Sharpening

Sharpening can be divided into two classes of operation.  One is to correct losses of sharpness due to processing and the other is to optimise the appearance of the image for the medium where it will be displayed.

Oversharpening can introduce artefacts into the image like halos around areas of contrast.

In this exercise I will examine the differences in sharpening for print and for on screen viewing.

To do this I have to take an image that I have shot with some edge detail and some smooth areas like a portrait.  In a portrait eyes have detail and the skin areas are soft, smooth areas.

My reference picture is as follows:

Reference image

No sharpening has been applied.   

I will make three versions of he above image with varying degrees of sharpening.  To determine the levels of sharpening I will be using Scott Kelby's The Adobe Photoshop CS5 book for Digital Photographers.  

To sharpen images in Photoshop you use the Unsharp mask feature in the Filters menu.

The Amount slider determines the amount of sharpness applied to the photo.

Radius determines how many pixels out from the edge the sharpness will affect.

Threshold determines how different a pixel must be from the surrounding area before its considered an edge pixel and sharpened by the filter.

It is important to note that the lower the threshold number the more intense the sharpening.

The recommended level of sharpening for a portrait.  
According to the book, the recommended level of sharpening for portraits is Amount: 75%, Radius: 2, Threshold: 3.  These settings provide a soft, subtle sharpening with enough punch to bring our the detail in the eyes.

Whilst viewing the image at 50% magnification I input these settings.

Maximum sharpening
For this image I used the maximum sharpening settings which are recommended for architecture and buildings.

Amount: 65%, Radius: 4, Threshold:3

Moderate sharpening

In this image I applied a moderate level of sharpening. 

Amount: 120%, Radius: 1, Threshold:3

Looking at these images on screen I don't see a dramatic difference between them.  The maximum sharpening looks the sharpest and I don't really like the fact that the hair and curves of the face are just as sharp as the eyes.  

The moderate level is slightly better with the skin having a softness to it.  

My preferred is the settings for portraits.  I feel that the eyes are adequately sharpened and the skin is soft.  

I will now arrange for my images to be printed so I can compare them in print.  Printed images require different levels of sharpening depending on the type of printer you are using and the paper you are printing on.  Based on the ideal print output sharpening can be calculated by looking at the normal viewing distance of the human eye resolving detail to around 1/100th of an inch.  This means that an image with a resolution of 300 pixels per inch will need a radius of 3.  

From my prints I have found that printed images can require a lot more sharpening that web outputs.  I have found that images that look distorted due to the level of sharpening on the screen look good printed.