John Paul Caponigro's "Sensor Dust"

March 27, 2011

John Paul Caponigro's "Sensor Dust"
...imagine retouching sensor dust in one test exposure, and then being able remove it in every subsequent exposure, automatically. 

Dust happens. 

Even in well-sealed professional digital cameras, dust can find its way into the mirror box, and from there to your camera’s imaging sensor. Once there, it appears in every shot you take.  You’ll tend to see any sensor dust when you look at a magnified view of an image on your computer screen, and/or if you make large prints.  The dark spots are most visible when they fall upon a light-colored area of the subject you’re photographing.  And they are sharper and more defined when you shoot images with wide-angle lenses, and/or at small lens apertures.  Thus, a photographer who shoots lots of scenic images with a wide-angle lens at f/16 or f/22 will be especially prone to seeing the effects of dust, while a sports photographer who typically uses a 400mm f/2.8 lens at wide apertures for indoor sports (which often have dark backgrounds) may scarcely notice it at all.  

This article centers around three fundamental issues:

1.    Minimizing the build-up of dust in your camera
2.    Testing for and removing any dust that may be present on your imaging sensor
3.    Simplifying the process of removing dust from your finished images


Minimizing Dust (in camera):

Reduce the amount of cleaning you have to do by minimizing the practices that can allow dust to collect on your sensor — changing lenses, using zoom lenses, etc.  Use body caps whenever a lens won’t be attached to your camera for longer than a few seconds, and be sure the cap is clean before you put it on.  Store and handle your equipment with care in as sterile an environment as practical.  I carry all of my cameras and lenses in sealable plastic bags to reduce exposure to dust and moisture.  Don’t be afraid to change your lenses when you’re on location, but use common sense, and do so quickly and efficiently.  Good habits will reduce the amount of work you have to do — before and after shooting.

Some digital cameras, including many current Canon EOS digital SLRs, have a built-in system to physically “shake off” any dust or foreign matter that may be clinging to the front of your imaging sensor.  These systems are generally quite effective at reducing the amount of dust you may see in your images, but no such system is 100% perfect.  From time to time you may begin to encounter small dark specks in your images.

Test for Dust:

For critical photographers, it makes sense to know when any sensor dust may be present — before you shoot pictures.  And the easiest way to find out is to take a simple test picture.

If you’re using one of the recent Canon EOS digital SLRs with the integrated cleaning system, you’re in luck — there’s a built-in system to automate the process of testing for any remaining dust that may still be clinging to the front of your sensor, and removing the dust automatically in Canon’s Digital Photo Professional software (version 3.2 and higher).  If you’re using an EOS model without this feature, or a competitive brand camera, all is not lost.  Below is a procedure that allows essentially the same thing, using a test picture you take, and RAW image conversion software from Adobe (other RAW processing software and/or image-editing software may also have similar capability). 

Here’s how you make a test picture for use with Adobe’s RAW processing software:

Make an exposure of a flat field of light color with little or no texture — a blue, gray or white sky, even a gray card;  it’s easier to see the dust on lighter colors.  One example that works is a shot of the clear north sky.  You should shoot this with a fairly wide-angle lens, and be sure to set a small lens aperture, such as f/16 or f/22.  ImagingExpo’s ExpoDisc™ (expodisc.com) is another excellent tool for this;  besides creating a test image for dust, it  this third-party accessory can be used to set white balance, correct for vignetting, and check and correct color consistency in a chip. 

Open the file on your computer and zoom into at least 100% on-screen, in either a browsing software program, or an image-editing program like Adobe Photoshop™.  Then sweep through the image quadrant by quadrant to make sure your sensor is clean.

Do this at the beginning of every photo shoot, and you accomplish two things:  you’ll know if your sensor needs to be cleaned, and you’ll also have a test image you can use to automate the process of removing dust from your images after you shoot them.  More on that in a moment.

Removing Dust from the sensor:

If a review of a test image on your computer screen shows some tell-tale dark spots, one obvious option is to clean your camera’s imaging sensor.  However, tread carefully.

1.    Your camera’s imaging sensor, and the multi-layer filters that are in front of it, are incredibly delicate and easy to stain, scratch or even crack if you touch them carelessly.

2.    Replacing the imaging sensor in a digital SLR is extremely expensive —it can easily cost well over $1,000.

3.    If you damage an imaging sensor by trying to clean it, it will never be covered under your camera’s factory warranty.

Canon’s service department recommends that you start by trying to blow off any dust, using air from a rubber bulb (similar to a large ear syringe).  These are frequently available in well-stocked camera stores.  However, do NOT use “blower brushes”, or any brush-like device to wipe off an imaging sensor — these are dust and grease magnets.  Likewise, Canon does not recommend using canned or compressed air to blow off the sensor.  It’s far to easy to either accidentally get some propellant onto the sensor surface, or to put too forceful a blast of air into the camera, which can damage the sensor or other components in the mirror box.

The camera’s shutter will have to be open, to be able to blow any dust off of the sensor.  EOS digital SLRs have a menu setting to do exactly this, called “Clean Sensor” or similar wording;  with newer models with the self-cleaning sensor (see above), the menu item you want is “Clean Manually”.  Have your camera’s battery fully-charged before you start — it requires constant battery power to keep the shutter open, and you don’t want it closing suddenly and unexpectedly as you try to clean inside the mirror box!  For the same reason, you shouldn’t use the camera’s BULB setting unless there’s no other option to keep the shutter open. 

Be sure to hold the camera so the open lens mount is facing downward when using a blower, allowing any dislodged dust to fall away from the sensor and out of the camera. 

Suppose you carefully blow off the imaging sensor, and after you take another test image, you still see some remaining dust in your images that’s clinging to the sensor and just won’t blow off?  That leaves you with three options:

1.    Send the camera to a trained repair technician to have the sensor professionally cleaned (highly recommended by Canon — Canon’s service department strongly suggests not to ever touch the camera’s imaging sensor)

2.    Try to clean it yourself, using commercially available sensor cleaning “swabs” or other products  (be aware that if you decide to do this, you’re taking a small but significant and potentially extremely expensive risk of permanently damaging the sensor). 

3.    Shoot images with the dust on the sensor, and take steps in the computer to “clone-out” the dust and remove it from finished images.

Dust Mapping

Here, too, you have a few options.  For years, digital camera users have known that they can use the “clone tool” in Photoshop and other image-editing software programs to remove small dust spots and other imperfections in an image.  The problem with this traditional method is that it’s very time consuming.  You may not feel it’s a problem if you only are trying to clean up one or two finished images -- but if you’ve shot an entire vacation worth of image files and find out there’s dust on your imaging sensor, you'll definitely need a faster and more automated method. 

Dust happens.  But imagine retouching sensor dust in one test exposure, and then being able remove it in every subsequent exposure, automatically.  Better yet, don’t just imagine it — do it.  All it takes is one preparatory exposure before each shooting session and dust in every image from the session can be retouched — automatically.  This is a process called Dust Mapping.

We’ll first describe what’s necessary to take a RAW test image and create an automated dust-removal workflow using Adobe’s RAW file converters.  Then, we’ll describe Canon’s built-in “Dust Delete Data” solution in recent EOS models with the integrated cleaning system.

With Adobe software

The latest RAW converters from Adobe, Lightroom™ and Camera Raw™, both include automatic dust removal tools:  the Remove Spots tool in Lightroom and Retouch Tool in Adobe Camera Raw.  Other third-party raw conversion software and image-editing applications may also have similar tools to automate the dust removal process.  We’ll show you here how to do this using Adobe’s converters.

Shoot a RAW test image of a flat field of color.  ImagingExpo’s ExpoDisc (expodisc.com) is an excellent tool for this.  Even a shot of a sheet of white paper will work acceptably well.  Just be sure the test shot is taken at a relatively small aperture, and that it’s done with a standard or wide-angle lens. Do this every time you change lenses, the most likely time that dust will collect or shift on your imaging sensor.  Download the file and open it in your RAW converter.  Use the dust removal tool to clone or heal (I favor healing) dust spots.  The cloning/healing can then be applied to all exposures made during the same session.

Let the programs pick the clone/healing source.  If you do, the program will automatically search for the best source in each individual exposure.  If you set the source manually, the vector and distance you define will remain the same for every exposure, increasing the likelihood that a cloning/healing source will be incorrect.  Even if the first cloning you do appears to be precise, those same settings may generate visible errors in other images with the same dust spots, but different surrounding subject or background matter.

In Adobe Lightroom, retouch the dust map (the test image).  Select the other exposures in a session.  Click Sync.  Click Check None.  Click Spot Removal.  Click Sync.  Click Check None, then click Synchronize in the Synchronize Settings dialog box.

In Adobe Camera Raw, retouch the dust map and click Done.  Select the other images to be retouched and open them in Adobe Camera Raw.  Click on the dust map first, click Select All second, click Synchronize third, check Spot Removal (unchecking all other fields) fourth, and finally, click OK.

Every selected image will now be retouched.

Taking a Test Image with Canon’s Dust Delete Data function:

This function is possible only on recent Canon EOS cameras which have the self-cleaning sensor and integrated cleaning system built-in.  It’s not possible to update previous EOS models to add this feature.

Even though the self-cleaning sensor function normally does an effective job of minimizing dust build-up on the front of your imaging sensor, it still can occasionally happen.  Before you take important pictures, you may want to take a special in-camera test image, called a Dust Delete Data image. 

You need to select a fairly specific target to do this;  you can’t just take a picture of a person standing next to you.  You’ll need to do the following:

a).  Have a solid, plain white object readily available (a piece of paper, a blank section of wall or ceiling, etc).

b).  Use a lens with a focal length 50mm or longer

c).  For the Dust Delete Data image, set the lens’s focus switch to MF (manual focus mode), and pre-set focus to infinity.  If your lens doesn’t have a focus distance scale, look at the front of the lens, and turn the focus ring clockwise until it stops.

d).  Position yourself no more than 1 foot (30cm) from your subject — even though you’ve pre-set your lens’s focus to infinity.

In the camera’s shooting menu, select “Dust Delete Data” (Fig. 1).  The LCD monitor will display the last time this was performed, and by highlighting OK (Fig. 2), you are telling the camera to take another test image.  The camera will perform its self-cleaning function automatically (Fig. 3). With some models, you’ll hear it “click” the shutter one or more times — this is not the actual test image.

Press the shutter button when prompted (Fig. 4).  Once the camera assesses the info (Fig. 5), the LCD monitor will display “data obtained” if your target and technique were sufficient (Fig. 6).  If not, try a different plain, white test target, and be sure your lens is set as described above.  Once you get the Data Obtained display, highlight OK and press the camera’s SET button.

You’ve now got test data that will be added to each image you take from this point onward.  If you have to change lenses in a dusty environment after this, you may want to repeat the procedure to account for any possible additional dust that may be present.  Please note that the Dust Delete Data test image is NOT a picture you’ll see on your memory card.  The camera simply records any data about dust size and location, and applies this seamlessly to each image you take from that point onward.  It has almost no effect on file size.

Unlike some other techniques, Canon’s Dust Delete Data can allow automated correction of dust to any image you shoot:  RAW, JPEG, or “small RAW” on cameras with the sRAW setting.

Automatic Dust Removal with Digital Photo Professional Software:

Once you perform the Dust Delete Data test image, there’s nothing more to do in-camera.  Each image you take from that point onward will have extra information added to it, identifying the size and precise location of any identified dust spots.

If you use the supplied Canon Digital Photo Professional software (version 3.2 or higher) to browse the images, you can select all your thumbnails on-screen, and in the Adjustments pull-down menu, select “Apply Dust Delete Data”.  Once you do this, all the images will be adjusted to remove any detected spots, whether it’s a few images or hundreds.

A couple of quick notes about the automated dust removal in Canon’s DPP software:

1.    In some cases, if dust is extremely small, it may not be identified by the Dust Delete Data system.  Any such spots will have to be manually touched-up.  (This can be done using DPP’s “Stamp Tool”.)

2.    If the Dust Delete Data test shot revealed no identifiable dust spots on your sensor, you’ll get a notice of this on-screen when you select “Apply Dust Delete Data” in the software.

3.    If no identifiable dust is present on images after you’ve taken a Dust Delete Data test shot, the dust removal tools in the Stamp Tool window in DPP (usually used to manually remove any dust) may be grayed-out and not available.

4.    Dust Delete Data results will show in DPP’s Stamp Tool window, but if you simply enlarge an image with the software’s Edit Window, any spots will be visible on-screen. This is normal.  When you transfer an image to Photoshop or another image-editing program, or ask DPP to save a file (JPEG or processed RAW image), the dust removal will be in effect.

Summary:

Regardless of how you perform it, any steps you can take to speed up your workflow and spend less time manually removing dust spots will be a big help in your photography.  After all, less time at the computer means more potential time to be out taking pictures!  Think about which procedure works best for you, and consider the importance of periodically taking a test photo and using it after shooting to automatically remove dust from batches of pictures.  You can’t always totally eliminate dust from latching onto our imaging sensors, but you can minimize the hassle in removing it from finished images.  It’s a powerful practice;  retouch one image and every image will be retouched.  Dust mapping saves you an enormous amount of time.

 

John Paul Caponigro is an internationally respected fine artist, a member of the Photoshop Hall of Fame, author of Adobe Photoshop Masterclass, the DVD series R/Evolution, and a renowned Canon Explorer of Light. Learn more about him and his work at The Canon Digital Learning Center, or at www.johnpaulcaponigro.com

The CDLC contributors are compensated spokespersons and actual users of the Canon products that they promote.

All images are copyright JP caponigro, Erika Silverstein, Rudy Winston

http://learn.usa.canon.com

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