I am looking for some guidance please on how to use deconvolution sharpening as a replacement for capture sharpening in Lightroom or ACR.
I am mostly OK with Photoshop techniques and I have read various threads on deconvolution sharpening but I am puzzled by the following and would value any insight people can share.
1) When to use a plugin such as Focus Magic in the workflow.
I understand that the capture sharpening needs to be done early in the workflow. I also have read that things like luminance noise reduction and vertical upright correction should be done after deconvolution sharpening. Normally I would do all these in Lightroom. If I need a round trip to Photoshop to run a plug-in what is the recommended workflow to tackle ACR corrections, deconvolution sharpening, luminance noise reduction and converging vertical corrections?
Yes deconvolution is best done early in the workflow, because it avoids additional alterations of the original source data. To better understand that, it's pehaps a good idea to recap what deconvolution does.
Deconvolution is a mathematical attempt to reverse a so-called convolution operation. A convolution is a mathematical description of how the (in our case) energy of point light source or a point from the surface of our subjects is also spread over several neighboring pixels instead of a single one. The model used to describe that convolution is called a Point Spread Function (PSF). Using the closest approximation of that PSF model, a deconvolution pulls that spread out energy back to the source position by subtracting it from the surrounding pixels and adding it to the central pixel. This is then repeated for all pixel positions.
A significant problem is that the signal levels in the central pixel under investigation, and the neighboring pixels, do not only come from the original subject's brightness at a given exposure time but also contain a lot of noise. In fact, light itself is noisy due to the random arrival times of the photons, and the camera electronics also add (a different type of) noise into the mix. That makes it harder to determine exactly how much signal energy to get back from neighboring pixels while leaving the noise mostly unaffected.
So, any alteration of the original pixels will change the somewhat predictable noise distributions and make it harder to only deconvolve the signal
and not the noise that was inherent or added to that pixel but not blurred. Noise reduction is often not a linear operation but it adapts to the image contents and attempts to preserve detail. Distortion corrections need to resample the image data, and resampling introduces its own blur (in a non uniform way!). This all needlessly complicates, or even prevents, successful deconvolution.
Even tone-curve adjustments or a gamma curve complicates the restoration of the original signal component, because it disturbs the RGB balance. That's why a linear gamma data set is preferred for accuracy and allows to use relatively simpler calculations.
2) Preparation steps
I have read that it is better to upscale an image and convert to a linear gamma before running a deconvolution plug-in. What is the recommended way to do these tasks? I have no idea on how to convert to linear gamma and back to pro-photo gamma 1.8.
For the better quality deconvolution plugins, that's usually not necessary. Who knows, they may already do some of that under-the-hood. Resampling only the scale of the image will usually introduce artifacts of its own, and we do not want to exacerbate those. However, if done with care, there might be some very slight benefits to being able to use a more accurate PSF model that can improve sub-pixel accuracy. But for most common situations, I'd let the plugins do their thing of the as-original-as-possible image data, which is hard enough given the amount of tonal adjustments that are applied by default during Raw conversion.
3) Which plug-in to use?
I have the Topaz suite so have Topaz InFocus. I am happy to consider an alternative plug-in. What is the preferred plug-in for carrying out capture deconvolution sharpening? I have heard of Focus Magic and Piccure +, are these worth the extra cash compared to InFocus?
Which plugin to use, is a bit of a moving target as new versions are introduced, and with per image effectiveness. But given that you already have Topaz Labs InFocus, you can already do a lot to improve the resolution of your images, even with the drawback of partially altered image data.
The benefit of FocusMagic is that it's somewhat less prone to generating artifacts and it makes a good separation between signal and noise, but with proper care, they can also be mostly avoided with 'InFocus'. Piccure+ is a bit of a different animal, but I'm not confident that it doesn't also reduce image gamut. How Color Management is affected by Piccure+ is not clear to me.
To get the most from your Topaz InFocus, I recommend trying the 'Unknown/Estimate' blur type with a blur radius of 2. That will often get you in the ball park, if you make sure to first zoom in on the sharpest part of the image.
If you want to use the 'Generic' or 'Out of Focus' blur types, it can help to find the proper Radius setting by first maxing-out the Micro Contrast and Sharpness sliders with a Sharpness radius set to 0.8. This will grossly exaggerate the sharpness of the deconvolution, but it will make it easier to see when artifacts due to too large a 'Blur radius' setting are created. I'd start with a 'Blur radius' of 0.5 and slowly increase it until halos and ringing artifacts start to appear, then back off a bit and further mitigate those remaining artifacts with the 'Suppress Artifacts' control. Then set the "Sharpen" amounts back to zero and slowly increase those to your liking.