I’ve been staying up much too late watching Ken Burn’s “National Parks” film and so I was rather annoyed to find that I slept in a bit late on a rare foggy morning. While wading around in the river looking for visual moments in the rapidly decreasing fog I found a nice splash of color and proceeded to shoot there for the next 20 minutes.
Unlike the grand, sweeping landscape shots of mountains or lakes, these intimate landscapes require a very high aperture setting to obtain any significant depth of field. Here I’m shooting at f/13 and the scene really called for even more, but tradeoffs start building up quickly after f/11 (diffraction, exposure time, etc.)
The physics of light giveth and they also taketh away. With a digital SLR cameras, high f-stop PLUS high contrast = dust spots. The increased aperture alone emphasizes spots on the sensor, but if you start raising the black point of your images to increase the contrast during post-processing, you can expect to see dust spots popping out of the woodwork — especially in areas of smooth tonality – skies and water, for example.
Getting rid of dust spots used to mean heading over to Photoshop for a session with the cloning tool, but both Lightroom and Aperture (and other photo software) have recognized the ubiquity of the DSLR dust spot problem and provide tools to help clean up your images. Here are a couple of tips for Lightroom:
1. Since all of the image changes in Lightroom are non-destructive, jack up the black point to over-emphasize contrast difference. The spots will make themselves much easier to find. You can do your spot cleanup and then just return the black point to the setting you want for creative purposes.
2. If you zoom in 1:1 on the image, you can use the PageUp and PageDown keys to quickly cycle through each segment of the image. This is far easier and more methodical than dragging the image around.
3. However, dragging the image around at 1:1 zoom has advantages that should not be overlooked. After you have paged through, use the drag tool to “jiggle” the image. There are two reasons for this — while the paging method is complete you will tend to not notice spots along the edges, so a quick dash around with the dragging tool takes care of the “seams”. The second reason is that the human vision system has special processing for movement and edge detection. By jiggling the image you will see spots that you might otherwise overlook. If you’re not using those neurons to stay alive on the savannah anymore, you can at least put them to use making nice photographs. 
4. As with the black point control, you can also find spots using another Lightroom tool in a way it wasn’t designed for: the sharpening tool’s masking setting. Zoom in 1:1. Then hold down the Option key (Alt on PC) and click down on the masking control. The display will change to a black and white contrast map of the image — essentially it is displaying all of the edges for you. By raising and lowering the masking you will find that at a certain level (it varies with the image) the dust spots will jump out at you. As a matter of fact it’ll probably find far more than you want to know about! Just as with the black point control, remember to reset the masking to your desired creative setting after you’ve hijacked it for this particular bit of problem solving.
5. Don’t forget that dust spot removal can be applied (synched) across images! Within a short span of time the spots are unlikely to increase/decrease/move on the sensor so once you’ve done the hard work of finding them, you can reapply them to multiple images. HOWEVER, unless the scene is completely static, you will want to review the fixes at 1:1. The dust spot tool is an intelligent clone tool, but you will probably find that you need to tweak the source of the fix from image to image.
How many dust spots do you see?
Here’s the Masking tool’s view:
You can avoid, or at least reduce, all of this work by keeping your digital SLR’s sensor clean. The Cleaning Digital Camera’s website is a great reference. If you are squeamish about cleaning the sensor you can often send your camera back to the manufacturer for a tune-up.
In my experience dust spots tend to be more visible in prints than on the screen. So before you commit ink and paper on an image, spend some time looking for dust spots. Your piggybank and the environment will thank you.
