Do sensor dust test shots need to be exposed to the right?
Asked 4/14/2020
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When checking a camera sensor for dust, I’ve seen some guides recommend the usual setup—low ISO, very small aperture, defocused or focused far away, and a plain bright surface—but also say to use a shutter speed that pushes the histogram as far right as possible without clipping. Other guides don’t mention this.
I compared both methods: one exposure chosen normally by the camera, which gave a mid-gray image, and another longer exposure that made the frame very bright with the histogram near the right edge. The darker test images consistently showed more dust spots than the brighter ones, even though the brighter exposures did not appear clipped.
Why would the number or visibility of spots change between these exposures? Is exposing to the right actually necessary for sensor dust testing, or only helpful in some cases?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
6y ago
2 Answers
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The main question - should you expose to the right? - depends on how thoroughly you need to clean your sensor, and that depends on your style of photography. To find every dust particle that could possibly appear in your images, exposing to the right will give you the best odds. Realistically, unless you're routinely shooting large areas of sky or other uniform, light surfaces, for the same reason that the faintest particles are difficult to deliberately detect, there's a good chance they won't affect your images.
Why expose to the right? Exposing to the right captures more light, and thus more information, up until the point where highlights clip. This can help find spots that would otherwise be difficult to see. When you expose to the right, you're increasing the amount of light that the sensor gathers compared to the amount of noise: in technical terms, the signal-to-noise ratio. For the same reason that details are lost in the noise in underexposed images but not correctly exposed shots, increasing exposure beyond the metered neutral continues to increase the information about a scene that your camera captures.
Why did it have the opposite effect? Straight out of the camera, there probably won't be any apparent advantage. When you overexpose an ordinary scene, details appear washed out or low in contrast until you reduce the exposure in post-processing to recover the highlights, and the same applies to this situation. Shoot in raw and open the image in your raw converter of choice, then reduce the exposure to a neutral level, checking for spots becoming visible as you do. Increasing contrast may help. Some software like Lightroom includes a spot visualization tool that can also make faint spots more apparent (though unless your wall has a really flat texture, it's likely to produce a lot of false "spots").
The short answer to why this adjustment is necessary is that raw conversion has to take lot of information spanning a large range of brightness levels and condense it into an image that probably has contrast and detail in the right areas. This process doesn't represent every part of the range equally in the final image, and adjusting exposure is one way to control which parts of that range contribute the most contrast.
Originally by user91780. Source · Licensed CC BY-SA 4.0
user91780
6y ago
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Generated from our catalog & community — verify before relying on it.
Exposing to the right is not strictly required, but it can be useful if you want the most thorough dust check.
The goal of ETTR here is to record as much light and image information as possible without clipping highlights. That can make faint dust more detectable when you inspect the file, especially if you’re trying to find every possible speck.
Whether that matters depends on your photography. If you often shoot bright, even areas like sky or plain backgrounds, a more aggressive test helps reveal dust that might show up in real images. If not, the faintest particles may never matter in practice.
If your brighter test seems to show fewer spots, that’s likely about how the image is being viewed or processed rather than dust actually disappearing. Exposure and contrast strongly affect visibility of low-contrast marks. The sensor has the same dust either way, but some settings make it easier or harder to see.
So: ETTR is helpful for a maximum-sensitivity test, but not mandatory. Use it if you want to find every speck; otherwise a normal dust test is often enough.
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