Dust on the sensor

Digital cameras, especially DSLRs cameras with interchangeable lenses are fun to use and transforming the way photographers work. They give you virtually instant feedback and can produce results to match if not better almost everything from 35mm film, but they do produce their own problems.

If, like me, you use a DSLR, you will be only too aware of the problem of dust on the sensor. Only one manufacturer has currently faced up to this problem. Olympus in the E1 use an ultrasonic vibrator to shake the dust off before each shot. The other manufacturers seem largely to have shrugged their shoulders and passed responsibility on to the photographers.

Even if you never change the lens on your DSLR you can have a dust problem. Some unlucky owners find the manufacturer has kindly included a fair amount of it on the screen of their new camera. One photographer of my acquaintance tested half a dozen cameras of one of the best-known marques from the shelves of a remarkably compliant dealer before he found a camera that was 'clean'. However it wasn't really worth the effort, as even if there is none straight out of the box, then it will gradually appear.

Perhaps through an electrostatic effect, the sensors seem to attract dust. Some camera models suffer less than others, possibly because of a better seal by the mirror when the lens is removed, possibly because the CMOS sensor used in some Canon models has less attraction for dust.

Digital and Film compared

Dust is not generally an insuperable problem in digital images. It requires only a brief wipe with the healing brush or clone tool in Photoshop to banish each spot from your digital file, but as it builds up this becomes more and more of a chore.

Even so, the problems with digital are generally less than with film, where few if any transparencies are entirely free of problems, and prints can require considerable skilled spotting. Transparency retouching was largely carried out by the repro houses as a part of their role (or simply not done) but with much material, whether originated on film or digital, being supplied by the photographer as digital files, it has now become a part of the photographer's job.

Reading the Manual

DSLR camera manuals are often less than direct about the dust problem, and sometimes clearly unhelpful. There is no entry under 'dust' in the manual for my D100, although there is a section on cleaning listed under the 'CCD' heading.

As Nikon point out, the actual CCD is protected from dust by a low pass filter, present in front of the CCD to reduce moiré effects (these are interference patterns caused through the interaction between regular patterns in the subject and the regular array of the sensor.) What we call dust on the sensor is actually dust on the filter.

Helpfully, Nikon tell you how to look at this filter, but the advice is less than useful. Most dust particles are too small to be visible with the naked eye, and they way they suggest looking at them is likely - unless carried out under clean-room conditions - to result in the filter gathering more dust.

A simple check for dust

Don't take off the lens and peer inside. The best way to see dust is to get your camera to record it. This will show you exactly how big the problem is.

Choose a featureless subject that will fill the frame. A clear blue sky, pointing well away from the sun, is idea. A reasonably sized plain white or light-coloured card will also do. Switch the camera to manual focus and exposure, fill the frame with an out of focus image of nothing, and make a correctly exposed picture. It should give an overall mid-tone. Load this file into your image editor and examine it at 100%. Probably dust particles will be obvious, but you can increase their visibility using any of several filters such as 'find edges', or by using the 'auto-levels' command.

Cleaning the Filter

Nikon's advice is simple. The filter is "extremely delicate and easily damaged." They recommend you to take it to be cleaned by "Nikon-authorised service personnel." Should you decide to take the matter into your own hands, they do tell you how to use a blower (not a blower-brush, as the hairs could damage the filter) to try to clean it. Unfortunately, in my experience a blower seldom removes much of the dirt, which is too firmly attracted to be removed by an air blast. If this procedure fails, Nikon tell you that "Dust that cannot be removed with a blower can only be removed by Nikon-authorised service personnel. Under no circumstances should you touch or wipe the filter."

This is not really a serious proposition for most photographers. When you need your camera cleaned there isn't usually a Nikon service facility standing by ready to assist. Dust isn't a blue-moon event, but an everyday occurrence, and if you want to keep working with your digital equipment, you are forced to learn to deal with it. The manufacturer advice looks suspiciously as if it is more concerned with avoiding replacement under warranty than with providing a usable solution.

Commercial sensor swabs

Some other manufacturers offer a more relaxed approach to the problem, giving advice on how to wipe the filter safely. You can buy swabs and solvent for the purpose - the same materials as used by Nikon service centres. Unfortunately the purpose made swabs don't seem very effective, and photographers report that having your camera cleaned professionally often doesn't work too well. Working with the commercially available swabs does remove some of the mess from the sensor, but a normal service involves only a single swab being used, because of their relatively high price - several dollars a wipe. The swab design is also not very efficient.

SensorSwipe

It was thus a problem left to photographers to solve for themselves, and numbers considered their thoughts on various online forums.

Any cleaning you attempt yourself is clearly at your own risk. Certainly if you own a Nikon like me, you are deciding to ignore the clear advice of the manufacturer. Nicholas now sells his 'SensorSwipe' worldwide for around 8 USD including carriage if you don't want to two minutes work involved in making your own. Whether or not you want to buy his device, make sure to look as the long and detailed illustrated instructions on his site - he gives far more detail than my short description above. He also gives links to places in various countries where you can buy the disposable lint-free PecPads and Eclipse methanol needed.

Advantages

There are two big advantages to using either the SensorSwipe or a homemade equivalent. First is cost, but more importantly it is simply far more efficient than the commercially available swabs I've looked at. So far as I'm aware, the approach outlined here is the only viable way to keep your digital camera images clean if you want to be able to change lenses - unless you are using an Olympus E1. I'd strongly recommend to other DSLR makers that they provide a device such as the SensorSwipe with every camera body they sell, together with instructions on its use.