Artifacts – Part 1

In digital photography artifacts refer to the undesirable modifications caused by the sensor, optics, and internal image processing algorithms of the camera. On low budget compact cameras these artifacts are more visible and easy to identify. On the other hand, top DSLR cameras will require a more careful analysis to identify these artifacts and most of the times they don’t represent a disturbing issue. In this first part we will talk about Blooming, Chromatic Aberrations, Jaggies and JPEG Compression.

Blooming

Every pixel in a digital image is the corespondent value of an electrical charge. This in turn is directly related to the number of photons that will fall on the pixel’s photodiode in time of exposure time. If the exposure time is to long, the electrical charge will reach it’s maximum and will overexpose the corresponding pixel. Blooming occurs when this extra electrical charge affects also the surrounding pixels, causing a loss of details in the photo. Here is an example:

Blooming

Blooming

Chromatic Aberrations

Chromatic aberration (also called “color fringing”) is caused by the camera lens not focusing different wavelengths of light onto the exact same focal plane. The amount of chromatic aberration depends on the dispersion of the glass. Purple Fringing usually refers to a typical digital camera phenomenon that is caused by the microlenses. Purple fringing is “chromatic aberration at microlens level”. Edges of contrasty subjects suffer most, especially if the light comes from behind them. Blooming tends to increase the visibility of purple fringing. Here is an example of purple fringing, one of the most common chromatic aberration in digital cameras:

Purple Fringing

Purple fringing

Jaggies

Jaggies (or aliasing) refer to the visible “steps” of diagonal lines or edges in a digital image. These steps are a consequence of the square layout of a pixel. As the sensor resolution increases, jaggies become less visible. The are also algorithms of anti-aliasing that will reduce this effect. On the other hand, sharpening will increases edge-contrast and make jaggies more visible.

Jaggies

Jaggies

JPEG Compression

JPEG is the most commonly used digital image format. It allows photographic images to be compressed by a factor 10 to 20 compared to the uncompressed original with very little visible loss in image quality. JPEG algorithm rearranges the image information into color and detail information, compressing color more than detail because our eyes are more sensitive to detail than to color, making the compression less visible to the naked eye. Secondly, it sorts the detail information into fine and coarse detail and discards the fine detail first because our eyes are more sensitive to coarse detail than to fine detail. JPEG allows you to make a trade-off between image file size and image quality. JPEG compression divides the image in squares of 8 x 8 pixels which are compressed independently. Initially these squares manifest themselves through “hair” artifacts around the edges. Then, as you increase the compression, the squares themselves will become visible.

JPEG compression

JPEG compression









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