It is generally desirable for image brightness and RGB color distributions to be uniform except where they change to form an image. There are factors, however, that tend to produce variation in the brightness and RGB color of a displayed image even when no apparent detail is present. In the most cases of practical interest, this variation is random, non-additive and has no predictable pattern. The variation reduces the overall image quality and is especially significant when the objects being imaged are small and have relatively low contrast. This random variation in image brightness is designated as noise. No imaging technique is free of noise, yet noise is much more prevalent and distracting in certain types of imaging procedures than in others.
The sources of noise in digital photography imaging are numerous, and a detailed assessment of their nature goes well beyond the scope of this short blog post. However, it should be noted their very nature can be incurred from the physical properties of the imaging sensor design and the analogue-to-digital (A/D) signal conversion steps during the image acquisition process.
Moreover, a consumer camera, especially used in an outdoor setting, is a subject to environmental factors, e.g. temperature, which even further affect the visual variability and the noise levels of the acquired images. This is why a number of hardware-related approaches have been developed to mitigate the imaging noise that arises from variable conditions. The most important built-in solutions are active sensor cooling and dark-frame noise subtraction, typically applied in long-exposure imaging.
As stated in the introductory part of this post, certain types of imaging are more prone to visible and distracting noise patterns, than others. In the most cases of practical interest, considering the sensitive CMOS-based cameras, long-exposure photography techniques are known to suffer the most from the noise-related variability.
Long-exposure night photography can be easily characterized by the use of long-lasting acquisition times, on the order of tens of seconds with the high image sensitivity settings (ISO), very often exceeding the value of ISO3200.
Although, the noise levels at a fixed value of ISO vary greatly from camera to camera, and are a subject to sensor design, it is fair to say, the imaging noise in general comes in “many flavors”. The dominant contributions to long-exposure image noise can be identified as: luminescence noise, chromatic noise, hot pixels and anisotropic noise, often referred to as “banding”.
It is in the identification and correct treatment of these noise patterns, where the key to image improvement truly is. Consequently, a comprehensive noise-reduction tool should offer a quick and intuitive means of dealing with the different “flavors” of noise. Capture One Pro 8 performs very well in that domain, offering a complete and logically organized set of noise reduction tools.
The applicability of Capture One Pro 8’s noise reduction methodology can be elegantly demonstrated on the long-exposure photograph attached below.
A mountaineer as seen preparing for departure for the summit of Mont Blanc on a starry and cold summer night in the very heart of Mont Blanc Massif. Photograph taken with Canon 5D Mk II, Canon 17-40 f4.0L at 17mm f4.0 30sec ISO3200.
The image has been taken with Canon 5D Mk II, which by today’s standards can be considered as a solid yet outdated CMOS-based DSLR camera. However, photographing at 3200m above the sea level with the ambient temperature of approx. -15C and the relative humidity of 20% permits a satisfactory result, in which a stunning amount of information in the highlights is retained. However, the mildly-processed RAW file, suffers from a multitude of noise artifacts, including high luminescence noise, apparent chroma noise in the shadow areas and severe banding artifacts, visible after a mild (+0.33EV) exposure adjustment.
BEFORE / AFTER – 200% zoom of a relatively representative subsection of the photograph, depicting luminescence noise adjustments, chroma noise manipulation and sharpness adjustments. Detail adjustment parameters: Sharpening – Amount: 0, Noise Reduction – Luminance: 44, Details: 71, Color: 50, Single Pixel: 10.
In this particular case, the dominant banding noise artifacts were suppressed by the selective color manipulation, bringing down the magenta channel saturation by -15 in the advanced color editor, whilst avoiding time-consuming retouching in an external photo editing software. The luminescence and chroma noise were subsequently minimized by incrementally increasing the reduction values and studying the effects of the adjustment in 100% magnified views. Lastly, hot pixel noise was treated in the reflective subsection of the original image.
Multiple FOCUS TOOL windows used to simultaneously study the effects of noise reduction.
Please note, the effects of the noise adjustments were carefully studied both for the close focal planes that include the person in front of the tent, as well as the starry sky that stretches about the mountains. Additional focus windows with 100% zoom levels were created in order to study the simultaneous effects of noise processing in different subsections of the image. A functionality that is very specific and unique to Capture One Pro 8.
In order to achieve the most satisfactory result, a delicate balance between the luminescence noise reduction, the detail enhancement and the application of global sharpening must be established.
Adjustments made in Capture One Pro 8: Exposure: +0.33, Contrast -15, Brightness 9, Saturation: 50, Highlight: 100, Shadow: 15, Clarity: 43, Structure: 30, Method “Natural”, Custom White Balance, Kelvin: 3200, Tint 2.5, Magenta: -15, Sharpening – Amount: 333, Radius: 1.0, Threshold: 1.0, Noise Reduction – Luminance: 44, Details: 71, Color: 50, Single Pixel: 10.
A concise and incremental application of noise reduction tools in Capture One Pro 8, combined with a powerful and intuitive color editor permit processing and manipulation even of the most demanding long-exposure images. It should be noted though, the aesthetically acceptable noise levels, as well as detail enhancement in long-exposure photography are very subjective, and depend solely on the experience and the personal preference of the photographer. Hence, even the best tools, applied without further refinement can have a decremental effect on the processed image.