This article appeared in the July 2019 issue of the Journal of the Photographic Society of America and is reproduced with the permission of the author.
Dr Greg Duncan, APSA, GMPSA/b, GPSA, EFIAP/p, AV-AFIAP explains why smaller apertures produce greater dust problems.
Issue 1.0
Why do I sometimes have dust spots in my pictures?
We all know that smaller apertures produce greater depth of field. Taken to extremes, a pinhole camera produces an image where everything is in focus from the front to the back.
In the days of film, using f22, f32 or even f45 may have produced a bit of diffraction and chromatic aberration but that was the only major problem. Not now with digital photography. The smaller the aperture the greater the depth of field and the greater the impact of sensor dust.
F22 is a Catch 22. If you want greater depth of field, then you will have larger and more pronounced dust spots. In Figure 1, the image on the left was taken at f2.8 and has no dust, but the one on the right was taken at f22 and shows significant dust.
Fig 1 Image on the left taken at f2.8, Image on the right taken at f22
In a digital camera the sensor consists of millions of tiny light receptors whose dimensions are measured in microns. Above the sensor is a transparent filter layer which not only protects the sensor but can also filter out various wavelengths of light such as infra-red. This layer is also where the dust collects.
At wide open apertures, the light comes from many directions and some of the light can get around the dust and hit the sensor receptors located beneath the dust spot. See Figure 2 (not to scale). As a consequence, at wide apertures dust does not usually cause a significant problem.
Fig 2. Light can pass around dust at F2.8
However, at smaller apertures, the beam of light from the lens is narrower and the dust will cast a shadow on the sensor as shown in Figure 3
Fig 3. Light casts a dust shadow at F22
At very small apertures, virtually no light will hit the sensor and the image will have a dark dust spot.
The two figures demonstrate that as the aperture is reduced to obtain greater depth of field, the light beam narrows, the dust casts a stronger shadow and your images will show dust spots.
As if that was not bad enough, the location of the dust in your image is dependent upon the focal length of the lens. Figure 4 shows that when you use a long lens the same piece of dust on the filter layer will create a dust spot which is closer to the center of the picture than when you use a short lens. This makes it difficult to use any automated dust removal process which relies on a dust map image - you would need a dust map for each focal length of your zoom lens!
Fig 4. Shows why the same piece of dust casts shadow in different places
Testing for dust
Although you could use a loupe to examine the sensor for dust, that method of checking for dust exposes the sensor to the air and therefore even more dust. Instead, you can test for dust using the following simple closed system.
Using a long focal length lens, set the camera to the smallest possible aperture of f22 or f32 or above and set the exposure compensation to +2 or more stops OVER EXPOSED. Turn off the auto focus and set the focus to infinity. If you normally shoot in RAW, then fine. If you normally shoot in JPEG, then set the quality to the largest best quality JPEG. Now photograph a plain bright evenly lit surface (such as a sheet of paper or a white wall) from about 10 to 15 cm away so that it completely fills the screen and is completely out of focus. Do not worry if it is a long exposure - this makes no difference whatsoever. (If you are outside, clear cloudless sky will work.)
Using the display on the back of your DSLR, zoom in as far as it will go and move the zoom point to one of the top corners. Now scan back and forth and down across the whole image. If you have dust, it will show as a dark spot. Repeat the test at f11 to see if the dust is going to be significant. If the dust still shows at f11 you will need to clean the sensor and repeat this simple test. It may take two to three cleanings to obtain a satisfactory result.
If you do your own cleaning (it is not difficult), remember to hold your camera upside down so that your cleaning brush is pointing up. Otherwise you risk dropping dust on to the sensor as you try to clean it.
Minimizing the dust effect
Although many cameras have built-in dust reduction facilities, there will be dust on the sensor. The following guidelines can help minimize the accumulation and effect of this dust.
· When taking landscapes on a tripod shoot the full range from f2.8 to f22 with image stabilization OFF. If there are dust marks on the f22 target image, it is often possible to clone in the affected part from a more open image. This process works particularly well if the dust has appeared in the sky.
· Test your sensor frequently using the simple test given above, especially if the shots are critical. Serious dust can re-appear within days.
· Take your cleaning kit when travelling and use the simple test described above to see if the cleaning has worked or needs to be repeated.
· Because apertures smaller than f11 can reveal dust marks, do not use small apertures unless essential for the picture. Try to arrange and compose the shots so that you can use large apertures of f8, f5.6 or f4.
· Reduce the chance of dust by being very careful about where and how you change lenses. Remove the current lens slowly (so that surrounding dust is not 'sucked' into the camera body) and never leaving the camera ‘open’ while hunting for the next lens.
· Be aware that live view means that the sensor is being exposed to the dust in the camera body. Zooming certain types of lens in live view can effectively blow dust on to the sensor!
· When doing a panning shot or photographing a waterfall or similar motion blur shot, it is common to use a small aperture to produce a slow shutter speed. However, from a dust point of view, it is better to use a neutral density filter and keep the aperture open wide.
