What is causing these blemishes?

There are spots on the photo below, but not on the photos before and after this shot. I keep the front and back of by lenses clean and I use a hood whenever shooting outdoors. I know I can eliminate these blemishes in post processing, but do you know what causes them? Edited February 26, 2019 by davidrosen

The heart of the digital camera is an imaging chip with a flat surface. The job of the lens is to project an image of the outside world onto the surface of this image sensor. The surface of the image sensor is fragile so it is protected by a flat cover glass. This cover glass not only protects, it is a filter that stops infrared light. This is because the digital image chip is also sensitive to IR light and this radiation will impart false colors. Each time you remove and replace a lens, you are exposing the surface of the imaging senor to dust and debris. Should any land on the cover glass, it will cast it's shadow and this is the artifact you are seeing. You can prove this by shooting some shots of a clear blue sky or other mundane uniform tone. Shoot each frame at various aperture setting. You will discover that debris records most clearly when the lens is set to tiny opening like f/22 or f/16 or f/11. Large diameter apertures like f/5.6 or f/4 will also show the artifact but its shadow will be highly diffuse. In other words, artifacts revealed using tiny diameter working apertures are must more prevalent. This is proof the artifact is due to debris on the cover glass. Check your camera manual for proper image sensor cleaning.

IMO - Spot on the sensor - have a similar one that I haven't been able to remove.

but not on the photos before and after this shot

Likely not a permanently dirty (as in oil spot) sensor, given its inconsistency. Could be a dust flake that fell off between shots, or if the OP has his camera set to "clean sensor" when powered-off or -on. Dirt that will require cleaning the sensor by hand will be consistent from shot to shot. (I should know, my D7100 is well past the need for a good cleaning.)

Dust on the sensor will show up more when shooting at smaller apertures as the other Alan said, so sometimes they show, other times they don't.

The heart of the digital camera is an imaging chip with a flat surface. The job of the lens is to project an image of the outside world onto the surface of this image sensor. The surface of the image sensor is fragile so it is protected by a flat cover glass. This cover glass not only protects, it is a filter that stops infrared light. This is because the digital image chip is also sensitive to IR light and this radiation will impart false colors. Each time you remove and replace a lens, you are exposing the surface of the imaging senor to dust and debris. Should any land on the cover glass, it will cast it's shadow and this is the artifact you are seeing. You can prove this by shooting some shots of a clear blue sky or other mundane uniform tone. Shoot each frame at various aperture setting. You will discover that debris records most clearly when the lens is set to tiny opening like f/22 or f/16 or f/11. Large diameter apertures like f/5.6 or f/4 will also show the artifact but its shadow will be highly diffuse. In other words, artifacts revealed using tiny diameter working apertures are must more prevalent. This is proof the artifact is due to debris on the cover glass. Check your camera manual for proper image sensor cleaning.

Thanks for that description. I have initiated the sensor cleaning on my Lumix GX85, but it appears to take just a moment. I am not sure it is working.

It does not take long at all to clean, it is a matter of seconds. So it may very well work as it should. Nikon cameras shake the sensor somewhat, so when I use the manual feature on my camera, I tilt it so the lens points towards the floor. Several Nikon cameras also remap hot/dead pixels if you activate the built-in sensor cleaning from the menys twice, which may also help reducing artifacts that can show up in your photos. For many cameras, you also have the option of leaving the shutter open to perform a mechanical cleaning of your sensor which is another feature.

 

I hope this cleared your dust spot(s).

Thanks for that description. I have initiated the sensor cleaning on my Lumix GX85, but it appears to take just a moment. I am not sure it is working.

 

The camera sensor-cleaning routine essentially just shakes/vibrates the front surface with the hope that the debris will lose its grip and fall off. So you might try several such cycles. But if the particles are very small, they are not easily affected by this. (I'm pretty sure, from the appearance, that your particles ARE small.)

 

The next thing to try is probably an air bulb with the nozzle held near the sensor, etc., but these are also mainly effective on larger bits of debris. If you're willing to touch the sensor there are a few other options, some need for you to know where the debris is. Particles that won't shake loose are generally too small to see with the naked eye; best is a long-working-distance stereo microscope so you can observe from outside the camera body.

 

The last resort is a wet-cleaning process (with "approved" cleaning materials and solvent), but this takes a certain skill to avoid leaving streaks, etc., along with the possibility of leaving a scratch. So I think that a lot of camera owners prefer to let a specialist do this. Or just leave things as is, avoiding the lens apertures where it is most objectionable. "Noisy" scenes, as opposed to clear blue skies, tend to camouflage the spots which is my guess as to why you haven't seen them before.

 

So you sort of pick your poison, so to speak. Personally I think that any serious pro should learn how to do emergency wet-cleaning (or have spare bodies with an assistant who can do the cleaning). There is little doubt that wet-cleaning reliably does the most thorough job, although in many cases a stamp tool is perfectly adequate.

 

Ps, I did a quick search for cleaning tools, and this website looks pretty legitimate to me; it reviews a number of tools: Cleaning Digital Cameras - Guide to cleaning DSLR and Mirroless cameras. - The Methods & Tools