Auto contrast threshold and auto radius are per picture.
Auto iteration limitation is by 32x32 pixels. The image is processed in tiles of size 32x32 pixels and each tile gets its own auto iteration limitation
Here’s an example raw file (shot using a 4.0/200 mm lens at f4.0) where a value of 0.20 for corner boost improves sharpness in the outer regions.
This file is licensed Creative Commons, By-Attribution, Share-Alike.
D700_20120801_0313.NEF (15.0 MB)
Just found this 
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Good point. A default programmed by /u/heckflosse is probably much better than a default set by some shmuck in a dreary Adobe corporate office who doesn’t even do photography.
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I have a similar approach.
Most of my photos end up on a hard drive with a select few appearing on Instagram, so generally my editing comes down to moving things until I like what I see.
To have a starting point with CS and a few other tools in RT with their default settings just makes editing a little quicker and overall more enjoyable. 
My Canon cameras make use of anti-aliasing filters which require some additional sharpening be made to the raw images. I do mostly landscape photography which requires the image to be very sharp say in comparison to portraiture.
So I have been using the default auto Capture Sharpening together with some USM and a tiny bit of edge sharpening on my landscapes. Before Capture Sharpening I preferred to use the RL deconvolution method for sharpening on low ISO images, but with CS engaged RL deconvolution sharpening produces noticeable artifacts. The USM method remains clean with a radius of 50 and strength of 150.
The strategy produces landscapes which appear pleasingly sharper right into the corners viewed at 100% and especially with corner radius boost enabled at 15 for lenses which could use a little help toward the periphery.
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Welcome aboard!
Yeah, that’s what I love about Rawtherapee. It is just so powerful, and keeps on getting even more powerful as time goes on. The images I can get out of Rawtherapee are so great, and if I need to do more advanced pixel editing things, such as compositing, dodge and burn, and removing things, I can get a well detailed and richly rendered intermediary tiff out of Rawtherapee.
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I’ve just taken it out for a spin now, and after setting the radius wrong a few times, I suddenly found some settings that did wonders in terms of sharpening.
Picture settings: Nikon D600, 85mm F1.4 shot at F5.6, iso 100. Sunlight.
Contrast threshold: 5
Radius: 0.66
Corner radius boost: 0.00
Iterations: 20 (with auto limit)
And holy cow, what a result! I think I might stop using the normal “Sharpening”, even though I’ve always liked the quality of that. This new thing seems to beat it! Okay, it certainly uses more CPU, but who cares if the result is better. So far I’m very impressed.
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It uses more CPU in preview mode because it is applied on the whole image, not only on the preview , but you get some speed back when panning around in >= 1:1 view because it’s already applied. In final processing (Queue/SaveAs) it’s faster than the normal “RL-Sharpening”
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Yeah I did notice that when scrolling around, it didn’t feel it at all. It’s only when you enable it or adjust it, that it’s slow. So yeah, not an issue at all.
I did have a weird mishap when I tried batch changing the Radius setting on three images at once from the file browser, using the batch edit mode thingie. But I might have goofed up. I haven’t done systematic tests of that yet. So far I’m just playing around and getting to know it.
I edited my reply. Batch processing was not what I meant. I sometimes confuse Batch with Queue.
Ohh cool. I didn’t find it hard to see the effect of the threshold even without that preview, but that’s a cool feature for sure.
Uh-ohh: The batch edit problem is real, I’m afraid. Example: Take three images. Set the the Capture sharpening “Radius” to 0.5 on the first image using the “Editor” tab. Now go to the file browser and copy the settings to image 2 and 3. Now they all have Radius=0.5. Next step: Select all three images, use the “Batch edit” mode to change them all to 0.7. Now inspect each image. Did you actually get 0.7? I didn’t. (Recreated the problem multiple times already.)
Works fine here. Maybe I miss something…
The forum doesn’t support mp4, so…
www.robotplanet.dk/f/rawth/batch_editing_capsharp_radius.mp4
As you can see, I start by having them all at 0.5. Then I change them all to 0.72 at once. But once I inspect the results, they’re all suddenly 1.22 … a much higher value.
You’re right, I was in “Add” mode. But “Add” mode seems to work differently for the Capture sharpening parameters. If we compare it to, for example, “Highlight compression” on the first tab (basic exposure stuff). This slider goes from 0 to 500. But as soon as I multiselect several images, the slider changes into a mode where it offers negative values. This way, the “Add” mode can also subtract, which makes sense if you want to lower the value for multiple images. This does not happen for the Capture sharpening parameters, so it is impossible to lower their values in batch edit mode. This seems odd to me.
Probably a bug. I will have a look.
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Just as an addendum to my above post, I find with the benefit of further use that applying 150 in the USM amount field produces over sharpening even in landscapes in most cases. 50 or 75 is better.