f/22, nearly a pinhole aperture|
slow, metered for good exposure
blurry as possible
|good sensor survey technique|
Cameras with removable lenses are very versatile. Put on a wide-angle lens and take photographs of great landscapes and beautiful ballroom scenes. Replace it with a telephoto lens and take a photo of that bird in the trees or your favorite baseball pitcher on the mound.
However, one shortfall of removable lens cameras is the fact that dust can get inside the body, close to the film or digital sensor. When dust is near the sensor, it can show up more clearly in photographs.
Many people often don't notice dust until they see the same spot in two photographs, but once you see it, it is an ugly distraction to the otherwise perfect image.
Here is a technique to perform a "Sensor Survey," which allows you to see where the dust might be lurking on your sensor, ready to ruin your next portrait or landscape photograph.
- Attach any lens to the camera, and turn off automatic focus.
- Set the camera to a Manual (M) or Aperture-priority (Av) programmed mode.
- Adjust the aperture to a near pinprick size, such as f/22.
- Meter your shutter speed for the open sky, or your ceiling.
- Manually focus to the blurriest possible image.
- Take a handheld shot of the sky or ceiling.
- If the shutter is slow enough, you can even wave the camera around during the shot to further blur all details out of the image.
The survey photograph should look like a featureless sky-blue or ceiling-gray frame. If you can see dust spots in this blurry frame, you know you have some cleaning up to do!
Usually, survey photographs have almost no contrast, and small dust motes won't show up very prominently. If you want to study the sensor a bit closer, you'll need to improve the contrast.
- In the GIMP or Photoshop, adjust the Curves for the image.
- You can drag the curve to adjust its shape.
- Experiment for the best contrast. A typical example is below.
- (Sometimes a simple automatic "Auto-Levels" feature works well.)
Just like the human eyeball, the film or sensor will capture the image upside down. A dust spot in the upper left corner of your survey image is actually caused by dust near the lower left corner of the sensor (as you look through the body's front opening).
If your survey was taken at f/22, and a dust spot was still blurry, you might want to check the rear element of your lens instead of the sensor.
Lines on the image may be more serious. These can indicate scratches on the protective glass filters on the sensor, or scratches on the rear elements of your lens. They may also be a sign that your cleaning fluid has left streaks.
Contact Ed Halley by email at
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