We Three Curves of A-C-R

11 November 2009

I was asked a question through Model Mayhem recently about curves settings in Lightroom as well as the impact of a linear vs. logarithmic conversion process. That got me playing with the conversion engine a bit more experimentally than normal and made some interesting observations. Most of these won't be new to the true photo geeks out there, but as they were to me... well, it was something to write about on a rainy Veteran's Day (btw, Happy VD guys).

So what am I talking about? Well, it's important to start by understanding the logarithmic nature of how we perceive light and dark vs. the linear way in which digital imaging captures it. Rather than rehash what's already been better written, I refer you to Bruce Fraser's excellent article over at Adobe on the subject. The gamma curve which he refers to is applied by the ACR engine as part of the camera profile, and looks like the image below in its default form. For the purposes of our discussion, this will be the Tone Curve. The next two curves in the processing chain are applied almost simultaneously, those being the Parametric and Point curves. It's important to realize that they act independent of one another, as well that while Lightroom and ACR are very much the same engine, they do not behave the same way here. In ACR, one can utilize the Point Curve just like a Curves adjustment layer in Photoshop, whereas in LR we are limited to the selections "Medium Contrast", "Strong Contrast", and "Linear." Indeed, sometimes ACR is better for conversion than LR. In any case, this affords you three levels of control over how your image ends up being presented to the user, and three opportunities to maximally extract quality detail over the image range.

The next question many folks will have is how to utilize that first Tone Curve to suit your purposes. This requires use of Adobe's DNG Profiles Editor. I will be honest with you that it's a bit cumbersome to work with, and requires opening of an image which has been converted to DNG in order to create a camera profile for that model of camera having the Tone Curve you desire. Details of its operation can be had from the Adobe page linked above, but if you have questions, please feel free to ask.

The photo at the top of this has almost nothing to do with linear vs. logarithmic conversion, but I thought was a nice demonstration of careful post work by Sarah. Milanya is a fantastic model, btw, and if you're in the DC / MD / VA area you should book her ASAP.

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