How can I tell if a scratch is directly on my sensor or on the filter above it?

Question

I have what appears to be a scratch on my sensor. I can't see it on pictures taken with apertures larger than F16, but I do see them at F32. How can I tell if a scratch is directly on my sensor or on the filter above it

Answer 1

I can't see this scratch on my pics in aperture range less than 16 but on range f32 I can notice them.

It's on the filter stack.

When the front of the filter stack has dust or scratches on it, we don't actually see the dust or the scratch in images taken with the sensor. What we usually see are the shadows cast by that dust or scratch. Sometimes in the case of a scratch we will see an area of increased brightness as the shape of the scratch on the surface of the filter stack refracts more light on a smaller area.

Whatever it is, it is on top of several filters in front of the sensor and is therefore not exactly in the sensor plane. Because of the much shorter distance between the lens and sensor than is typical between the lens and subject, the distances on either side of the sensor plane are much more critical than the distances near your focus point on the subject. How blurry an object appears when it is a set distance away from the point of focus varies at different apertures. There is also a varying "depth of field" in front of the sensor for different apertures.

As the aperture is closed down the amount of light coming from the edges of the lens and striking the sensor at an angle is reduced and only the light coming from closer to straight on is allowed to reach the sensor. When a wider aperture is used some of the light rays coming from wider angles manage to strike the pixels directly behind dust spots or scratches. When a more narrow aperture is used the effect of the dust or scratch is more concentrated because a much higher percentage of the light is coming from straighter in front of the sensor.

The wider angled light rays at large apertures also effectively spread the shadow of the dust spots or scratches over a greater number of pixels. This reduces the contrast between the shaded and non-shaded areas until at some point the shadow is so spread out that the difference is no longer perceived. Much the same thing happens in reverse when you point your camera at a bright star and defocus the lens until the light from the star is so spread out that you can no longer see it.

The real question, then, is what is it? It might be a scratch. But it just as likely may be a carpet fiber or other fabric fiber of some type or even a small hair, possibly left there by a cleaning brush.

Clean the sensor properly if you are comfortable doing it yourself or have it cleaned by a camera shop if you are not.

There is no single "best" way to clean the sensor of a digital camera. To put it more precisely, which way is the "best" can depend on a number of variables. The most significant variable, by a fairly large margin, is determined by what, exactly, needs to be cleaned from the sensor. The same method that is most effective and safest for removing lightly attached dry dust particles is not the best way to remove a sticky wet substance that has managed to get onto the filter stack in front of the sensor.

Another consideration that can affect what is the "best" way to clean a sensor can be summed up by answering the question, "How clean does it need to be?" A less risky method that is effective enough to shoot 'complex' scenes with lots of detail at wide apertures with long focal length lenses may not be the best answer if the camera needs to be used with wider angle lenses at narrower apertures shooting fairly uniform fields of brightness and color. For the second scenario a more risky but more effective method might be a better choice.

There are more effective methods, and there are safer methods. They are generally inversely proportional to each other. The methods, in order from the lowest to highest risk factor are:

• Automatic dust removal system
• Air blower (with a filtered intake). Be sure to use a blower that doesn't spray dust into your camera's light box. Good blowers have a one-way intake valve with a dust screen to prevent sucking dust into the blower along with the air.
• Dry brush
• Electrically charged brush such as those made by Arctic Butterfly.
• Dry cleaning products, such as the LensPen brand's SensorKlear that use a combination of microfiber and microscopic carbon beads to remove oil and smudges.
• Wet cleaning systems that use swabs and cleaning fluid
• 'Tape' method that uses a cleaning instrument with a sticky surface that attempts to capture hard to remove dust without leaving residue behind.

Further Reading

For more about why the shadows of scratches and tiny dust and other things on the front of the sensor's filter stack are more distinct at narrow apertures than at wider apertures, please see:

Why use a small aperture when trying to see sensor dust?
Question About the D600 Dirty Sensor Test - Why Use a High f-Stop?

For help in determining what is causing the effect you are seeing, lease see:

How can I remove the hair on the sensor from this picture?
Dust spot or scratch?
What are these slightly-translucent, branching squiggles on the top of my photo?
What are these purple pixels on my Canon 60D and how do I get rid of them?
Dark spots - sensor toast?

For more about scratches and how they affect your photos, please see:

Sensor scratch: how likely is this one to cause problems and would it worth a replace
Light scratch on sensor: is that fixable