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07-24-2012, 04:24 AM   #1
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Shadow and highlight correction

Hallo!

I have Pentax k-x for more than a year. The camera has such features as shadow correction (high, medium or low) and highlight correction. I would like to know more about these features: how exactly they work, when they are useful, how to use them properly, ect. Unfortunately, there is very little information about these features in userís manual.
I have tried shadow correction. In some situations this feature (high correction option) seems useful as it reduces dark shadows. I presume that shadow correction (and also highlight correction) operates at the expense of something.

I highly appreciate any advice and information about these fetures.

Best regards,
Alberts

07-24-2012, 05:02 AM   #2
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Highlight correction darkens the parts of images that are extremely bright, shadow correction lightens the parts that are dark. It's like editing the image and increasing/decreasing exposure, only here it's done to parts of the image.

Highlight correction doesn't really have any bad points, but shadow correction can create noise in the darker parts of images. How much depends on the settings and ISO value. With higher ISOs, every change to images can introduce more noise, at low ISOs this won't be as bad. If you set the sha. corr. to 'high', it will introduce more noise and possibly artifacts than the 'low' mode. Are you shooting JPEG?
07-24-2012, 05:42 AM   #3
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A try for a most simple explanation: Each pixel of the sensor responds to light by providing a certain voltage. This will be amplified and digitized. These values are mapped to luminosity values of the pixels of the picture. The mapping of the sensor values to pixel values of the picture is fixed in the firmware.

The camera can:
1) Modify the amplification globally by "faking" ISO (showing ISO 200 to the user, but internally using ISO100 or 400)
2) Modify the parameters of the digitalisation (maybe value dependent)
3) Modify the standard mapping in RAW after digitalisation (the form of the gradation curve will change), as usually it is done at conversation to JPEG
There may be more possibilities, and a camera may use a combination of the above.

As an example, the K200D (which offers highlight correction only) is faking the internally used ISO by 1 step without showing the user, then tries to compensate for now missing details in shadows by modifying the gradation curve. This can lead to slightly increased noise in darker areas, max. as if one higher ISO step was used.

As all the mapping is artificial anyway, there isn't a disadvantage as such (the result decides about improvement).
Whether the resulting image looks more or less natural than without shadow/highlight correction, cannot be answered universally. At least highlight clipping usually doesn't look nice, that's why Pentax offers a (mild) compensation for this also in the K200D.

Dependent on the kind of correction and the situation, the price you may pay can be slightly increased noise (1 ISO step). Preventing of highlight clipping and/or showing more details in dark areas may lead to slightly less contrast in the middle range. The manipulations do not expand the possible total amount of dynamic range! In some cases the effect also may vastly differ between computer monitor and print.

The Pentax manuals I saw do not make really clear which of these manipulations change the JPEG conversion only, and which are affecting also RAW.

Last edited by RKKS08; 07-24-2012 at 06:08 AM.
07-24-2012, 02:29 PM   #4
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Perhaps I will explain things a little clearer, using the histogram as an example.

When you shoot JPEG, the histogram has a range from 0-255, which is divided into roughlyn3 segments. The middle, from about 25-230 is roughly linear, with each count of 45 greyscale (all settings neutral) being one stop. Below 25 or above 230 the next stop has 15 greyscale, the next 7 and the next 3.

Using shadow protection adds about 1-1.5 additional stops into the range between 25 and 128 but does not impact above 128. Adding highlight protection (at maximum) a similar compression between 128 and 230. There is additional compression above 230 and below 25 but I have not measured how much.

The net result is you give up a little on the incremental changes in the dynamic range for a broader range by about 3 stops (with both active)

It is super contrast reduction, because going from max contrast to minimum contrast only adds about 1/2 a stop on either side of 128 as opposed to 1 1/2 stops on either side

07-25-2012, 12:16 AM   #5
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Thanks for answers very much! I almost always shoot JPEG (I have shot RAW only experimentally). I have noticed that images have quite dark and distinct shadows in sunny days when I shoot normally (without exposure corrections and other corrections). In such situations I have tried correcting exposure (overexposing). However, it rarely gives acceptable result as it usually results in too bright sky and so on. So I have given up this option. Whereas, shadow correction seems to work better as the darker parts become more detailed but brighter parts remain normally bright (not become too bright). According to my quite short practical experience with Pentax kx, it seems that strong shadow correction works better than week or medium. Perhaps, I have not tried these two options of shadow correction sufficiently.
If I correctly understand, shadow correction (also highlight correction) works like post processing (PP) when the histogram is affected selectively. And the major disadvantage is risk of higher noise (especially if high ISO values are used). As I hardly use PP, shadow correction could be good option for me in sunny days when dark shadows in an image become real problem. Perhaps, fill-in flash is better option but is not applicable in landscape and similar shooting.

The reason for starting this discussion about shadow/highlight correction is the problem with dark shadows when shooting in sunny days. May be there are other ways how to deal with this problem? I know only exposure correction, fill-in flash and shadow correction. There is also an option to use HDR mode. However, I do not take it seriously and it is very complicate to use it with Pentax kx (it is not possible to use timer in HDR mode).

Best regards,
Alberts
07-25-2012, 12:22 AM   #6
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QuoteOriginally posted by Alberts Quote
The reason for starting this discussion about shadow/highlight correction is the problem with dark shadows when shooting in sunny days. May be there are other ways how to deal with this problem? I know only exposure correction, fill-in flash and shadow correction. There is also an option to use HDR mode. However, I do not take it seriously and it is very complicate to use it with Pentax kx (it is not possible to use timer in HDR mode).
Think about shooting in RAW more often, especially when lighting is harsh. You will get better results manipulating shadows and highlights in post processing than you will by fiddling around with in-camera settings while shooting.
07-25-2012, 04:40 AM   #7
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I am not a fun of PP. And the only PP software I have is Pentax Utilities. I am not very strong in it. This software also eats quite a lot of computer resources. So prefer to run it occasionally not regularly.
I prefer shooting in JPEG as it allows taking many photo in short time (if necessary) and consumes less memory of the card. So I am interested in solution that allows shooting in JPEG and does not require PP. I understand that JPG offers less than RAW. However, the quality of JPG usually satisfies me. At the moment I am not ready to shoot every photograph in RAW. I reserve RAW for special cases.

Alberts

P.S. Perhaps, it is subject of different discussion. Pentax KX offers two RAW modes – Pentax (by default) and Adobe. At the moment, my camera uses Pentax RAW mode as I have not changed this default setting. Which mode is better?
07-25-2012, 04:52 AM   #8
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Aren't we talking about those special cases?
Anyway, whatever. Take my advice or don't. I used to think in-camera jpegs were all I needed, and now I have concluded I was wrong.
As for which RAW format to use; don't take my advice - even Pentax has ditched their proprietary format. Choose DNF.

07-25-2012, 06:21 AM   #9
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You could bracket exposure and then combine shots on your PC with software, but since you don't want to do that and you don't want to shoot RAW, you only have 2 options: HDR or in-camera highlight/shadow correction.
07-25-2012, 08:04 AM   #10
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QuoteOriginally posted by Alberts Quote

The reason for starting this discussion about shadow/highlight correction is the problem with dark shadows when shooting in sunny days. May be there are other ways how to deal with this problem? I know only exposure correction, fill-in flash and shadow correction. There is also an option to use HDR mode. However, I do not take it seriously and it is very complicate to use it with Pentax kx (it is not possible to use timer in HDR mode).
Alberts
Albert

there are many ways of dealing with shadow detail, but in general, even with highlight and shadow protection enabled, or if performing similar powt processing functions in RAW as some have suggested, the real issue is that the tonal range from bright sky to and white puffy clouds to the typical strong shadow that you can get is about 15 stops, and highlights begin to burn out after about 10 at best with the latest digital sensors, so no matter what you do, you have a problem. HDR, or graduated ND filters are the only way to go., although having said that, a good polarizing filter can at least save the sky.
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