Recognizing CMOS spots

How do you know when it’s dust on your CMOS and not a particle you’re seeing on your camera’s mirror or lens?

Dust on your mirror won’t appear in an image at all; it will just affect what you’re seeing through the viewfinder. Conversely, as shown in 10-4 and 10-5, the most telling feature of a CMOS dust spot is that it appears consistently from image to image. If you change lenses, it’s still in the photo at the same location. And you won’t see a CMOS dust spot when you look through the viewfinder — it only appears in the digital image. You can occasionally see a CMOS dust spot when you look at a photo on your camera’s LCD screen, but it’s usually very difficult to see there.

The two photos in 10-4 and 10-5 were taken at two different times of day from the window of my hotel room on Copacabana Beach in Brazil's Rio de Janeiro. Note that the bright sky is where the spots are most visible. (Can you see other spots other than in the sky, particularly in the daylight image?) Some of the particles are obvious, while some are more subtle; these latter ones are more visible in the daylight image. Both images taken with a 1D Mark lln and an EF 24-70mm f/2.8L lens. Figure 10-4 was taken at ISO 100, 1/1000 second at f/11, and 10-5 was taken at ISO 100, 1/500 second at f/11.

Dust particles on the CMOS often travel in packs, and you’ll see multiple spots in your image. Some will come off easily, while others may persist and require more serious intervention to get them off the sensor.

In many cases, you can use image-editing software to remove dust spots after the fact. Taking a small dust spot out of a blue sky, for example, is a very simple task in Canon Digital Photo Professional, Photoshop, ACDSee Pro, and other image-editing applications. However, when the dust spots collide with complex elements in your photograph, things get tougher to repair. For example, take a look at this image of Rio de Janeiro’s Sugar Loaf Mountain (see 10-6), which features a number of spots on the CMOS. While most are in the clear blue sky and come out easily, the spot that is sitting directly over the gondola cable is anything but easy to remove with software. Almost any alteration you make using a Healing Brush, Clone Stamp, or other tool will cause you as many problems as you try to repair.

This image of Rio de Janeiro's Rao de Agucar (Sugar Loaf Mountain) in the early morning is marred by a CMOS spot just over the gondola cable. While it appears to be a simple spot, fixing it using Photoshop's Healing Brush or similar tool is difficult because of how clear the sky is behind it, and because the cable will move and change as it's being fixed. Taken with a 1D Mark lln using an EF 24-70mm f/2.8L lens at 1/1000 second, f/5.6, at ISO 50.

Sometimes it’s difficult to tell if there’s really a spot on your sensor, especially if you’re shooting photos with a lot going on in the image (as opposed to, say, a clear sky). If you suspect you have a spot, then you’ll want to test to see if it is indeed the case by shooting something bright and clear: a blue sky without clouds, a large light source (if you have a studio soft box, that’s perfect), or any subject that provides you with even light against which a dark little particle will show itself. Then download the image and look at it on your computer screen to see the dust.

Category: Camera care

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