The image above is a crop from a portion of sky which clearly shows
the nasty greasy looking marks that dust on the sensor of a digital
This is a problem that film cameras do not suffer from as a new
portion of film is used for each exposure. If you don't notice the
marks, you can produce hundreds of images that will later on have
to be repaired using the healing brush tool
as described in the tutorial on this subject.
So what causes the marks? The image sensor on a digital slr camera
is exposed when the mirror flips up as part of the exposure process.
The sensor is reputedly charged electrostatically and so it can
attract oppositely charged dust particles in the air - like your
hair being attracted towards a plastic comb.
It is not actually the sensor itself which gets the dust, but the
low-pass filter in front of it. Dust on the filter casts shadows
onto the sensor - and that is what we see.
The dust inside the camera has come from the outside air during
lens changes, but some is generated internally by the camera itself
as the shutter and mirror move about.
What can you do about it?
There are new cameras such as the Canon 400D which now have a sensor
which vibrates when you turn the camera on, and a sticky pad attracts
the dislodged dust onto it's surface and holds it there. The technology
is apparently effective and will doubtless find it's way onto other
cameras as time passes - roll on that day! (Apparently it is going
onto the 1D mk III in a new up rated form).
The first thing that you can do to prevent the dust problem is
to turn the camera off when changing lenses. This switches the charge
off from the sensor and theoretically will decrease the amount of
dust attracted to it.
Always keep lens changes to a minimum and do this as quickly as
possible. every second the lens is off of the camera dust can enter
! Dust tends to settle down from above (in still air) so if you
can hold the camera upside down as you change lenses this will also
be of benefit. Get the new lens onto the camera first and as quickly
as possible and then worry about putting lens caps back on after
- but do this as soon as you can to keep external dust away.
Try to not change lenses in dusty situations if at all possible
- having a second camera body with a different lens on it can be
very useful in extreme conditions - such as on safari. If you must
change the lens, try and do it inside a plastic bin liner.
When you return home after a days shoot, clean your camera and lenses
and also clean the inside of the camera, including the pentaprism
and mirror. Use a combination of a soft brush and a rubber blower.
This must be done in advance of sensor cleaning - otherwise the
dust will be attracted onto the sensor as soon as it is exposed.
Many people overlook this vital step, and this is why I'm sure a
lot of people have major problems.
Clean the sensor following the manufacturer's instructions for
the camera in question. This usually involves selecting the "clean
sensor" option from the menu, and pressing the shutter button
to lift the mirror which reveals the sensor. At this point, hold
the camera over your head with the sensor pointing down towards
the floor and using a rubber air blower such as the Giotto Rocket
Air (obtainable from a camera shop). Give the sensor some vigorous
puffs across it's surface. Do not put the tip of the blower too
far into the camera as you may touch the sensor!. Most cameras need
to be switched off in order for the procedure to finish. Immediately
put a lens or blanking-off cap onto the camera as it is once again
susceptible to attracting dust. Apparently some older camera blanking
caps were made of a plastic which sheds dust as it is screwed in
- so it could be worth getting a new one if you think that this
is a problem for you.
When you have cleaned the sensor take a picture and check again
to see if you have successfully removed the dust as described in
the next section. You may have to repeat this a couple of times.
To test for dust
Periodically take a picture a piece of white card (or of the sky
if you prefer) and look for dust marks by previewing the image and
scrolling around at 100% zoom on the back of the camera. If you
want to really be alarmed and want to see just how dirty the sensor
is, take a picture at a small aperture (e.g. f22) then load the
image onto your computer and view at 100% (actual pixels).
The easiest way of taking an image of a plain white card is to set
the camera to manual focus (the manual/autofocus button usually
located on the lens) this stops the autofocus hunting around as
it struggles to focus on the low-contrast blank card. You actually
don't need to focus on the card at all, in fact it is better if
the image of the card is not in focus - as it will cause confusion
if you are not sure if the marks that you are seeing are sensor
dust or marks on the card! Also, there is no need to use a high
shutter speed or use a tripod, as the dust on the sensor will move
at the same time as the camera shakes or moves.
The white card will fool the camera's exposure meter into under-exposure,
so I recommend dialing in about +1.5 stops of exposure compensation
to keep the card looking bright to show up the grey dust against
It is pointless trying to clean every spec off the sensor - it is
hard to do and more dust will appear soon after. Be satisfied that
you have no major spots if you take your test shot at f-11. Be prepared
to remove the odd remaining visible spot in post-processing.
By taking these simple precautions I have never had to resort to
risky and expensive sensor cleaning methods. Most manufacturers
don't recommend cleaning sensors with commercially available sticky
pads. So you may need to return your camera for cleaning if you
neglect to do this bit of housekeeping!
Sensor dust update - Jan 07
Despite using the precautions described, my 1d mk2 developed a
number of small dust marks towards the upper edge of the frame which
refused to be blown out.
I have now invested in an Arctic Butterfly sensor brush (available
from Visible dust
) - which I have used just the once with very good results. The
brush appears ridiculously expensive for what it is, but it does
work. You have to fit batteries into the body and when you switch
it on it spins the brush head. You have to spin the brush three
times for a few seconds before using it on the sensor. Spinning
charges up the high-tech filaments of the brush and also throws
off any dust collected.
After wiping the sensor once with the brush, you have to quickly
turn off the camera and refit a lens onto the body to prevent more
dust getting in. Then you spin the brush again a few times to shake
off the dust
collected and get it back into it's (far too tiny) cover without
touching it with your greasy fingers !
I found that the brush removed every trace of dust in a single
pass - so I give the Arctic Butterfly a thumbs up! After a few uses
the brush has to be cleaned - so be prepared to fork out for another
expensive bottle of cleaning fluid after using it a few times.
I hope to only use the Arctic Butterfly a few times a year, so
I hope that I don't need to keep cleaning the brush.
Sensor dust update October 2008
I should start by saying that the in-built dust reduction systems
of both my 40D and 1DIII work pretty well. They don't eliminate
dust, but they definitely extend the time period required between
However, just before departing on an overseas trip, I decided to
clean the sensor on my 1DmkIII camera.
I used the Arctic butterfly as usual, but to my dismay I must have
touched the side of the sensor chamber and accidentally transferred
a drop of shutter lubricant onto the sensor - arrgh!
I therefore had to resort to the thing I had wanted to avoid at
all costs - wet cleaning! I found it took several attempts with
Eclipse fluid and Pec pads to remove all traces of oil and dust,
and I would still not recommend wet cleaning as a matter of routine
as some people advocate. I reserve it for emergency use only when
all else fails, such as when there is a really stubborn piece of
dust or something greasy. Rather than describe at length the procedure
of cleaning the sensor with Eclipse fluid, here is a link to a website
which describes it fully using what has come to be known as the