Mastering Sharpness

What is Sharpness in Image Editing?

See the youtube video version of this at: Mastering Sharpness

Definition

Sharpness in terms of image editing (i.e. not optical sharpness in camera) is actually a process known as Acutance.  You can find the definition at the link, but essentially, it is the process of applying contrast at edges.  So how does PaintShop Pro determine edges?

Edge Detection

We can begin to understand this by using the Effects > Edge Effects > Find All tool.  Applying this effect on a number of different experimental images, what we find is that PSP detects edges based on contrast lines.  These contrast lines can differ by luminance or hue.  For each image, the left side is the original, and the right side is the effect of Find All.

 Luminance Contrast

Hue Contrast

Gradient Contrast 

Note for the gradient, that as the delta between pixels becomes less, the strength of the edge detect becomes less.  If we take a blurry image of a pineapple, what we see is that there are no detected edges where the detail should exist.

The most important idea to recognize from this experimentation is that acutance can't fix a blurry image, it can only enhance the perception of sharpness of an already optically sharp image.

What are the PaintShop Pro Sharpness Tools?

Understanding how acutance works, this is a summary of how the PaintShop Pro tools differ in applying acutance.  Note that the term "radius" indicates how far from the detected edge that the contrast is applied.

•  Sharpen - applies contrast at a fixed radius
•  Sharpen More - applies more contrast at a fixed radius
•  High Pass Sharpen - applies contrast at a variable radius
•  Unsharp Mask - applies contrast at a variable radius, but with greater strength

Optical Sharpness Techniques

Finally, since acutance can't fix an optically blurry images, here are some tips for taking sharper photos.  

The Lens

Here are some general tips for lenses:

• Better quality (read more expensive) lenses will produce sharper images
• The center of a lens is sharper than the edges (What is an MTF Chart?)
• For prime lenses, it is typical that 2 F-stops back from wide open is the sharpness "sweet spot"

Technique

Auto-focus on a camera can help you get sharp images if used intentionally.  First let's consider how auto-focus works.  For most digital cameras, there is a sensor in the light path that collects light for the sole purpose of detecting contrast.

The auto-focus will adjust the focus ring, and analyze how the pixels in the sensor change.  As they approach a solution that is of greater contrast, it presumes it is moving to sharper focus (this is why often it will overshoot and then back up).

Knowing this, it is important to control which focal point in the array is used.  For full-auto, the camera will decide, which will yield unpredictable results.  My suggestion is to force it to always use the center focal point. 

In this way the guess work of full-auto is removed.  Superficially, one could argue that this works out nicely only if your subject is always in the middle of the frame.  To overcome this, simply follow this approach:

   Aim Focal Point > Half Shutter > Reframe > Full Shutter

This way you lock in the focal point, then refarme to get the desired composition.  This technique can also be used for locking in exposure or white-balance as well.

One final note on this topic.  Since the auto-focus is trying to detect contrast to determine if the object is in focus, you want to have the focus point set on a point of detail and contrast.  Hence this is an example of a good aim for focusing.  Eyes are great for this.

This is an example of bad aiming for focus.  The camera will have a hard time detecting contrast when everything in its little view is the same color.