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Author Topic: Exposing for the mid-tones??  (Read 1412 times)

Hening Bettermann

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Exposing for the mid-tones??
« on: March 25, 2017, 05:13:37 PM »

Hi!

When I watch the live histogram on my a7r2 and increase exposure, I see the histogram move to the right. However, before it touches the right side, it is being skewed to the right. This must mean that an in-camera software compresses the upper mid tones to prevent highlight clipping. As far as I can see, this in turn must mean less separation between tones in that range.
 
IF this is correct, then I will in the future typically expose for the "flattest" histogram rather than to the right, since I have no problems with noise. Well, maybe I will get them if I follow my idea...

Any thoughts?
 

Doug Gray

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Re: Exposing for the mid-tones??
« Reply #1 on: March 26, 2017, 01:12:00 PM »

Hi!

When I watch the live histogram on my a7r2 and increase exposure, I see the histogram move to the right. However, before it touches the right side, it is being skewed to the right. This must mean that an in-camera software compresses the upper mid tones to prevent highlight clipping. As far as I can see, this in turn must mean less separation between tones in that range.
 
IF this is correct, then I will in the future typically expose for the "flattest" histogram rather than to the right, since I have no problems with noise. Well, maybe I will get them if I follow my idea...

Any thoughts?

Your analysis is correct but I don't recommend your solution.

The tone curve is compressed. Partly to simulate the response of film and partly to raise the brightness in the midtones relative to "white." This process is a standard part of the conversion from camera sensors to a digital space known as "output referred."  It compares with "scene referred" which is used in photography for reproduction.

Said another way, if you, using normal photography, take a picture of a picture. Print it and put it side by side with the original, it will look quite different. The high end of the tone curve is compressed in the print.

If you use scene rendered photographic processes to make a colorimetric match of an original, then the print will closely match the original. This is a specialty because, except for repro work, the prints produced tend to look dull and less colorful than standard "output referred" prints.

That's how it works. Don't try to defeat it, you will not like the prints or you will wind up boosting the brightness. You also incur higher image noise.  If you use RAW files, none of this compression occurs in the camera. The camera imager operates linearly and the best exposure is where the highlights just come within a few percent of the sensor clipping point. RawDigger is a tool that can tell you exactly where this occurs.
« Last Edit: March 26, 2017, 01:21:32 PM by Doug Gray »
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Hening Bettermann

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Re: Exposing for the mid-tones??
« Reply #2 on: March 26, 2017, 06:46:17 PM »

Hi Doug!

Thank you for your comment. If you are right in that this compression does not occur in the camera, it means that the live histogram is based on an output referred in-camera jpeg. So I'll go back to exposing to the right...

Thanks again!

qwz

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Re: Exposing for the mid-tones??
« Reply #3 on: April 21, 2017, 06:37:26 AM »

Firstly, A7r-II and most other cameras shows histogram for JPEG (after gamma correction, white balancing and so on) even if it set to record RAW file.
Secondly, yes typical curve for JPEG  intented to conserve highlights and make a smooth roll-off like on film.
Thirdly, Sony's compressed ARW moves most tones to middle areas 'cause compression algorithm also includes a gamma-like tone-curve.

Is there an easy solution?
Nope. Only understand your particular camera limits and 'features'.


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