There’s a collection of stories out there by photographers who’ve mistaken image detail for sensor dirt when trying to clean their DSLRs. Photographers want sharp images and are used to doing whatever might be necessary to produce them. Sometimes, this instinct can work against you and checking for sensor dust is one of those times. Here’s the thing to remember:

When you’re checking for sensor dust, keep the camera moving during the exposure.

Any detail that remains in the shot represents something that’s moving with your camera and is not part of the scene. Spots on a wall that you’re pointing at, for example, or flecks on a sheet of recycled paper, will all be blurred away to nothing as long as you move the camera enough during the test exposure.

Here’s how most people recommend that you identify sensor dust:

1. Set the aperture of your lens to the smallest available. (Beginners might need a reminder that this is the highest f-number; quite often, it will be f/22 or f/32.) The idea is get the greatest depth of field.
2. Aim the camera at an area of low detail, like a white piece of paper or a smoothly painted wall.
3. Take a well exposed shot that’s bright enough to have the paper appear as a middle-to-bright grey
4. Scroll slowly through the image at a high magnification (ideally, on your computer rather than on the camera) and identify the dark marks that are the tell-tale signs of sensor dirt.
5. Clean the sensor using your chosen method (there are many but be careful: even though it has a very thin, protective glass coating, the coating and the sensor are easily damaged).
6. As step 4; with luck, the sensor shows fewer marks now or perhaps even none.
7. Repeat the whole process if necessary till the dust has gone, moving from dry cleaning to wet cleaning if that’s what it takes to shift the dirt.

There are variations—sometimes you’re advised to put the camera on a tripod, sometimes to point at the sky rather than at a wall or piece of paper—but you get the general idea.

The illustration at the beginning of this short article was shot at f/22 with the camera was pointing at an indoor wall lit by daylight and tungsten. The light was low enough to require a 30-second exposure. (In good daylight, you’d need only a fraction of a second.) You can see the sensor dirt quite clearly in this cropped section, displayed at 1:1, even though there wass plenty of texture and detail on the wall. The camera was moving round and round in circles for the entire exposure, meaning that the photographer (me, in this case) met his daily exercise quota as well.

Incidentally, we’ve found that we see the sensor dirt more clearly when manually setting focus to the nearest distance available instead of infinity, at least on the lenses that we used when experimenting with this. That makes sense but again goes against much of the advice out there. Try it and let us know how you got on.

One last thing about sensor dust: when you shoot at wide-open apertures to achieve a shallow depth of field, sensor dust will not appear clearly in your images but as long as it’s there, it will always be reducing contrast and clarity in some part of your image. In short, it’s worth eliminating. (Read your camera’s manual and seek advice if unsure. You can break or scratch the protective layer over your sensor doing this and repairs won’t be covered under warranty.)

Coming up: some information on Lightroom 3, public beta 1, which is free to install and download, even if you’ve never used Lightroom before. (It’s a beta version, which means it will expire.) Also, we’re putting some prices and information together on our courses based on Adobe Photoshop Lightroom—we cover Lightroom at every level, from beginner to advanced. To continue to receive all the free articles here on the blog when they appear, you can subscribe via RSS or by e-mail. Both options are completely free and we do not pass your e-mail address on to third parties. If you find the articles useful, please pass on our URL to others: shootraw.co.uk