Barrel & pincushion distortion

Lens technology has come a long way since the days of simple point and shoot cameras of a few years ago. It is now possible to correct many lens faults and color shifts because qualities of optical glass have improved considerably over the years.

It is a fact though that many compact digital cameras still suffer from barrel and pincushion distortion.

Blooming effect

When light passes the lens of a digital camera and is captured by the CCD it is converted into an electrical charge.
There is a limit to how much charge each pixel or photosite can store. If there is too much charge for one photosite it will overflow to its neighbouring pixel causing an effect which is called blooming or streaking.

Manufacturers try to eliminate this effect by “anti-blooming gates” which can be compared to vertical drainage channels running beside each row of photosites. These allow the overflowing charge to flow away without affecting surrounding pixels.

Though these anti-blooming gates are fairly successful at avoiding the problem, very extreme exposure situations can still lead to blooming. Especially where a very bright edge is next to a very dark edge, as seen with leaves or branches of a tree shot against a bright sky. It will be visible as a white halo or vertical streak which extends for several pixels.

The effects of blooming often make chromatic aberrations more visible. These are the purple lines along dark edges in an image, caused by the effect of blue light bending more than red light.

Chromatic abberation

If you have ever looked through cheap children's binoculars the effect of chromatic aberrations will be all too familiar to you.
This “purple fringing” can sometimes be found on digital cameras as well. To what extent it becomes visible is dependent on the sort of image and the presence of dark or light edges in it.
Different wavelengths of light have different focal lengths and chromatic aberrations develop because of the camera's lens inability to focus these different wavelengths of light onto exactly the same focal plane.

Often the effect of chromatic aberrations is amplified by what we call blooming – the overflow of charge from one pixel to its neighboring pixel on the camera's sensor.
Chromatic aberrations are reduced if special lens systems such as achromatic or apochromatic doublets are used. These use two or more pieces of glass with different reflective characteristics. Not even these are completely perfect however.

In your digital darkroom it is possible to reduce the saturation of the specific magenta hue that causes the "purple fringing". Although this doesn't completely solve the problem it certainly makes it less visible as it replaces the magenta with a shade of gray which is far less prominent when looking at the image.

Jaggies & noise

Enlarging a digital image or examining it from close by will reveal visible steps in diagonal lines in an image. Since pixels are square in layout every diagonal line has to be built up by square blocks of color stepped on top of one another. These “steps” are called jaggies and are usually visible on lines at slight angles against a contrasting background. The effect is more prominent in low resolution images since less pixels make up the image and consequently the “steps” to form diagonal lines will be larger.

Visibility of jaggies can be reduced by aliasing lines against their background so they look more like a smoother line. When applying unsharp mask however, they may reappear since sharpening is often based on selective contrast adjustment which removes aliasing.
Other forms of noise can be compared to grain or imperfections in conventional film. They are caused by electronic errors or intererference of the camera's CCD or CMOS sensor. Certain color channels are more affected by noise than others because digicam's sensors are more sensitive to certain primary colors than others.
Visibility of noise is often affected by ISO setting or temperature. Higher ISO speeds or temperatures amplify noise while long exposures can also introduce noise in an image. Since JPEG compression reacts badly to noisy images this can amplify noise even more.

JPEG quality levels

If you save images in JPEG format some original image information will be lost, due to how JPEG compresses images. Considering that our eyes perceive small color changes less accurately than small changes in brightness, JPEG averages the color data in blocks of 8 by 8 pixels. An advantage of using JPEG is that file sizes will be considerably smaller than with lossless compression methods such as TIFF.

Although digital images can be compressed at up to 100 JPEG quality levels most digital cameras that use JPEG let you save the images at three settings, usually marked Fine, Normal or Low, though the exact terms used may vary from one manufacturer to the other.
This means that you have a choice as to how much information will be lost during compression and consequently what the final quality of your image will be.
As it is not always possible to determine beforehand what an image will be used for, we would advise you to always use the highest quality setting, since information lost can never be retrieved. Also, with more image information available, correcting or sharpening an image without perceivable quality loss will be much easier.

Repeatedly opening and saving a JPEG image will lose a little more image quality each time, since an already compressed image will be compressed again, causing even more information to be lost.