For January's monthly project, Tom Persinger is joining us again to examine how depth of field and experimenting with point of focus can add interest to your photography. Please add your photo to the...
Back in the days when dinosaurs roamed the earth and photographers used totally mechanical cameras, it was common to have a camera serviced every few years. The springs in the shutter assembly had to be adjusted to keep shutter speeds accurate and various parts of the camera and lens could need lubrication. A CLA (clean, lubricate and adjust) was something that kept a camera in top condition.
Current cameras (and indeed most cameras made in the last few decades) really don’t need a regular CLA. The shutters are electronically timed and in general no lubrication is required. Wiping off external dirt is about all the servicing that most film cameras ever needed. With digital SLRs the situation is slightly different since and dust or dirt on the sensor will be recorded in the same place on every image and despite automatic cleaning systems, you will eventually find dirt stuck to the sensor that shows up in your images.
When (and what) not to clean
If you see dark spots on your digital image which you suspect come from dust and dirt, remember the following. It’s 99% certain that they are due to particles sitting on the front of the filters which cover the digital sensor. Dust elsewhere in the camera or in the optical path generally won’t show up in the image as dark spots. Even though most current sensors have systems in place to minimize the amount of dust which is attracted to the sensor (by using an anti-static coating) and to displace it (by vibrating the sensor) if it does land there, these systems aren’t perfect and it’s very likely that sooner or later a physical cleaning of the sensor will be required.
So, for example, if you see dark dust spots when you look through the viewfinder, they will be on the viewfinder screen, not the sensor. Since the viewfinder screen isn’t in the optical path of the light when an image is recorded, dust on the screen doesn’t affect the image in any way. These dust spots may be annoying, but they won’t cause spots in your images.
If you remove the lens and look at the reflex mirror inside the camera, you may see some dust on the surface. Again, these won’t appear in the image. When an image is recorded, the reflex mirror is safely out of the optical path, so no amount of dirt on the mirror will show up in the image.
If you look though your lens and see dust spots on the elements (which you will), again they won’t record on the image. Though in this case they are in the
optical path while an image is being recorded, they will be so far out of focus that they will not be visible in the image. About the only exception to this would be dust on the front element of a circular fisheye lens when shooting stopped down at minimum focus distance. In that case the DOF of the lens might just get the front element in focus!
The upshot of all this is that cleaning the viewfinder, mirror and lens surfaces won’t help in removing spots from your image, and in fact may actually cause harm if you aren’t careful. The SLR mirror, for example, is fairly delicate. It’s a front surface mirror and can easily be scratched. The viewfinder screen has a fine, rough texture and if you try to clean it by rubbing it with any type of cloth of lens tissue, you’ll get fibers all over it.
If you must clean the mirror or viewfinder screen, the safest way to do it is with a blower bulb. With luck this will blow any dust particles off the surface. Pretty much the same applies to lens cleaning. You can only clean the front and rear elements anyway. A blower bulb and a brush should remove any surface particles. It’s also safe to use a lens cleaning cloth and lens cleaning fluid on lens (and filter) surfaces as long as you use items intended for cleaning photographic lenses. The tissue and fluids used for eyeglasses may contain unwanted ingredients such as anti-fogging agents.
How do you know if it’s dust on the sensor?
The defining characteristic of dust on the sensor is that the smaller the aperture you shoot at, the more visible it becomes. At large apertures dust spots blur out and often are very hard to see. At f/29 they will show up as definite and well defined dark spots. The way to see just how much junk there is on the sensor is to take an image of a uniform target with the lens defocused and set to its minimum aperture. You can use the blue sky or a sheet of white paper or a wall as your target. You don’t need to hold the camera still so long exposures don’t matter. You’re simply trying to uniformly illuminate the sensor.
To really see all the debris on the sensor you can enhance the image contrast by stretching the histogram. If you do this be prepared to see all sorts of junk on the
sensor, as well as image vignetting and even possibly some sensor non-uniformity, so don’t worry if the image looks really bad. The important thing to remember in this case is that if you don’t see it in an actual image, then it doesn’t really matter if it’s there. You don’t do extreme histogram stretches or contrast enhancements with actual images. This is just as well, since it’s virtually impossible to remove all detectable marks when cleaning a sensor, and even if you did, after a few days (or maybe even minutes) of use, you’d start to see a new set of minute dust spots.
When cleaning a sensor your goal is to get an image which doesn’t show anything objectionable when shooting your uniform target at the minimum aperture you
actually use and with no more contrast enhancement than you would be likely to use on real world images. Attempts at repeated and excessive cleaning to remove every last speck of dust seen in a histogram stretched image are far more likely to do more harm than good.
There’s an alternative way to detect dust on the sensor and that’s to use a loupe (magnifying glass) to directly see the dust particles. Magnification from 5x to 10x is enough to visualize the dust. There are loupes with built in illumination which makes this process easier (e.g. the VisibleDust Sensor Loupes). Of course the ultimate test is to make sure the actual image has no dust spots on it, but using a loupe can certainly be faster than taking images during the cleaning process to see if that last stubborn dust spot is still there!
So if you have spots on your images that you have confirmed are due to dust on the sensor, how do you clean it?
DSLR Sensor Cleaning Mode
Pretty much every DSLR has a mode for sensor cleaning in which the mirror is flipped up and the shutter held open. This then allows direct physical access to the sensor (or rather the filter which is over the sensor). The only thing to remember here is that it’s wise to have a fully charged battery when you do this. The last thing you want is for the battery to drain, the mirror to return and the shutter to close while you have something stuck inside the camera!
Blowing dust off the sensor
The first thing to try is blowing the dust off the sensor with a blower bulb. Don’t use compressed air or any of the “canned air” products. Too much air pressure can cause damage and the various forms of “canned air” can sometimes spit out liquid. A small blower bulb which you squeeze to get a puff of air is sufficient. Most of the time you’ll find it really doesn’t do a lot of good, but since it’s cheap and has essentially zero possibility of doing harm, it’s a good first step to try. You can find Inexpensive blower bulbs (e.g. the Giottos “Rocket Air Blaster”) or you can pay more for a high-tech version with filters and antistatic properties (e.g. the VisibleDust “zeeion”).
Brushing dust off the sensor
When you find that you still have dust on the sensor after trying the blower bulb, the next step is to try one of the various commercial brush systems which are designed for use on digital camera sensors. A brush can dislodge particles resistant to be air and it can also trap the dust particles and remove them from the camera. Of course you need a very clean brush (or it will deposit more on the sensor than it removes) and, if you believe the advertising, it doesn’t hurt to have a super high-technology brush using special materials and specially shaped brush fibers.
Visible Dust (www.visibledust.com) specializes in such high-tech cleaning appliances. Most use static charge and ionization in the cleaning process since dust is most easily moved around using electrostatic charges. The Visible Dust brush spins under battery power and this is said to both remove dust from the brush by centrifugal force and create a small static charge on the brush to attract now dust from the sensor. You can read the technical details on their website if you are interested in the science behind the products. As a scientist, I’d say their explanations make some sense, though I’m somewhat sceptical about how much better the high-tech devices are then “low-tech” blowers and brushes. Personally I still use “low-tech” products because (a) I’m cheap and (b) I have quite a bit of experience in cleaning of delicate optical systems. The advantage of the “high-tech” route is that you’re buying products designed for delicate cleaning work and from a manufacturer who can give technical support and who guarantees their product. The dedicated products may cost more than “generic” versions, but they are less likely to cause damage and may do a better job.
Wiping dust off the sensor
If the specs on the sensor are still there after a high tech brush cleaning of the sensor, the final “nuclear option” is to clean the sensor with tissue and a solvent. Again there are companies who specialize in providing materials designed for this task. One such company is PhotoSol (www.photosol.com) who have a line of solvents and tissues (Sensor Swabs) marketed under the “Eclipse” name. Again the advantage of dealing with a company which produces products designated for digital sensor cleaning is that you can get technical support from them and their products are guaranteed to cause no harm when used as specified. Cleanliness is paramount when using swabs and cleaning fluids. Use the swabs only once and use the minimum amount of solvent required to dampen the swab. Pressure is generally not needed (and may be counterproductive), so a gentle wipe across the sensor is the way to go. After one wipe check the sensor and only repeat if necessary. As I said earlier, there’s no point in trying to remove every last minute speck of dust. If nothing shows up in real images, the sensor is clean enough.
Personally, I use conventional lens tissue and methanol to clean my sensors, but again I have experience and I’m willing to take the risk. So far I’ve had no problems, but I’d recommend going the commercial product route to anyone who is at all nervous about sensor damage.
What’s the worst that can happen
The worst that can happen is that you scratch the sensor while attempting to clean it. If you are careful and you use professional cleaning tools this is highly unlikely. Scratching can occur if you get a small piece of grit on the tissue you use to wipe the sensor. Commercial products for sensor cleaning are made in clean rooms and are very unlikely to be contaminated. A typical book of lens tissues is certainly more likely to be contaminated. They are cheaper, but the risk of damage is higher. The sensor covering filters isn’t especially delicate, in fact it’s similar to the coated filters you’d use on a lens, but even so it can be scratched.
If you do scratch the sensor (or more accurately the front of the filter stack that covers the sensor), you’ll get a black line on every image and the filter over the sensor (or in some cases the whole sensor assembly) will have to be replaced. This is a job for the factory service center and will cost several hundred dollars for most cameras. This is clearly not desirable and for older digital SLRs it may not even make economic sense.
The commercial solution
If you’re really worried about damaging the sensor you can find service centers that will do it for you. Some camera manufacturers will do it for free (at least for a new camera), though of course for most users that means shipping the camera off and waiting a week or more for it to be returned (plus you’ll pay shipping costs). A number of camera stores and independent repair centers will also clean sensors with a cost somewhere in the $30-$50 range being typical. Some may do a good job, others not so good. In general, I’d recommend doing it yourself because it’s much faster, more convenient and in most cases cheaper. However, if you decide not to then commercial cleaning is available.
The software solution
Quite a few cameras have a software based “dust deletion” feature. These generally work by first taking a blank frame which the camera uses to record the position of any dust particles on the sensor. This information can then be used (normally during RAW processing) to automatically clone out dust spots on the image. If you’re stuck in the field without the chance to do a sensor cleaning and you know you have a problem, then using this software solution can save you a lot of work later manually cloning out each dust spot on each image.
Minimizing the need for sensor cleaning
The best way to clean the sensor is, of course, not to get it dirty in the first place! While it’s not really possible to totally eliminate sensor dust, there are some things you can do to minimize it.
Dust gets into the camera when you change lenses, so it’s wise to be cautious when changing lenses, especially in dusty conditions. You can change lenses quickly, with the camera pointing downwards for example. If you must change lenses in very dusty conditions, you can do so in a protected environment, such as inside your car rather than outside, or even inside a protective plastic bag. Another alternative if you know the conditions are going to be bad is to choose a wide to telephoto zoom, which may minimize the number of lens changes that you need.
I’d give two pieces of advice about sensor cleaning. First, don’t do it unless you really need to and if you do, apply the minimum amount of cleaning that’s required to get the results you want on real world images. Second, use products designed for use on digital sensors, even if they are more expensive than “generic” cleaning products. Cleaning a digital sensor really isn’t difficult and it’s something pretty much anyone can do. Eventually all DSLR sensors will need cleaning and it’s much more convenient (and cheaper in the long run) to do it yourself than having your camera cleaned by a repair shop. You may even do a better job too!