Sensor dust Pt.1
/You have polished and polished your precious lenses just yesterday and yet, you still can’t seem to get rid of those darn dark spots in your photos. Sometimes they seems to magically vanish only to return in the next couple of photos. Your camera is likely suffering from a bad case of sensor dust so let us investigate what we can do about it.
When I set out to write an article on sensor dust spots and sensor cleaning I found it grew larger than a single post. So I decided to split the article up in two distinct parts. This first part will deal with how you identify your camera sensor is suffering from a dirty sensor and how to minimise dust from getting onto you sensor in the first place. The second part will deal with the actual cleaning of the sensor itself and what tools and methods are available.
Why does sensor dust happen?
Sensor dust is nothing extraordinary and happens on a regular basis as you change your lenses. Even if you try to be as careful as you can dust and other particle will eventually get into the camera body and stick onto the sensor. But not all particles are dust, small metallic or plastic particles from the cameras lens mount can rub off and drop into the body of the camera and attach themselves to the sensor. Likewise you may see small oil or grease spots caused by the lubricants inside the camera chambers moving parts. Condensation is another enemy that can leave drying marks on the sensor if the camera is heated up too quickly coming in from a cold environment.
No camera is immune to sensor dust, even cameras without interchangeable lenses will suffer from this as the lenses themselves breath when you zoom. Take the lens off your camera and hold the palm of you hand close to the rear of the lens, and with you other hand zoom the lens in and out. You will feel a draft of air when you zoom the lens out on your palm. Now as you zoom in and out with the lens mounted to the camera, dust particles are sucked into your camera body and has a chance of ending up on the sensor.
Some lenses may suck in more air than others depending on their design but all zoom’s do to one extent or another.
How sensor dust affect your photos
Here’s the thing with sensor dust, you may never see it in your photos even though your camera has it. This is because at lower apertures say that of f/1.4 - f/5.6 the lens opens up to let more light in. Subsequently the sensor of the camera gets hit with a wider beam of light. This causes particles and dust specks to cast less of a shadow onto the sensor surface. In contrast to this closing down the aperture say between f/8 - f/36 or higher causes a narrower beam of light to hit the sensor. This smaller light source causes dust specks to cast harsher shadows onto the sensor and the spots will become much more prominent.
How you develop and shot your photo has a big impact on how the spots are seen in your photos as well. If you have a photo that is very bright even though it is shot at higher apertures the brightness will conceal some of the spots as it washes out the shadow of spot itself. Likewise if you darken a portion let’s say the sky with a graduated filter or apply HDR this will instead bring out the spots on your photo.
Both of the above photos was shot at f/22 and the sensor was just as dirty if not dirtier in the one shot straight into the sun, but because that one is brighter, less spots are seen vs the bridge photo where we can easily see spots in the sky, both on the left and right sides of the arch, as well as in the water.
If you do not remove spots in post-processing and then print a photo, this can have disastrous effects. Some spots you may not see on your computer screen because a computer screen is so much brighter than what a print is not to mention, backlit. So while a photo could appear clean on your computer monitor, once printed you may see spots in all kinds of places you didn’t see on your screen. Keep in mind the bigger the print is, the more apparent even the smallest of spots will become.
How to identify sensor spots
So how do you know your camera is suffering from sensor dust and not just a dirty lens?
LED sensor loupe's with magifying glass allows you to see the state of your camera's sensor.
If you already own a sensor loupe simply remove the lens, lock the mirror up for cleaning and investigate the state of the sensor through the loupe. However, chances are if you are reading this article you don't own a sensor loupe, and why would you? I most certainly didn't own one until I started cleaning my camera's sensor on my own. The sensor loupe slides over the camera’s lens mount and illuminates the sensor using LED lights. It has a magnifying glass so you can see most of the dust and other particles on your sensor, but thinner lighter dust may not be visible even via the loupe.
The second way is a bit more involved and for this to work best, the first step I recommend is to ensure that your lens is clean, both the front and rear elements. You don't want to do this with a dirty lens as this will make it more difficult to identify spots created by the lens vs a dirty sensor.
Take a photo of something that is a single uniform colour, subjects such as a clear blue sky, a wall that is white or close to white, a piece of white paper or even a computer screen with an empty Text Edit document will work. The key is that you fill the frame with only the subject and nothing else. Set your camera to the highest aperture the lens allows it to go usually f/16 - f/32 the higher the better. Also you don't have to worry about focus it is actually better if the photo is out of focus, the spots will show up anyway. Download the photo to you computer and review it at full scale.
The above was shot at f/22 against a white A4 paper. We can easily see the larger spots and some smaller additional spots when viewing at 100%. If you own more than one lens you can verify it is not a lens issue by taking the exact same photo with both lenses at the same aperture and if the spots doesn't move then it is the sensor.
To take this a step further, Lightroom has an option called ‘Visualize Spots’ to help see spots when the Spot Removal tool is selected. To active this feature, in Lightroom select the Spot Removal tool, ‘q’ on your keyboard, and with the spot removal tool selected press the ‘a’ key on the keyboard which will turn on the Visualize Spots feature. There is a slider on the toolbar ’t’, allowing you to increase and decrease the strength.
Below is a series of exposures taken ranging from f/2.8 up to f/22 using a blank A4 paper. The photo’s where then imported into Lightroom and I switched on the Spot Removal tool with the Visualize Spots strength slider turned up about ⅔. These demonstrate the effect sensor dust have on a photo as the aperture is increases. Let’s just say it’s time for me to clean my sensor.
The above clearly captures the effects of how a dirty sensor will affect your photo's as the aperture is increased.
Preventive measures
While you cannot completely eradicate sensor dust there are some things we can do to minimise it from happening.
Changing lenses
When changing lenses always point the camera down towards the ground. It is harder for dust particles to flow up into your sensor, than them falling down into it. This technique also allows for any small metal or plastic being scrubbed off the lens mount to fall away from your camera. Switching your camera off will ensure that any static charge that builds up are dispensed and there is less chance of it drawing particle in.
If outside and it is windy, try to find shelter close by before swapping your lens. Use your camera bag or a plastic bag dedicated to swapping lenses. If you are parked not too far away perhaps walking back go the car to change lenses inside the car is an option.
Deserts, beaches and other dusty environments
Consider using a weather & dust cover for your camera as you are shooting. These come in different varieties and are sold by your local camera vendor and can be found online at Amazon. This way fine grained sand won't be creeping into the crevices of your lens and camera as easily. Ensure to use a rocket blower and a brush to wipe any excess dust or sand off the camera before changing lenses.
Cold locations
To prevent condensation when coming home resist the urge to take the camera out of the bag directly. Instead let it sit in the backpack with all the zippers closed. The bag will gradually warm up as will the gear inside. This slow warm-up will prevent condensation inside the camera housing as well as inside the lenses which can otherwise occur. Similarly if you are going shooting in the cold you should attempt to gradually cool the bag and the gear the same way. If you have a high balcony you can put the gear out the night before or put it in the boot of your car.
The Takeaway
While you will never be able to completely prevent dust and other particles from ending up on your sensor you can now at least minimise the amount that gets accumulated on the sensor. This article may also have answer some questions why you still see spots on your photos even though you have cleaned you lenses, and why they seem to go away only to return later.
In the next part of this series I will talk about the different methods that can be used to clean the camera sensor.
Until then take care and thanks for reading!
