A major dust-up

So there I was, late one winter’s afternoon on the Lac de Joux - a lake in the Jura mountains that freezes solid - when this guy in a balloon comes in to land on the ice.

Everyone rushes forward to watch. Great composition! I take a couple of steps to one side to get the sun behind balloon and fire away.

But, when get home and look at the images ... what’s that far off in the sky? Above the silhouette and to the right. A second balloon?

No, it’s the dreaded sensor dust.

This is a problem that particularly affects digital cameras with interchangeable lenses.

Some people get very stressed about it. They change lenses as little as possible (which rather defeats the object of having such a camera) or they buy little lens-changing bags.

I’ve seen these bags on sale in photographic shops. You put the whole camera and replacement lens into one, zip it up, put your hands into sort of glove-like things and change the lens. Then you take the whole lot out again.

Meanwhile, whatever you were going to photograph has long since vanished. Oh ... and every time you open the bag, dust gets inside, anyway.

Face up to it. Dust on the sensor is inevitable. Even if you don’t change lenses, a zoom lens acts like a giant air pump, sucking air (and dust particles) into the body every time you shift from wide angle to telephoto. Then the shutter mechanism wears with use, producing microscopic particles.

If you want to see just how much dust is on your sensor, try this. (Warning. This is not for the faint-hearted.) Set your camera to aperture priority and stop down to the smallest aperture possible (probably f22). Take a picture of something completely plain and light-coloured – a clear blue sky is best, but a white door or wall will do. Don’t worry about the shutter speed. It will be horrendously slow for hand-holding, but that doesn’t matter. What you’re looking for is already sitting in the camera, jiggling about as you do!

Examine the image in your image editing program. If you’re feeling really, really brave you can apply the ‘Automatic Contrast’ setting to the image. That’ll show up every last speck.

But maybe I shouldn't have told you that. Provided your sensor isn't like Miss Havisham's wedding table it's not so important. Most of the time the spots will be invisible and the few times that they do intrude (like above) you can easily remove them with an image editing program.

In fact, dust even used to be a problem back in the old darkroom days. You had to be sure your negative was spotless before printing, but that was well-nigh impossible in the red-lighted gloom ... coupled with the fact that most of the darkrooms I've worked in were not exactly the most dust-free environments.

Then, when the spots and hairs showed up on your print as little white dots and squiggles you had to remove them with a camel-hair brush and some retouching pigment.

It's a whole lot easier nowadays.

4 comments:

Anonymous said...

My first real camera was a Pentax ESll. Through the 50mm you could see a bug strolling around, trapped between the 10 or 11 optics which made up the lens. It never showed up on the slides. It should be dead by now.

Getting back to your previous posting. I think the use of a tripod in Her Majesty's Parks is forbidden simply because it symbolises the difference between an amateur and a professional photographer.

Alistair Scott said...

Poor bug! How did it get in there I wonder?

As for HM's Parks ... although the public is allowed free access they are, in fact, private property. Therefore the owner, or anyone authorised by her, can place any restrictions they like on photography.

The same goes for National Trust and English Heritage properties, inside churches, RSPB bird reserves and so on.

As far as I can tell from the video, the guy who gets hassled by the police is on public property. Provided he is not causing an obstruction, he is free to photograph what he likes.

Livia said...

What is your opinion about automatic (built in camera) sensor cleaning? Is that enough to reduce or maybe eliminate this problem?

Alistair Scott said...

I have no personal experience of automatic sensor cleaning. None of my cameras have it.

However, reading of others' experiences, on blogs and in forums, it seems to be a very useful facility.

It's not the complete answer. For example, some dust can 'weld' itself to the sensor if there is a trace of moisture present (e.g. condensation when coming in to a warm house from the cold exterior). That can probably only be shifted with a sensor swab.

But, certainly, the self-cleaning sensors are one more weapon in the constant battle with dust.