Repairing damage to Micro Four Thirds Camera Images

I have a Sony a6000 which is a micro four third camera. It worked fine until I bought some extra lenses. Ever since I have been completely unable to guarantee after a lens change that there won’t be image damage. If a tiny, tiny piece of dust gets in it will make sometimes quite dramatic marks, particularly on blue ski or snow backgrounds.

The main reasons I have installed your software is that I hoped it would be a better way than GIMP to repair these images. Can anyone explain to me the basic steps of what I should do? I occasionally use GIMP to crop images and that is the limit of my image editing experience.

Here is a sample image for those who have not heard of the problem.

Hi @mde,

You have dirt on your sensor.

The main reasons I have installed your software

What software might that be?

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Yeah, that is pretty obvious. Thanks for pointing it out though.

Is this not a dedicated forum for RawTherapee?

Yes, but not the whole forum. I’ve moved the post to the correct category.

You may want to invest in a blower, I have a rocket blower for blowing dirt off lenses and sensors

After realising the built in sensor cleaner is only vaguely reliable, I bought a box of pricey little brushes. They might work and they might not. The have a good chance just to move the spot elsewhere. My experience is unless you open the camera under lab conditions there is a good chance of something getting it. The screen on the a6000 is not so big or clear that you can see error until too late. I am tempted just to get a new camera(s) but am already about 1,500 euros into this one and you never get back anything but a fraction of your money selling secondhand. Hopefully, someone can guide me through the basic steps for tackling these sorts of blemishes.

GIMP is my go-to for precisely this. The Heal and Clone tools do a fine job blending out such abberations, and are quite simple to use. Just Shift-click to select a source area, then paint the offending area with Drag.


I was using this. It is immensely time consuming and doesn’t necessarily do a great job. Depends very much how well you select by eye. I thought there is surely some more automated way of doing it which can use a bit of computing power.

Hi, as far as I know there is nothing magical to do this (well, maybe in principle you could train some machine learning algorithm, but it would take you much more time and computing power!).

You really want to clean your sensor. A rocket blower should be a good start, and unlike brushes it won’t touch the sensor so it doesn’t risk damaging it.

I’m not usually very careful when I take photographs of machinery and often what would have been easy to do before I end up doing in post. Back in film days we called it “spotting”. I will often go through an image a cut a piece of my image and paste it over a spot of dust or dirt. It takes forever (but it’s faster than it was 50 years ago). Most tools have a “cloning brush” that will allow you to do this. For dust on your sensor you should deal with the dust first and foremost, then patch up those images that need fixing.

So much time spent on this! Its an art form of its own.

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Just as an aside:

No, it’s not. It has an APS-C sensor of 24×16mm (Micro Four Thirds is 17.3×13mm) and uses the Sony E lens mount, not Micro Four Thirds.

Olympus and Panasonic are the main two manufacturers of Micro Four Thirds cameras. Sony does not produce any.


Okay. It is one of the new generation of changeable lens cameras then. Whatever they are called I would strongly not recommend them. At least not for anyone who wants to phtoograph outisde or in conditions which are not those of a studio.

Millions of people, me included, use Mirrorless Interchangeable Lens Cameras without problems after changing lens. You only have to take care that the body is not open for too long, the mount should direct downwards to avoid dust coming in, and like in DSLR cameras the sensor should be blown out or wet cleaned from time to time.

It is a little silly to blame one mark of camera for a problem that all cameras with interchangeable lens’ , and many without, have suffered for the last 200 years!

How do you get your sensor so dirty? I am only use my camera (Sony a6500) outdoors, in the mountains, on the street , as sea sailing etc… and have not suffered excessively from dust on the sensor of any of my digital cameras… Lost a few lens’ due to mildew in the tropics though.

That said here is video on sensor cleaning.

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Existing photos

To “repair” an existing image, you’d want to use “healing”, “retouch”, or a clone tool. It depends on the software you’re using for the edit. Healing / retouching tries to match to surrounding areas, cloning is a simple copy and paste from one area to another, often with feathering.

For what it’s worth, I couldn’t find where this might exist in RawTherapee, either in the app itself, in the docs, or in a web search. (I know darktable and GIMP both have both features, the simple clone tool and a smart tool called “healing” or “retouch”.) Perhaps someone else could chime in on how to do this with RawTherapee. (The answer might be to edit externally, like in the GIMP.)

Future photos

To “fix” new pictures you’ll snap in the future, consider using a wider aperture (a lower f/stop number). The higher the f/stop, the smaller the aperture, the more depth of field you’ll get in the image. In other words, you’ll have everything from the closest subjects (in this case, your dust flecks) all the way to the mountains. If you change the depth of field by adjusting the aperture to a smaller number, you can change what’s in focus — and move the focus to around infinity (far away from the dust on your sensor)… ideally, set it to the hyperfocal distance, which is before infinity and let the depth of field (which would include infinity) do the rest, to make everything in the photo sharp (that you want sharp).

Besides, a Micro Four Thirds camera has a diffraction limit of around f/8, so you’ll have overall image quality losses above that (like f/16 or f/32). Around these apertures you’ll see more pronounced dust spots anyway.

There are circumstances where you do want a tiny aperture (high f-stop number). It still can make sense to shoot f/16 on a MFT lens, as everything is a tradeoff. But note that when people are talking about shooting scenery with f/11, f/16, f/18, etc. — they’re usually talking about full frame or APS-C sensor sizes. It’s usually around f/5.6, f/8, f/11 (depending on the subject(s), lighting, lens, etc.) instead for MFT.

As Micro Four Thirds cameras have a smaller sensor than APS-C and “full frame”, you do get some depth of field “for free” (for either good or bad, depending on what you’re going for). For scenery, it’s great — you’ll have more in focus at once at a lower f/stop. A lot of people do shoot at f/8 on MFT for scenery. (But it all depends on lots of tradeoffs, just like the rest of photography.)

If you do want to use a high f/stop number, then you’ll basically have to clean your sensor glass (some cameras have this built in for typical amounts of dust) or post process to take care of the dust. This is just one of the downsides to digital cameras with interchangeable lenses that we all live with. Film cameras don’t have the sensor dust problem as much (as the film advances and often takes dust flecks with it). There are, thankfully, a lot of upsides to being able to swap out lenses, of course. :wink:

If you are shooting a landscape where the subject matter is far away, you can make everything simple: You could drop the f/stop number down a bit (open the aperture wider) to a sweet spot between aberrations (lens irregularities on a “wide-open” aperture setting on a lens) and diffraction (when a lens starts making everything a bit blurry) and simply focus on infinity.

TL;DR: Consider shooting at f/8 or lower on a Micro Four Thirds camera to keep your subject(s) in focus and minimize diffraction and dust spots.

(For what it’s worth, throw out all these rules of thumb when you’re doing macro. For that, you often want a deep depth of field (high f/stop number) and have to do focus stacking to get an even deeper depth of field. And long exposures are also a bit different, especially when not shooting the sky. But for most standard photos, the above applies.)

His camera is APS-C anyway

The subject itself states “Micro Four Thirds”.

…But pretty much everything still applies to APS-C and every other digital sensor size too. The only thing that would change between various sensor sizes is the suggested f/stops for balancing between depth of field and visibility of dust on a sensor.

I have owned and used Micro Four Thirds, APS-C, and full frame cameras and have experienced dust on high f/stops on all of the sensor sizes. As the camera tech for automatic sensor cleaning (which can only go so far) has improved over the years, it’s become less of an issue, thankfully. (My Fuji X-T2 is great; dust is not really an issue on it. On my old full frame 5DmkII, it was quite bad. Before that, when cameras didn’t have any form of sensor cleaning built-in, it could get pretty horrible.) Again, this is at very large f/stop numbers (small apertures).

Actually, if anything, Micro Four Thirds is less susceptible to it than APS-C or full frame, for two reasons:

  1. smaller f-ratio for equivalent depth of field, therefore higher angles of incident light on the sensor (f-ratio = 1 / 2·sin(θ)), so the light goes “around the dust”

  2. thicker sensor stack, so the dust is further away from the actual sensor, which helps with 1.

On the other hand, specks of the same size would occupy a larger portion of the frame, but apparently it’s not enough to compensate for those effects.

That was somewhat explored in this DPReview thread.

@mde First of all, like others have hinted, preventing this ‘damage’ is easier than repairing it. Please take better precaution against dust when changing lenses. You really don’t need laboratory conditions for that. If you keep the mount pointing downwards at all time when changing lenses, gravity helps to prevent dust to settle. And if your sensor is really dirty (which it is), you might be better of to have it professionally cleaned by your local photolab.

Having said that, if your dust hasn’t moved since you took your photos, you can also use RawTherapee’s flat-field correction to get rid of the dust spots. and in particular this section
If your dust has moved, this method is useless and you should resort to the clone brush in e.g. GIMP, as RawTherapee doesn’t have a proper spot removal tool (yet). You can also read more here RT: No spot removal tool?

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