Actually Ash,
That's exactly what I did I am using the k-x and the way I overode the 400 iso is in the expanded iso range.
Is there a reason not to do this?
Thanks!
05-14-2011, 04:30 AM   #8
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To understand the problem it helps to look at a dynamic range graph.
Below is the dynamic range graph that I measured on my K-7 from a series of RAW photos. (how I did this will be published in another post)
This is not the same as on your camera but the principle is the same.

One can see that the graph shows an extended toe. This means that the sensor can record fine gradations of near black.

But the opposite is true at the shoulder of the graph. Here there is an abrupt transition to saturated pixels. This means that the sensor cannot record fine gradations of nearly white. If you want to do that you must slightly underexpose the highlights so that they do not lie on the shoulder of the graph.

I presume this is what highlight correction does. There is no magic involved, the camera can't change the properties of the sensor, as seen on my graph.

When I need to preserve highlights accurately I prefer to spot meter on a chosen highlight and enter a corresponding exposure compensation, according to what brightness level you want to represent that highlight. The second illustration below shows the principle.






05-14-2011, 06:46 PM   #9
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Thank You Labnut!
I understood the first illustration, but and understood your explanation about the second as well. However I don't see how you brought this out in the picture.
Regardless, I don't understand why the iso setting is limited to the 400 & higher.
Also based on your opinion, I presume that shadow correction works the same way, but on the darker side of the spectrum, if so, it seems that you would not have the need to correct as much, as in your graph illiterates, the sensor is capable of recording fine gradations of near black.
Thanks again! Jake
05-14-2011, 09:36 PM   #10
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FrumPilot - there is no ISO setting limited to 400. The highlight correction will require ISO 200 and shadow compensation can work at ISO 100.
05-15-2011, 09:07 AM   #11
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Laurentiu, I beg to differ, (at least in the k-x)
When highlight correction is engaged, you can not set the iso lower than 400. When didengaged you can set it to 200. The exception is when used in expanded iso range.
05-16-2011, 03:36 AM   #12
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QuoteOriginally posted by FrumPilot Quote
Thank You Labnut!
Regardless, I don't understand why the iso setting is limited to the 400 & higher.
That's because highlight correction works by secretly using an ISO one stop lower than what you set in the user interface. In effect it underexposes one stop and lifts everything except the highlights back up in the JPG generation.

The downside of highlight correction is one stop higher noise. That is a good tradeoff in plenty of light (low iso), but typically not in low light.

Sincerely,
--Anders.
05-16-2011, 01:51 PM   #13
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Thank you asp!
05-16-2011, 07:25 PM   #14
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QuoteOriginally posted by FrumPilot Quote
Laurentiu, I beg to differ, (at least in the k-x)
When highlight correction is engaged, you can not set the iso lower than 400. When didengaged you can set it to 200. The exception is when used in expanded iso range.
That's because your lowest ISO is probably set to 200. If you enable 100 (option C1.3 - Expanded Sensitivity), then you'll be able to shoot with highlight correction at ISO 200.
05-17-2011, 01:27 PM   #15
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Agreed!
But I still wondered why highlight correction forced the higher iso, & ASP answered my question.
Jake
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