Sensor Dust Problems: Detecting, Preventing, and Curing

Who knew that digital sensors would be magnets for dust? They are, acting like little static electricity machines that attract minute and sometimes grain-sized dust particles that are a pain in the neck. You can “spot” the particles out later (which record as black dots and streaks) using cloning tools and various healing brushes and blur tools, but the frustration of having to spot images is simply not a good use of time, especially when the problem can be eliminated or diminished with a bit of care.

Manufacturers have not denied the problem and many cameras now incorporate dust “cleaning” systems that turn on every time you start up the camera. Generally these “shake” the sensor in the hopes of vibrating the dust off; sometimes this works OK and other times you might have to use more drastic measures. The best bet is to prevent dust from catching on the sensor in the first place.

Dust is pernicious and can work its way inside the camera in more ways than one. Here’s some advice about keeping it to a minimum.

1) Always store the camera in a case or bag. It might look handsome on a bookcase or shelf but you are needlessly exposing it to ambient dust.

2) Never change a lens with the camera turned on. Having a charge across the sensor plane when it is exposed will act as a dust magnet.

3) If possible, change lenses inside a vehicle or changing bag, especially if there is any wind. This is where most dust problems occur. Turning your back on the wind to shield the camera when changing lenses does not seem to help: if you lack a changing bag, shielding it with a coat is better than nothing.

4) If you like to photograph outdoors, when shopping for a camera get one with a dust reduction (shake) system, and one that bills itself as having extra seals to prevent dust.

This shot looks “innocent” enough and seemed fine when  inspected on the LCD after exposure. 

Later, with close inspection, the dust nightmare it contained was revealed. This was at the end of a long day’s shooting with a loaner camera and many of the images were unusable as a result. (Note: enlarge this image on your screen to see the blizzard of dust it contains.)

An experience like this should  teach you a good lesson, as it did me. For some shots the dust had to be meticulously retouched in software. What a waste of time!

Note: If you look through the viewfinder and see dust you might not have dust on the sensor—it might be on the lens itself or in the viewfinder pentaprism ( the optical finder in DSLRs) or on the mirror. Check the lens, and if need be clean the front and rear element. Dust inside the lens cannot be cleaned without sending it to the manufacturer--this can be very expensive. If the pentaprism has the dust it can be bothersome, but that dust will not show up on your pictures. If it's on the mirror clean it very carefully with a commercial kit and remove the lens indoors to get access to the mirror--CAUTION: Be very tender when doing this as too much pressure or force could damage the mirror mechanism.

Mirrorless cameras, have no (optical finder) pentaprism or reflex viewing system. If the dust is not on the lens, any dust marks showing up on images means that there is dust on the sensor. Check and clean the lens itself first, both front and rear elements. Dust on the lens should be quite evident, as minute dust on the lens will usually not show up on the image.

The only way to know if you have a sensor dust problem is to take a picture and inspect it by using the magnifying tool in your playback menu, or download it and enlarge it (though that might be too late for that day’s shoot). It can be tough to see dust on your camera's LCD during playback. If you suspect you have dust, before you go out on your shoot make photo of a blank white sheet of paper and enlarge the image on a monitor.  Move around it and enlarge it  to check—this will make the dust spots quite visible.

If you do have dust on the sensor try restarting the camera (if it has a dust reduction system) a few times to see if you can have the system “shake” it away for you. If your camera has a “clean now”  feature you can use this instead of restarting it. A few cameras have a "dust delete" protocol, which creates a kind of mask to spot out dust from a series of recorded images with the same dust pattern. I find this to be a very hit and miss affair.

Lastly, you can try cleaning the sensor yourself with commercially available kits, although I would CAUTION you to be very, very careful if you go this route as damage can easily occur. You can send the body back to the manufacturer for cleaning if need be, which is perhaps the safest route, or take it to a certified camera clinic.  That’s your call, but if you clean it yourself you need to use the camera’s self-cleaning setup (this is not available on many cameras; if it is instructions will be available on the company's web site or in the instruction book) and be sure to use an AC adapter, because if the power goes out during the process severe damage could result. 

Get in the habit of checking each time before you go on an extended trip. Checking periodically, and especially if you have been photographing in the desert, at the beach or in windy conditions.

Focus Lock How To

While you can maximize the depth of field with a shorter focal length lens, a narrower aperture and standing back from the foreground subject, it is all for naught if you do not attain focus on the foreground subject itself. Depth of field at narrower apertures does “come forward” a bit from the point of the actual focusing distance, but making sure the foreground is in focus at any aperture is your best bet.

Finding that right point of focus can be a problem when the front-most point of the scene sits at the side or edge of the frame, Care must be taken to either focus on that point using AF and locking focus, or to manually focus the lens on that point. The AF targets in your finder may cover a considerable amount of the frame, and setting single AF mode usually defaults the camera to closest subject focus priority, but even that might not snag the needed focus point in the shot.

There are two ways to insure that you have caught the proper close focusing point. One is to simply switch to manual focus and focus on the foreground subject, then set your aperture to get deep depth of field.

If using autofocus, choose one focusing target, move the camera to grab and confirm that you have focused on the foremost subject, lock focus (using the AFL button--autofocus lock),  recompose as you wish, and shoot.

Many cameras have an AFL button or, in most cases, you can lock focus with light pressure on the shutter release.

Being able to first find focus and hold it, and then recompose as needed may require some practice, but as you gain experience it becomes second nature and allows for a good deal of compositional control.

This photo of a thermal pool at Yellowstone poses numerous focusing problems for any AF setup. The water and mist might well cause the AF system to search back and forth because it lacks a definable (contrast) target. The composition requires focus at the near edges of the pool, so depth of field has to be from 4 feet to infinity.

To attain focus throughout the frame, the AF target in the center of the viewfinder was used and the camera was pointed down so that focus was confirmed (by a beep or lighted target) at the closest point; focus was then locked. Using this technique, a very narrow aperture with a very wide lens delivered depth of field from the edge of the pool to the row of trees in the background.

Settings: 20mm lens: at ISO 200, first, focus was set and locked at the lower edge of the frame, then exposure settings were made by taking a spot reading from the mist area with a +1 EV exposure compensation. f/22 at 1/60 second.

It is critical in this photo that the tree and rocks on the left edge of the frame are sharp. That made this area the important starting point of focus within the frame. To accomplish this, focus was locked on the foreground rock in the very lower left of the frame. A spot exposure reading in aperture-priority exposure mode set at f/11 was taken of the blue area of the sky and locked. With a 28mm lens, depth of field ranged from 5 feet to infinity.

Settings: 28mm lens, at ISO 200, f/11 at 1/250 second. Focus lock on lower left corner; exposure lock using spot metering off the blue area of the sky.

Exposing for Directional Lighting

While having scenes where the sun fully illuminates the subject can be pleasing and informative, more dramatic renditions can usually be captured early or late in the day, or when a seasonal sun is low in the sky. Known as cross- or directional lighting, it relies on the conditions, be it stormy or low sun, to create a kind of spotlight within the frame. It can skim across surfaces and create bright areas that become natural attractors to the eye. Because readings should be made for these brighter areas, the shadow areas in the frame tend to darken, another bonus, since the contrast created adds even more to the impact.

When making light readings in these conditions rely on spot or Center-weighted averaging metering patterns as this will restrict--or concentrate on-- the light being read to those crucial, brighter areas. Do not hesitate to bracket exposures, to make exposures plus and minus from the actual reading; although differences can be minor, slight changes in exposure can make a big difference in attaining success with this technique. Keep an eye out for directional lighting and use it to add extra impact to your images.

The sun had just risen on this quiet street scene in Delray Beach, Florida. The light revealed a rich blue sky with billowing clouds and spotlighted the soft, rounded edges of the building. The exposure reading was made from the bright area of the building, which made the dark, shadowed building on the left a strong counterpoint.

Settings: At ISO 100, f/8 at 1/125 second. Spot metering pattern reading off the bright area on the round building.

Catching the mood of directional light requires some nuanced exposure techniques. The sun was slanting through some low clouds onto the Ile St. Louis in Paris. When light is fleeting you may have to work fast; that’s when familiarization with exposure techniques pays off. A spot meter reading was made from the building face on the left and locked, and then the image was reframed to include the boat on the Seine. 

Settings: At ISO 200, f/8 at 1/250 second, spot reading off the bright area on the left.

Shooting Through: Use Depth of Field to Eliminate Foreground Obstructions

Depth of field techniques can create some magical effects, from making soft, pleasing backgrounds in portraits to having everything from a foot to infinity appear sharp throughout the frame in landscape work. One of the tricks that can be played with aperture settings is to eliminate distracting foreground elements by focusing “through” them while using a wide open aperture setting to create a shallow depth of field. This can be used to “eliminate” bars at a zoo, a chain link fence at a baseball game or, in this illustration, a rope that surrounded a merry-go-round in Madrid, Spain.

The ride was closed for the winter, and the owners had surrounded it with a rope fence to keep kids from hopping on and playing inside (though knowing kids you can be sure they found a way through!) To illustrate this technique, two equivalent exposures were made with the same lens from the same point of view. The lens is set on manual focus and focused on the closest horse. (Autofocus might have “snagged” on the rope.) A fast lens with a maximum aperture of f/1.4 was used. The first exposure was f/11 at 1/60 second (above) An equivalent exposure was made at f/1.4 at 1/4000 second (below). Note the very soft background. In essence, the very wide aperture setting put the rope so out of focus that it “disappeared.”

Settings: 50mm f/1.4 lens. Both images focused on the foreground horse. Right: f/1.4 at 1/4000 second. Above: f/11 at 1/60 second.

Silhouettes

While you usually want to get good exposure on most subjects throughout the frame, there are times when a silhouette can add dramatic impact to an image. For our purposes, a silhouette is an almost graphic form within the scene that shows no detail. It can be recognizable or abstract, but it should play a part in the scene that makes it a strong visual element.

While you should generally avoid making images that have you pointing the camera at the sun, a silhouette is created by doing just that. The difference is that the silhouetted form should block the direct rays of the sun, yet your exposure reading should be made off the background, not the foreground silhouetted subject. If the light in the background is bright enough and you avoid the foreground in your exposure reading, you can be assured that the foreground will become silhouetted. This is best accomplished using a spot metering pattern.

 The sky was hazy and the sun was near the horizon and I spotted this lighthouse that served as a great sun blocker and potential silhouette. I moved around until the structure blocked the sun and made a reading from the brightest area in the background.

Settings: With a 210mm lens: at ISO 200, f/22 at 1/500 second. Spot exposure reading made and locked off the bright background.