A question of sharpening: Translating PS to GIMP

[Donald]

I'll try to get to my point as succinctly as possible.

In Canon 70-200 L f/2.8 IS vs. 70-200 L f/4 IS, Colin referred again to Capture Sharpening and pointed us to his January 2009 posts at When/How to Best Sharpen a Digital Photograph.

Back then, just into the Forum, I didn't know enough to know what to ask. But now I know just enough to be dangerous!

So..... The question is one of translating scales/ranges.

Colin wrote "Canon recommend 250 - 300%, 0.3 Radius, and varying thresholds depending on the ISO (thus noise) of the image (I normally use zero)." What does that mean in GIMP?

250 - 300 of what? GIMP uses a scale of 0-10. So, if in PS 250-300 is set on a scale of 0 - 1000, I could start with 2.5 - 3 and see where that takes me.

GIMP Radius scale goes from 1 - 120. So, what does 0.3 represent on the PS scale for radius; 0 - ?.

Now, I'm not assuming that there will be a direct read-across in terms of point on the respective scales. But I think it will serve as a useful starting place.

And as Colin once again wrote previously, there's lots and lots of places to go to learn about PS etc. GIMP requires a bit more effort. But, hey ho, life's a challenge!

[Dave Humphries]

Hi 'dangerous' Donald,

On the % scale, I believe 300% is maximum in PS, however that's not to say PS max = GIMP max
EDIT: It is 500%, see posts below.

I typed "sharpening Photoshop GIMP" in google, it came up with these, which I haven't read in detail.

http://www.gimp.org/tutorials/Smart_Sharpening/ This is a Smart Sharpening tutorial which it says is based on Photoshop techniques.

That's it, spent ages (ok, more like 20 mins) looking at other results and they were useless.

Baling out for now as hungry,

[Jim B.]

Hi Donald,

I use PSE7 here's #s for sharpening,
0-500 for the Amount settings
0-250 for radius settings
Also has a slider for threshold,but I never have used it.

[Dave Humphries]

I use PSE7 here's #s for sharpening,
0-500 for the Amount settings
0-250 for radius settings
Also has a slider for threshold,but I never have used it.

I stand corrected, I just checked, my PSE6 is as Jim's 7.
The threshold goes from 0 - 255, not unreasonably since your are setting a level below which it will not sharpen and the levels are in 8 bits. Even when you are working on a 16 bit import, which I find odd!

Cheers,

[Dave Humphries]

Radius is in real pixels; so the incremental size should be the same regardless of s/w.

Somehow (oversampling?) PS and PSE can work in sub-pixel increments when applying the width of a correction.

However, if GIMP starts at 1, that implies GIMP won't be able to apply as subtle a sharpened edge as we commonly use in PS.

Or am I missing something?
Does anyone with both GIMP and say Elements want to try an experiment?

[Jim B.]

Hi Donald,

Don't know if you have this.
http://docs.gimp.org/2.6/en/
Photography on the Net has a Gimp forum also.

[Dave Humphries]

Good find Jim,

From there, I found this;

Amount
This slider and input boxes (0.00-5.00) allow you to set strength of sharpening.

Radius
The slider and input boxes (0.1-120) allow you to set how many pixels on either side of an edge will be affected by sharpening. High resolution images allow higher radius. It is better to always sharpen an image at its final resolution.

Threshold
This slider and input boxes (0-255) allow you to set the minimum difference in pixel values that indicates an edge where sharpen must be applied. So you can protect areas of smooth tonal transition from sharpening, and avoid creation of blemishes in face, sky or water surface.

So it seems GIMP can do sub-pixel sharpening!

As to conversion, I interpret the above as follows;
Amount 0.00 - 5.00 (GIMP) equates to 0 - 500% (PS), so just divide PS values by 100.
Radius and Threshold are both the same units and directly comparable (but with different maxima).

Anyone disagree?

[Donald]

Gentlemen - you're magnificent.

I had noticed that docs.gimp.org is actually not totally up-to-date. Latest versions of the GIMP have a 0 - 10 scale for amount. But I think I can work out the maths given Dave's last post above.

The selective sharpening tutorial on www.gimp.org is one I have gone back to a number of times and I reached a point of feeling that I had a good understanding and reasonable command of sharpening theory and practice. But this now, hopefully, takes it all up a few gears.

Thanks again.

[Dave Humphries]

Latest versions of the GIMP have a 0 - 10 scale for amount. But I think I can work out the maths given Dave's last post above.

Gulp, I hope I'm right

[Donald]

Further to my post a few days ago, resulting from Colin's posts about Capture and Content Sharpening, asking about scales in PS and translating these for the GIMP, fellow users might find this a useful starter.

After much work with a calculator and the application of that well tested scientific method called trial-and-error, I find the equivalent figures (for Wavelet Sharpen) are:

Capture
Amount: 3 - 4 Radius: 0.01 (the %equivalent amount would suggest 6, but this seems too high)
Even an amount of 3-4 is getting to the high end on some shots. Might be better at between 2 and 3
Content
Amount: 0.8 Radius: 0.01 (can't get radius any lower, but it seems okay)
LCE
Amount: 30 -100 Radius: 0.3 (Figures from a post by David)

If this helps - good. If not - oh well !!