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A better camera calibration profile for stacking (RAWs)

 
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Beatsy



Joined: 05 Jul 2013
Posts: 1391
Location: Malvern, UK

PostPosted: Thu Sep 20, 2018 10:28 am    Post subject: A better camera calibration profile for stacking (RAWs) Reply with quote

After getting a new Eizo monitor and taking on a product photography project requiring "spot on" colours, I delved quite deeply into colour calibration. During this, I stumbled onto the mechanics of camera calibration as applied to RAW files by the likes of Lightroom and (in my case) Capture One. This was a major revelation as I finally understood why I struggled to get good tonal balance for insects with very bright features amongst dark surroundings. Until then, I basically exposed to the left (underexposed) to protect the highlights and allow leeway for PMax stacking (which tends to pick brighter tones). The downside of this it that it robs dynamic range from the final image - even with RAW files.

In short, the default profiles applied to convert RAW files use a curve that boosts the mid tones at the expense of crushing the highlights. That's the opposite of what I need for controlling and adjusting highlights in insect stacks.

The solution is to apply a "linear profile" on RAW conversion instead (straightforward in Capture One, requires a bit of initial setup in Lightroom). As soon as you do this, the histogram leaps a couple of stops to the left leaving far more headroom for highlights. Now I can (correctly) expose to the right and still get perfectly controllable highlights in the output (i.e. highlights retain their full captured tonal range instead of being compressed into a smaller range after conversion).

I was going to write up my wonderful discovery - but as is often (always) the case, I've only re-discovered something the whole world apparently knew about already. Oh well - I'm still pleased I discovered it. This article describes what I'm talking about quite well and gives instructions for setting up Lightroom for a linear camera calibration profile too

https://trentsizemore.com/blog/using-a-linear-camera-calibration-profile/

Here's a couple of quick stacks I did today (just for looking, not picture-making). Far more "bright and airy" than I used to end up with! Even the strongest highlights are controlled and not blown. Just goes to show, you never stop learning - especially if you're as late to the realisation as I was here Smile


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Lou Jost



Joined: 04 Sep 2015
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Location: Ecuador

PostPosted: Thu Sep 20, 2018 10:51 am    Post subject: Reply with quote

Very interesting. It seems that in Adobe Photoshop the default tone curve is linear when developing a RAW file. Does this mean the issue you mention doesn't arise in Photoshop?
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Beatsy



Joined: 05 Jul 2013
Posts: 1391
Location: Malvern, UK

PostPosted: Thu Sep 20, 2018 11:39 am    Post subject: Reply with quote

Lou Jost wrote:
Very interesting. It seems that in Adobe Photoshop the default tone curve is linear when developing a RAW file. Does this mean the issue you mention doesn't arise in Photoshop?


No Lou, that's an adjustment applied after raw conversion. I'm talking about the camera calibration profile during conversion.
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Lou Jost



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PostPosted: Thu Sep 20, 2018 11:51 am    Post subject: Reply with quote

Steve, it is a choice made in the RAW conversion dialog, before conversion.
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RobertOToole



Joined: 17 Jan 2013
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Location: United States

PostPosted: Thu Sep 20, 2018 2:20 pm    Post subject: Reply with quote

Steve have you considered just setting a custom calibration profile, its really easy:

http://xritephoto.com/colorchecker-passport-photo


Also FYI, for Adobe users at least, I am pretty sure there are still tonal changes being applied even with a linear profile and also even when you have the controls zero'd out believe it or not.

Very good information here:

https://www.rawdigger.com/howtouse/overriding-raw-converter-default-adjustments-settings

And this is an excellent source for info on the exposure comp control in Adobe ACR/LR:

https://www.rawdigger.com/howtouse/deriving-hidden-ble-compensation
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Beatsy



Joined: 05 Jul 2013
Posts: 1391
Location: Malvern, UK

PostPosted: Thu Sep 20, 2018 4:15 pm    Post subject: Reply with quote

Lou Jost wrote:
Steve, it is a choice made in the RAW conversion dialog, before conversion.


It's a long time since I used ACR directly Lou, I still think it's not quite the same. If selecting that linear profile makes the histogram jump to the left (a lot), then perhaps it is. If it doesn't - then it's not.
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Lou Jost



Joined: 04 Sep 2015
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PostPosted: Thu Sep 20, 2018 4:21 pm    Post subject: Reply with quote

Yes, if I start with a curve like the one in the article you linked to, and then switch to linear, the histogram moves to the left. So I think it is the same, though I don't know what pre-processing goes on before this. I need to read Robert's links.
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Beatsy



Joined: 05 Jul 2013
Posts: 1391
Location: Malvern, UK

PostPosted: Thu Sep 20, 2018 4:22 pm    Post subject: Reply with quote

RobertOToole wrote:
Steve have you considered just setting a custom calibration profile, its really easy:

http://xritephoto.com/colorchecker-passport-photo

Thanks Robert. Yes, it was because of setting custom profiles that I discovered this.

Quote:
Also FYI, for Adobe users at least, I am pretty sure there are still tonal changes being applied even with a linear profile and also even when you have the controls zero'd out believe it or not.

Very good information here:

https://www.rawdigger.com/howtouse/overriding-raw-converter-default-adjustments-settings

And this is an excellent source for info on the exposure comp control in Adobe ACR/LR:

https://www.rawdigger.com/howtouse/deriving-hidden-ble-compensation


Good references - thanks for the links! That was my main point! Adobe (and others) are applying curves to convert RAWs (called camera calibration curves) and they all rob dynamic range at the top end. That's where you need to inject the linear curve to get "proper" RAW output to work with.

Edit: admittedly only skim read this, but I think it's apposite...
https://www.photo.net/discuss/threads/linear-processing-of-raw-image-files.346798/
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cactuspic



Joined: 26 Dec 2006
Posts: 432
Location: Dallas, TX

PostPosted: Fri Sep 28, 2018 9:03 am    Post subject: Reply with quote

I am a bit confused. I presently shoot with a camera profile based upon my camera, lighting, and lens derived by using a Passport Color Checker. I noticed that the Passport has several gray as well as white and black pigment chips. Is the resultant color profile linear? If not is is possible to combine the color balanced profile with a more linear profile?
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