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Deep Rock 
Sensor Dust

Scenic photography seems to be plagued by sensor dust. It shows up most visibly against skies and clouds and when the lens is set to a small aperture, which it often is when you're shooting a landscape and want a large depth of field.

Take a look at this photo from my last post. This is the output straight from the HDR step. The HDR process "enhances detail" and so it tends to worsen these dust spots. Here's a zoomed-in look at an especially bad part of the sky. Yuck! It's not the end of the world, though. Even basic editors like Picasa have tools for removing dust spots. So the next step for me is to go in and heal all of these nasty little spots. Sometimes they can be tough to spot; I find that it helps to drag the photo around--your eye seems to notice them better when the picture moves.

Here's a screenshot from Lightroom showing all the places I had to remove dust. What a pain! So what do you do about it? Well, many of you probably have newer camera bodies which have some automatic sensor cleaning and "dust-delete" technology built in. Our 20D does not, though, so I just constantly gripe about it and say to myself all the time "man, I really need to clean my sensor sometime!".

I'm finally getting around to cleaning it, and I thought I'd share the experience with you as well as record some notes for myself for future cleanings.

For starters, cleaning your sensor sucks. It's difficult, scary, and not very cheap. I haven't been perfectly happy with my results, either. The first time I did it, a little fiber managed to make its way into some part of the viewfinder and I haven't been able to get it out. So, with that cheery outlook...

Test shot
The first step is to take a test shot that clearly shows all of the dust on your sensor. Do this with a long focal length (I used our 50mm f/1.8 lens), the lowest ISO (100 for me), and the smallest possible aperture. Focus at infinity and take a picture of a blue sky or an evenly-lit, light colored wall. With these settings, it will probably be a really long exposure, but don't worry about it. Motion blur doesn't affect the dust particles on the sensor, and could even help by blurring out any unwanted detail in the wall.

Here's my initial test shot of a cream-colored wall. You can zoom in on this photo and spot the dust. To really highlight the dust, though, bring the photo into Photoshop or the GIMP.

Go to Image->Levels Adjustment and adjust the levels to create some crazy contrast. Bring the black and white sliders all the way into the edges of the histogram, like so: This should get you a crazy looking photo that really accentuates the grossness. The color can be distracting, so at this point I turn it to grayscale. Here's the same test shot with those adjustments: Now you have a baseline and you can try different cleaning techniques and see how much they improved things.

Cleaning the sensor
I find that anyone who says anything online about cleaning a camera sensor always adds a scary disclaimer to avoid any liability. So here's mine: do not treat this post as instructions for cleaning your sensor. If it reads like instructions, it's because I'm writing them to myself for cleaning my sensor, not for you to clean yours.

First off, you need an environment to work in that's as dust free as possible. I've read suggestions to use a tiled bathroom or kitchen with the windows closed.

Our home was built in 1928 and the weatherproofing is hopeless. The first time I cleaned my sensor, I did it in our bathroom, and I literally watched a few dust particles fall into the camera body. This time around, I did it in my cube at work at the end of the day--definitely a pretty sterile environment, provided I cleaned off part of my desk.

You'll need a tripod to hold the camera while you work. There are a variety of methods for actually cleaning your sensor. There are two that I've been using: a blower and sensor swabs.

I start with the blower. I use the tripod to point the camera at an angle downward to encourage dust to fall out and discourage dust from falling in. Your camera should have a function for locking the mirror up and out of the way so that you can get to the sensor. Mine is labeled "Sensor clean." and is one of the very last menu options. On the 20D, this locks up the mirror until you power off the camera.

The first step, then, is to lock the mirror up and use a few forceful blasts of air to try and blow dust off the sensor. I did this and took another test shot; here's the before and after comparison.

It's hard to tell with the small images side by side, but this did make a difference. Still pretty bad overall, though.

The next step is the sensor swabs. At this point, unfortunately, I don't really remember the research that went into choosing a brand of swabs and cleaner. If I remember correctly, I picked both the blower and the swabs up from a local camera store because they weren't any cheaper online. Also, I went with an off-brand sensor swab.

Using the swabs has never gone that well for me. The instructions are to apply the solution to the swab, then wipe in one direction across the sensor, then the other direction using the opposite side of the swab. I think this means that you should only have to put the swab in and out of the camera once, since you're basically wiping one way then wiping back the other.

I find the edges of the sensor to be the hardest part. After swabbing and taking a test shot, I ended up with what looked like some moisture residue along one edge of the sensor, plus a fairly large fiber. I decided to go in for a second swab, and it didn't get much better. I got rid of the fiber, but a couple large spots showed up in the middle, and the residue was still there along the edge. Unfortunately that's pretty much where I left it. I tried a few more blasts of air but no luck. I've done a little more reading since then and found some other techniques to try. There's a relatively cheap "sensor pen " that looks promising. I have such a tough time with the swabs and the edges; the pen seems like it would be a lot easier to use.

In the meantime, I guess I'll just keep doing spot removal when I process the images!

There are a lot of articles out there on sensor cleaning, so I'd suggest reading up before you try it. Here's one that I read this time around which had some helpful tips.

Posted by Chris at 7:00 PM No comments: Labels: Canon 20D, Dust spots, Sensor cleaning, Sensor dust, Sensor swabs


Knapp's Castle The HDR output One of the original exposures, edited to enhance the sky 
The Curves Tool 200mm, 1/1250sec. at f/8.0
Curves is a powerful but complicated tool that is available in more advanced photo editing software like Photoshop or the GIMP. It deserves a thorough, well illustrated, and well thought-out explanation, so I'm not going to aspire to do that here. What I do want to do, though, is bring it to your attention and show you how it helped me greatly enhance this recent photograph. Then, I'll point you to some well written tutorials where you can learn about it in more detail.

I'm going to oversimplify things a bit and say that curves is a tool for adding contrast to your images, as well as controlling the relative brightness or darkness of different parts of your image.

Here's what the tool looks like in Photoshop. Understanding the real meaning of this graph is valuable, but it's a hard place to start--it's easier to start with a more superficial but practical understanding. In the curves tool dialog, you can click and hold anywhere along the line, and drag up or down to add a gentle bend to that part of the line, creating a "curve". The simplest application of this tool is to create a slight "S" curve which will add some nice contrast to your image.

(The curves screenshots in the rest of this post are from Lightroom, which is where I normally use the tool.)

Hehe, that's a great photo. Jess took it; you can read about it here if you're interested.

This basic contrast is great, but curves may be overkill for just adding a little contrast to your image. A simpler photo editor will usually have a contrast slider which may be just as effective and easier to use.

Curves becomes a much more exciting tool when you think of it as a way to control the brightness of different tonal ranges in your image.

In my morning outing to Hendry's Beach that I posted about a few weeks ago, I took the photo at the beginning of this post. I'm almost always shooting wide-angle at the beach, but this was a moment where I was glad to have a 55-200mm zoom lens in my bag.

Take a look at these before-and-after images, showing the original image and the tone curve that I applied.

Using the curves tool, I essentially told it, "make just the 'light' part of the clouds a lot lighter, and make the 'dark' areas a little darker". This creates a much more dramatic and higher contrast image.

Both Lightroom and Photoshop also provide a great intuitive tool for working with curves. It allows you to click anywhere on the image, and adjust the part of the curve which corresponds to the point in the image where you clicked.

I'm tempted to try and explain the curves tool more, but I know I won't have time to do it justice. If you're serious about editing, you should look into curves. Take a look at these great tutorials, and I'll do my best to answer any questions you leave in the comments.

I have to admit I haven't fully read through this one, but it appears to be very thorough and well illustrated, and I'm looking forward to reading it and maybe picking up a few new things. It goes to show just how much background is required in understanding this tool.

This one is from one of my favorite sites for tutorials, cambridgeincolour.com. It's a little shorter and more to the point, but also more technical. You may be able to get going quickly with this one without completely understanding it, then re-read it over time and gradually build your understanding. 
End of a day 

Happy Easter Weekend!
Posted by Chris at 3:20 PM 2 comments: Labels: Chase Palm Park, HDR, Jess, Logan, Palm Trees, Santa Barbara, Sunset