Today’s Question: Could you provide a more detailed description of how to capture a test shot to check for dust on a camera’s sensor? I appreciate that a loupe provides benefits, but sometimes I don’t have a loupe with me when I am traveling.
Tim’s Quick Answer: You can create a test capture to check for dust by stopping your lens down all the way to the smallest aperture size, and then capturing a normal exposure of a “clean” subject, such as a blank wall or sheet of paper. The photo can then be evaluated for dust spots and other indications of blemishes.
More Detail: As noted in a previous Ask Tim Grey eNewsletter, you can capture a test photo when you’re not able to use a loupe to evaluate the sensor directly for dust. I generally prefer using a sensor loupe to evaluate the surface of the sensor directly, as noted on the GreyLearning Blog here:
However, obviously at times you may need to check the sensor for dust when you don’t have such a loupe available. In that case a “manual” approach can work very well.
The first thing to do is locate a subject you can photograph that won’t have much in the way of texture that could cause ambiguity about where any dust spots might be. I typically use an empty wall or a blank sheet of paper for this purpose.
Then, configure your camera for a normal exposure of that subject using the minimum size lens aperture (such as f/22 on many lenses, for example). In many cases (especially if using a white subject for your test photo) it can also be helpful to increase the exposure by about one stop above what the meter indicates.
So, for example, in Aperture Priority mode with the lens aperture stopped down completely, you could set one stop of “plus” exposure compensation. Then point the lens at the intended subject, and capture the photo while moving the camera constantly during the exposure. This will help ensure that nothing in the photo appears sharp except for dust spots on the sensor that are blocking light from reaching the sensor.
After the exposure is completed, you could review the image on the camera’s LCD, zooming in and panning around to check for any blemishes. However, I recommend downloading the photo to a computer so you can get a bigger and closer look at the image.
Based on this evaluation, you can clean the sensor as needed, repeating the process of evaluating and cleaning until you have a thoroughly clean sensor.