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Editor's note: With this article we welcome Bob Atkins to the Adorama Academy team of photography experts. Bob combines a personal enthusiasm for photography with a deep technical knowledge of cameras and lenses that is legendary among long-time participants in photography forums online. We look forward to his sage, practical advice every month--starting now!
--Mason Resnick, Editor, Adorama Academy
Do you see mysterious dark spots on your DSLR images?
If
the answer is no, ignore the rest of this article and
be happy. But if you have seen these spots, read on. Your DSLR probably has dust on its sensor--a normal, but inconvenient, fact of life when using digital cameras with interchangeable lenses. Removing this dust from your sensor requires a delicate touch.
I'll show you how.
A tough spot: 100% enlargement of highlighted area in above image at right reveals two spots caused by dust on a DSLR's sensor. It's a common problem, and fixing it yourself comes with risks...unless you do it right. (Photo: Mason Resnick)
Where did those spots come from?
Spots on a digital image are sometimes attributed by users to dirt on the lens or SLR
mirror, but this is almost never the case. The mirror is out of the optical path when an
image is made, so even large amounts of dirt on the mirror will not show up in the image.
Dirt on the lens elements will also very rarely show up as spots on the image since it's
so far out of focus. You might see dirt on the front element of a fisheye lens if it's set
to its minimum focus distance and stopped down, but the optics of normal lenses won't
produce images of dust particles inside the lens.
Dirt can potentially enter your camera whenever you change lenses. That's why this isn't a problem with digital cameras with built-in lenses. One way to minimize dirt is to point the camera down while changing lenses. It won't stop all dirt, but it may minimize spots.
Check your sensor
If you don't know if your sensor has dust, here is how to check. Put
a lens on your camera, set focus to infinity, exposure compensation to +1, and the
aperture to its smallest value (largest number)--at least f/22, f/32 if you have it.
Hold the camera a foot or two in front a uniformly illuminated target that fills the frame and shoot. Exposure may be several seconds, but that's OK. You don't need to hold
the camera particularly steady. You want the exposure to be uniform, which is why you've
deliberately defocused, so camera motion will make no difference. Look at the image. Do
you see dark spots? If so it's 99% certain you have dust on the sensor.
At wider apertures, the dust specs will be much less visible. At f/11 they may be very faint and at f/8 or wider
apertures they may be undetectable. If the aperture setting doesn't affect the appearance of the spots, and they are very sharp and well-defined, they are more likely to be sensor defects than dust.
Can you spot spots?
Below are three 100% crops from a test image. On the left is a portion of the original
image, which was shot at f/5.6. It's very, very hard to see any evidence of a dust spot. In the center is
the same shot, but with an extreme histogram stretch to bring out the very low contrast
detail. You can now see a large circular area that's somewhat darker the the background.
That's the shadow of the dust spot on the sensor. Finally on the right is a crop from
exactly the same position in a test frame shot at f/32. The dust spot is quite obvious
without any manipulation of the image.
Some cameras, notably the Olympus Evolt DSLR bodies and the new Sony Alpha DSLR, take some
pains to remove dust from the sensor by rapidly vibrating, "shaking"
the dust off the surface. By all accounts this system works pretty well, but it doesn't
help owners of Canon, Nikon, Pentax, Leica, and earlier Konica-Minolta DSLRs. Even Olympus
and Sony DSLR owners might one day encounter dust/dirt on the sensor that can't be shaken
off!
If you see dust on your sensor (and eventually, everybody will) you really then have only three
choices:
- Ignore the dust spots and clone them out if and when you see them. This will get
very tedious and time consuming, so it's really not a very good option if the spots are
visible under normal conditions. If you only see them at f32 after increasing the image
contrast then you can probably live with them. All sensors will show some dust if you look
closely enough.
- Send the camera back to the manufacturer or your favorite repair shop for
cleaning. It will probably cost you at least $50 and you may be without your camera for
2-3 weeks, but there's a good chance it will come back dust free. There's also a chance it
will come back with less dust, but still a few specs. Even if it does come back clean it
will probably pick up dust with a few weeks of use. Then what do you do?
- Clean it yourself. This would be the obvious choice if it
were not for a very small chance you could do some damage and end up with a $200 repair
bill. Many, many people have cleaned their sensors themselves without any problems, but
if you are clumsy, you don't use the right cleaning tools, or are just
very unlucky, there is small chance you could have problems.
Why I vote for option three
Since even after you (or someone else) has cleaned your sensor it will eventually get
dirty again, learning how to clean it yourself will save you a lot of time, money and
frustration in the long run.
The surface of the sensor (see the diagram at right, which shows the construction of the Canon EOS D60's sensor) is about as delicate as the surface of a multi-coated filter.
In fact it is pretty much the surface of a multicoated filter since the actual sensor
itself sits behind anti-aliasing and IR blocking filters in almost all DSLRs. If you can
clean a muticoated filter without damaging it, you can probably clean a digital sensor
surface. The only caveat here is that even the smallest scratch will show up on the
sensor, while a small scratch on the filter may go unnoticed.
Technical note: The dust isn't actually on the sensor surface. It's on
the surface of a filter which is in front of the actual sensor itself. This is why dust
shows up more at smaller apertures. Since the dust spots are some distance from the actual
sensor pixels, a wide aperture lets in light which can "go around" the dust
spot. It's a bit like using a large softbox for lighting. Shadows (and what shows up in
the image is the shadow of the dust spot) are light and soft. At small apertures it's like
using a small pinpoint spotlight and shadows are dark and hard edged.
The following images
show small dust spots as f/22 and f/11. They have been greatly processed to show the dust
spots as clearly as possible. On the straight images the spots are totally
invisible at f11 and only very faintly visible at f/22
Here's what very dusty sensor looks like at f/29. This is a 100% crop from
an original image, no processing applied. After cleaning no dark spots could be seen.
There are several ways to try to remove dust from the surface of a digital
sensor. All require first raising the DLSR mirror and opening the camera's shutter. Most
DLSRs have a Cleaning mode which does both of these things, leaving the
sensor visible if you remove the lens from the camera.
Method 1 - The Blower
This is the safest way of trying to clean a sensor, though it's probably also the least
effective. Canon, for example, recommends a blower bulb, with the tip of the
bulb held no further into the camera than the lens mount. The theory is that with bursts
of air you can blow dust off the sensor. What you may actually do is redistribute the dust inside the camera. If there's
more dust on the sensor than around it, you'll may end up with less dust on the sensor.
However my attempts at this method usually resulted in more dust on the sensor than I
started out with!
- Good points: Zero possibility of damaging sensor. Even if the shutter
should close you're OK if you're outside the body with the blower. Easy to do, so it's
always worth a try.
- Bad points Doesn't seem to work very well; may result in more dust
than you started out with.
Caution: Do not use any sort of "canned air" for this. It may
be too powerful and the types that are liquid driven can "spit" liquid in the
gas stream and if that happens you may find residue left on the sensor.
Method 2 - The Brush
If you can't blow the dust off the sensor surface, maybe you can brush it off. The brush actually picks the dust up and removes it from the
camera, rather than just dislodging it and having it land somewhere else inside the camera
body.
 A company called Visible Dust introduced a line of very high-tech brushes which have bristles made of specially-shaped
materials and are treated in a high-tech way (in a plasma chamber), producing a brush that won't damage the
sensor but will attract dust onto the bristles rather than just pushing it around on the
sensor. From all accounts, the Visible Dust brushes do a very good job of sensor cleaning,
though they are not inexpensive (up to $280 for complete brush kits for 1.3x, 1.6x, and full-frame sensors.
- Good points - Usually works, very little chance of doing any damage
- Bad points - Commercial brushes aren't cheap, but they're cheaper than
a new sensor and cheaper than having a professional clean the sensor a few times.
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The Brush technique--DIY version
The research scientist in me wondered if I could get away with using a
cheaper brush. The basics of a desirable brush are that it should use
fine, soft, synthetic fiber bristles and the handle should be insulating. This will, in
theory, enable the brush to hold a small electrostatic charge which will attract dust and
"vacuum" it off the surface of the sensor as the brush passes over it.
 I found a very inexpensive makeup brush at Wal-Mart (shown). I don't remember
what it cost. Probably less than $1.
The most important property of the brush isn't that it removes dust, but
that it doesn't mark the sensor. The brush has to be clean and free from grease and oil.
You can wash it with a mild detergent and thoroughly rise it with clean water, then test
it by brushing it over a clean multicoated filter a number of times. If you see any marks
left, it's not clean enough to be used on a sensor. You can put a small static charge on
the brush by blowing air through the bristles (which also should blow out any trapped
dust) using either a blower bulb or compressed gas. The small static charge attracts and
holds dust particles without risking sensor damage.
For me, this brush worked quite well in removing the dust on my
sensor as shown by the series of images above. Since the brush is only about 1cm wide it
took a couple of passes to go over the whole sensor. Sometimes it's necessary to do a 3rd
or 4th pass to remove all the problem dust particles. The Visible Dust brush is wider and
can clean the sensor in a single pass.
Is it better than a Visible Dust brush? Probably not. Is it safer? I don't
think so, in fact it's probably more risky. Is it cheaper? Yes, it is. Could it damage the
sensor? I can't say for sure it won't, but it hasn't yet.
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Method 3 - If all else fails...full contact
If the dust refuses to move with either a blower or a brush, somewhat more drastic
measure may be called for. You may have to make harder physical contact with the sensor surface with
some sort of cleaning device. Since this method has the most physical contact with the
sensor, it's the most effective at removing stubbornly attached dust or dirt--but it also
has the highest chance of damaging (scratching) the surface of the sensor assembly.
 The first method of removing stubborn individual dust specs is to use a
SpecGrabber, made by Kinetronics. This is an insulating plastic rod with a
small pad on the end. The pad provides a high adhesion surface for individual dust specs,
higher than the force holding them to the sensor surface (if you're lucky!), so it will
lift the dust off the sensor. The pad leaves no residue and uses no solvent. It can be
washed and reused. The only trick here is finding the individual dust specs! They are
usually too small to be seen with the naked eye and the tip of the SpecGrabber is only a
few mm in diameter, so you may have to hunt around for them, checking the sensor
each time to see if you've got them.
 The second full-contact method is to use a lens tissue in conjunction with a solvent
cleaning fluid. This will clean the whole sensor at once so for a sensor with a lot of
dust will be faster and easier than trying to remove each with with a device like the
SpecGrabber. You can make your own tools to do this, or you can buy them.
To make your own
what you need is a soft but fairly rigid support, about 15mm wide by a few inches long and
1-2mm thick. This could be a Popsicle stick, a modified plastic knife, a modified kitchen
spatula or a strip cut from an old credit card. The idea of a soft but rigid support is
that if you should turn out to be physically inept and instead of wiping the tissue across
the sensor you tear the tissue and/or scrape the support across the sensor, you'd rather
it was soft like plastic than hard like steel. You then take a sheet of lens tissue, fold
it lengthways several times until it's about 15mm wide, then fold it in half and place it
over the stick as shown in the figure below. 15mm is the approximate height of an APS-C
digital sensor.
Do not touch the tissue in the area which will contact the sensor. Not
only will you transfer grease onto it from your fingers, you may transfer dust and grit.
Running a greasy, dusty, gritty tissue across the face of the sensor is not recommended.
You then moisten the end of the "swab" with pure methanol or other
high-quality, no-residue cleaning fluid such as that made for cleaning muticoated optics.
Do not use Windex or any similar household cleaning fluid! The
key here is "moisten". You don't want liquid dripping off. It should be wet, but
not that wet.
Hold your breath, engage the sensor cleaning mode on your
camera to expose the sensor and gently wipe the swap across the sensor from one side to
the other. The idea is to contact the whole width (height) of the sensor as you wipe
across it from edge to edge. If you have to do it again you will use another clean
sheet. "Clean" is important word here. Don't try to economize by using the same
tissue twice.
You don't need to scrub the sensor or apply too much pressure, but you do
want to apply enough pressure that the tissue fully contacts the sensor surface. The trick
is that you don't know how much pressure this is until you've done it. Perhaps a couple of
practice swipes across a filter might give you a better feel for what's required.
Obviously you use a fresh piece of lens tissue before cleaning the sensor.
Now, do another dust test by taking a shot of a uniform bright surface with your
lens stopped down to f/22 or smaller. If you are lucky you will see very few (if any) dust
spots. If you are unlucky you will still see multiple dust spots.
If you see some dust left (and you don't see any damage!) you can repeat the process
and hope to remove it. However the trick here is not to push things too far. Removing
every last tiny spec of dust that's visible at f/32 is not necessary. Even if you do manage
to do it, within a day or two I'll bet you'll find a few small dust specs back on the
sensor. You do not want to clean the sensor any more frequently or vigorously than
absolutely necessary. The less contact you have with the sensor, the lower the risk of
doing any damage.
You can buy special cleaning swabs for this purpose such as those sold under the name
Sensor Swabs made by Photographic Solutions Inc. They are assembled in a
clean room and sealed. The chance that they will scratch the sensor is very, very, very,
very, very low (unless you get dirt on them yourself after you unwrap them). The downside
is that they are $4 each and sold in packs of 12, so they will cost you $48. However this
is much less then the cost of replacing a scratched sensor. You can also buy special
cleaning fluid (such as Visible Dust Smear Away Solution Kit, Tough
Sensor-Stain Remover (shown), which costs around $20 and will not damage the sensor or leave residue.
Lens tissue is much cheaper (about $2 for 50 sheets of Kodak Lens Tissue).
You want optical grade lens tissue. I've used lens tissue on lenses for years
without a problem, including cleaning some pretty delicate optics such as dielectric laser
mirrors, so I'd say the chances of a problem are very, very small, but they are never
zero. I've also cleaned my DSLR sensor maybe a dozen times with then, again with no
problems.
Note: Be sure you buy untreated lens tissue, not silicone
impregnated lens tissue sold in eyeglass stores. If you smear silicone oil on your sensor
you will not be happy and you'll be spending quite a bit of time trying to clean it off.
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Caution and Disclaimer
While many people have reported success in cleaning their sensors using these
techniques, I have seen one report of someone who scratched their sensor trying to clean
it using a tissue. They probably weren't working in a clean environment and picked up a
tiny spec of grit on the tissue and they may have used too much pressure on the sensor. If
you don't have good mechanical skills and a delicate touch you might want to think a
fewtimes about whether you really want to do this yourself.
I've used the
brush technique many times and I've also used lens tissue supported on a modified plastic
knife. There was no damage and the sensor ended up very clean in each case If I hadn't
managed to do it without damage or it hadn't removed the dust, I wouldn't have written
this article. When using the proper professionally produced cleaning tools designed for
digital sensor cleaning (rather than home made devices) the risk of damage is very low,
however there are no absolute guarantees.
A scratched sensor isn't a total
disaster, but replacing the sensor (or more likely the covering filter) will probably cost
around $200-$300.
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