I started to write this post back in September 2010 but it’s been sitting in my drafts folder since then, waiting for me to check my facts. Even so, as I found myself taking up more of my friend Andy Gailer’s time than I suspect either he or I would have liked (as he helped me to clean the sensor on my DSLR a couple of nights ago), I knew it was time for me to finally put pen to paper (or fingers to keyboard) and finish this off…
It’s a fact of life that, the more often a lens is changed on an SLR camera, the more likely it is that some dirt or dust will find its way into the chamber. Back in the days of film it was easy – grit would scratch your negatives, but a few specks of dust were rarely a problem (indeed, the action of winding on the film moved the dust/dirt away from the active area). Ask anyone with a DSLR though, and they will almost certainly regail stories of frustration as they try to remove dust spots from their sensor (or at least the low-pass filter immediately in front of the sensor).
This post has a few hints and tips that might help you if you have ugly spots appearing on your images. I also recommend reading Thom Hogan’s excellent article on cleaning sensors.
Dust off reference image
I use a Nikon DSLR and it includes a feature called teh dust off reference image. The idea behind this is that, by taking an image that shows the dust spots, this can be compared with others and changes made automatically. It’s a nice idea, but it requires the use of Nikon’s Capture NX software. I don’t use Capture NX (I use Adobe Lightroom), so this feature doesn’t help much… I’m not sure what Canon (and other manufacturers) do, but probably something to be aware of if you have a Nikon DSLR.
In camera sensor cleaning
My Nikon D700 has the ability to clean its own sensor at startup/shutdown but I’m not sure how effective this is. Even so, it’s probably worth leaving the option enabled – it won’t do any harm.
One tool in my friend Andy’s arsenal is his Arctic Butterfly. Basically a selection of brushes with a motor to spin off any dust, this kit allows skilled operators (i.e. not me!) to lift away dust by breaking the static bond that is attaching it to the sensor. You’ll need to lock up the mirror (the camera will usually have an option to do this in its firmware) in order to access the sensor.
It’s a useful tool and on at least two occasions now Andy has helped me to clean away most of the dust (there’s always some left behind). The downside is that the Arctic Butterfly is quite an expensive piece of kit.
I spent at least half a day working through a multitude of boxes, drawers and even some more unlikely places hunting high and low for my Giottos air blower but I can’t find it anywhere. If it doesn’t turn up soon, I’ll almost certainly replace it as it’s an excellent investment for blowing loose particles away. The trick is to hold the body with the lens mount facing down, then blow upwards (so that any dirt falls away from the camera and towards the ground). If you’re lucky, this is all you need to do to clear away the dust, but never use compressed air blowers (the propellant can sometimes get squirted onto the sensor) and, certainly never be tempted to blow with your mouth! I found, to my cost, that even a dry mouth will result in saliva on the sensor… which leads me onto the next tip…
Sensor swabs can be used for removing stubborn stains (like saliva… or grease). Available in specific sizes to suit full frame 35mm or APS-C sized sensors, I have used the Photographic Solutions Sensor Swabs Pro product previously, but my swabs seem to have gone AWOL with my rocket blower…
As it happens, Andy had some swabs from Visible Dust that probably did a better job – the main difference was that they needed to be moistened with a special fluid instead of being pre-moistened and sealed in a foil packet.
Checking for the presence of dirt on the sensor
Regardless of the technique(s) used to clean the sensor, it’s necessary to check for the continued presence of dust/dirt on the sensor. Some spots will be too small to view with the naked eye but, thankfully, it’s relatively straightforward to take a photograph that will show any problems.
- Take a picture of a plain object (e.g. a sheet of paper) from about 10cm away in good light. Make sure that you use the following settings:
- A narrow aperture (e.g. f22) for maximum depth of field.
- Zoom in as far as possible.
- Focus to infinity (you may need to do this manually).
- Some people suggest setting the exposure to +2.0EV but I tend not to do this as the dirt will still be visible on a grey image and over-exposing may blow out the image leaving no dust spots visible.
- View the image at 1:1 scale in your favourite image editing software. It may take a while to view the whole image (with several scans across and up/down) but it should be possible to see if there are any remaining dust spots. If the largest ones have gone and there are only a few left (especially at the edges), it may be advisable to cut your losses and leave them there…
Disclaimer: I feel the need, in today’s increasingly litigious society, to point out that this information has been given in good faith but that I can’t be held responsible for any damage to equipment as a result of following the advice on this website.