Image Sharpness

The concept of “Sharpness” in photography consists of two aspects. One is purely objective the other is somewhat subjective and relies on human interpretation of an image. The purely objective one is “Resolution”, it is measured in lines per mm. How many lines per mm can be distinguished before they blend together. The other is “Acutance” and is about distinguishing between edges. Increasing “Acutance” is what is typically done during digital enhancement. The process is aptly called “Sharpening”

May 20, 2017 – We went out for a morning drive through rural country along the Jumping Pound Road, west of Calgary and north of the Trans Canada Highway. Along the way we saw this blue heron at the edge of a small lake.

Original format RAW – 400 mm lens 1/250 sec, f/11, ISO 100.
Converted to JPG and resized to 1600 pixel wide
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The image you see here is somewhat processed by the editing software through the RAW to JPG conversion. One aspect of converting to JPG is that the color depth is significantly reduced. Typically professional photographers color adjust their pictures in RAW before converting them. The following screen shots are a more accurate representation of the post processing I have done to demonstrate the effect of sharpening.

Unprocessed Heron Detail – Zero Sharpening applied
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Notice how soft the edges are in this image. This is typical for RAW images. In this enlarged image you can just begin to notice the individual pixels of the sensor. Pay close attention to the trailing black head feather as I apply more sharpening. First I will apply maximum sharpening while holding the “detail” to zero.

Heron Detail – Maximum Sharpening Applied
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Here is a side by side comparison

Heron sharpnes – Side by Side comparison
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If you pay close attention to the trailing black feather you will notice a “halo” effect, a  slightly lighter tint than the blue water background. This is the digital technique used to give edges more contrast.

Another technique to enhance sharpness is to add “grain”.  This was well known in the black and white film era. Film emulsions with larger grain would appear more “sharp” than finer grained film emulsions. Below I have applied 50% detail. Again notice the “halo” effect around the trailing black feather.

Heron Detail – 50% “Detail” applied
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Here is the final comparison. If you look at the overall picture the right one appears sharper. If you look at it closely by enlarging until just the head and beak shows, you see the “flaws” (Use the “Ctrl” and “+” keys to enlarge ).

Original (left) and Sharpened (right)
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Of course these are subtle enhancements. Let’s take the sharpening to the extreme.

Maximum Sharpness, Radius and Detail applied
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This looks overdone when you consider the width of the halo around the trailing head feather. This is controlled by the size of the Radius slider. Also the simulated film grain is quite pronounced, this is controlled by the Detail slider. However when you look at the overall picture the result is still quite acceptable. Here is another comparison between the original and the extreme sharpening. Look at the trailing head feather, you can now see the halo quite well.

Extreme Sharpening comparison
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