Digital Imaging & Dust - sensor cleaning

Since the dawn of photography, dust has been an enemy of the photographer. Dust sometimes (and somehow!) managed to get into the processing tank and onto the surface of the film during processing. Dust frequently got onto the surface of the film when it was place into the carrier of an enlarger, and just as frequently, dust somehow landed on your sheet of photographic paper before or during the print-process stage.

The result usually meant either re-printing, or re-touching the final print. Re-touching was the norm, but for Forensic Photographers, re-touching could mean that an image subsequently becoming non-admissible in a court of law.

For the digital photographer, dust has only one chance of entering the process and hence, spoiling a photograph. This is of course, getting onto the surface of the sensor. The occurrence of dust on the sensor is impossible to see on the image display on the rear of the camera, and can have disastrous consequences. Dust is electrically attracted to the sensor, and is usually stubborn and difficult to remove.

The sensor is a precision piece of equipment and is extremely delicate, therefore a careless or heavy-handed approach may result in breakage. Dust can only become attracted to the sensor when the lens is removed, and it would be an ideal world if we never had to change lenses! Of course we all need to change lenses, add extension tubes, or attach our cameras to other devices such as microscopes. Therefore we will all end up with dust on the sensor at some stage.

A clean, careful approach to changing lenses is the best point at which to start - Keep the camera body facing downwards during lens changes, always replace rear lens caps immediately after use, and never be tempted to blow into the lens mount.

Even with this type of approach, dust will inevitably end up inside the camera "throat" and eventually become attracted to the sensor. A cleaning method or device must therefore be employed, in order to combat the problem. There are a number of methods and products available to the despairing photographer however, and here I will briefly discuss a few. Please feel welcome to contribute your own findings. - We all need to know if they are successful, but equally we need to be aware of unsuccessful methods, in order to avoid repeating others' mistakes. You only need to see the result of the nozzle flying off the top of a can of compressed air to see the disaster of a smashed sensor! Reference "Clean up your image - Automatically" Henshall J. John Henshall's chip shop."The Photographer" March 2004.

Of course, cleaning with a can of compressed air is the natural choice for photographers, as it is a familiar method to us all. The above example is just a cautionary note - I'm sure the nozzle becoming detached is not a frequent occurrence. However, I have personally found this method does not always remove all particles. Some dust just does not want to be moved. Additionally, it can sometimes seem that particles are just moved around the sensor, from one place to another!!

Converse to this method, some manufacturers produce a miniature sensor vacuum-cleaner. A friend and colleague purchased one of these, and initially swore by it. However, my only trial of this device was far from convincing. Perhaps there are other models available, which have been found to be more successful.

An obvious alternative to compressed air is the blower-brush - a simple piece of equipment which is in every photographers kit bag. The blower-brush can be used successfully, but again is often found to merely move dust around the sensor, rather than removing it completely. It is also possible to use the brush alone, without the blower. Care must be taken, not to apply too much pressure when employing the brush, as the sensor is easily damaged.

Cleaning agents and specially designed sensor swabs are a popular method, and again, there are several suppliers. One example of these can be found at:

meraclean.co.uk/main/technical/sensor_swabs.php

I have personally used the "Eclipse" fluid and swabs, as have several other photographers I know. They are fairly successful, but again must be used with caution. Heavy pressure may damage the sensor, and sometimes the fluid leaves a drying mark, or even a "tide mark" of dust.

Another method which exists is the "Triboelectric effect" Which involves electrically charging a specially made brush, by the technique of blowing your can of compressed air at the tip of the brush to charge it. The brush then "attracts" dust off the sensor.
Reference: "No more dirty pictures" Henshall J. "The photographer" July 2006.
To employ this technique, the brush hairs need to be of a particular type (see article) Brushes can be purchased from "Clean-skies sensor cleaning brushes". I anyone out there has used this method - please post a reply.

Of course some camera manufacturers are now designing cameras with technology which prevents dust from landing on the sensor, by means of a clear "filter" which is electrically charged so as to repel dust. Those of us without such cameras though, just have to keep our chip clean!!!!!

When dust particles are stubborn, perhaps the best approach is a combination of two or more of the above methods. For example; firstly remove free dust within the lens mount, using compressed air (mirror down). Then lock-up mirror, remove loose particles on the sensor surface with compressed air, then clean with swab & cleaning agent. Finally allow to dry, unlock mirror, and clean again with compressed air.

A final word of warning for those new to sensor cleaning. Never set your camera to the "B" or "Bulb" shutter setting, always use the mains power adaptor and navigate the menu to "sensor lock-up" in order to access the sensor. A finger can easily slip from the shutter release, and the shutter may then catch the swab or brush.

Imaging wounds in context

It is important for the crime scene investigator or forensic photographer to understand the appearance of different wound types, their significance and the implications for the technical aspects surrounding the lighting and composition, for effective demonstration and recording.
For an introduction to wound morphology, see forensic medicine for medical students.

Book review

There are few (but gradually increasing) books dedicated to Forensic Photography. Redsicker is sometimes regarded as the "standard" text, but in my view it was dissapointing and poorly illustrated - particularly as quality of imaging is key to effective Forensic practice. Read the full review at www.forensicmed.co.uk.

New Forensic Imaging Blog

Welcome to this new blog dedicated to all aspects of Forensic Imaging. Although Forensic Science is well catered for on the web, Forensic Imaging is not so well represented. This blog will aim to promote discussion and interest in this diverse and exciting field, drawing on the professional literature and web-based resources.

Contributions from other professionals in the field are greatfully recieved, as a sharing of information can only enhance the progression of knowledge and skills - which is at the core of continuing professional development.