Sensor cleaning

Big respect to the guys over at the London Camera Exchange in Winchester for doing such a good job of cleaning the sensor on my Canon 350D.

I’d originally found that the Southampton (Civic Centre) branch provide the service for £35, but they wanted it for about 3 days. That means travelling into Southampton, dropping it off, then going back to collect it at a later date, which isn’t easy. I just happened to call into LCE Winchester the other day, and the staff looked rather apologetic when they said “ooh… we’ll need it for about an hour”. An hour? Brill. Same price too. So the other day, I dropped the camera off, called in to see a bloke about a mortgage and then went and collected it straight away.

The difference is wonderful – I had a nasty residue spot on the sensor, which appeared in several of my photos, even at wide apertures. You can see the difference in the two images below. You can see the residue spot about a third of the way down on the left, along with lots of very prominent dust flecks. Note that you’ll never get every little speck of dust removed, but that’s more than ample for me.

Before

After

How did I get that picture?

You may be wondering how I got an image of the sensor of my digital camera. Best way to do this is to put the camera in aperture priority mode (‘Av’ mode on Canons), and then stop the lens right down as far as it will go. On my lens, that’s f/22. Set the ISO to be as low as possible (on my Canon 350D that’s ISO 100), and add somewhere around +1EV or +2EV exposure compensation. I would recommend saving using the RAW image option.

Point the camera at a smooth, evenly lit surface and take the picture. You’re probably going to end up with a long shutter speed with that combination, but that’s not a problem. In fact, it’s probably good to move the camera around a little to make sure that what you’re seeing in the image isn’t just some dirt on the surface of whatever you’re looking at.

Once you’ve got that taken, upload the image to the PC and in a tool like Photoshop, apply the “Auto Levels” command and prepare yourself for a shock. What that does is highlight every minor dust detail on your sensor, which won’t necessarily show up in your images. This is the worst case. Try looking at some real-life shots and deciding if that fleck in a clear blue sky lines up with the ones captured here. If it’s becoming really noticeable, you may want to clean your sensor. Search the web for options, or take it to LCE.

Why use those settings?

Here’s an explanation of why I recommended each of the camera settings:

Aperture priority
Aperture priority is designed to expose correctly, but use a specific aperture. For this we need to set the aperture to the smallest available.
Smallest aperture possible
We need the smallest aperture possible to make the light from the back of the lens appear as a point source. Point sources produce the hardest-edged shadows (we’re trying to photograph the shadows of the dust here)
Low ISO
Low ISO means low noise. We’re not fussed about a long exposure because we don’t care about motion or camera blur We would much rather have a picture with minimal digital noise present.
+1EV or +2EV compensation
The surface being photographed will undoubtedly be fairly well lit and we want to avoid trying to bring dirt data out of the shadows too much. Your surface to photograph shouldn’t encounter any blown highlights at +2EV.
RAW images
This avoid problems with overzealous JPEG compression. Probably the least thing to worry about here, but I recommend shooting in RAW mode permanently anyway. Slightly longer workflow, but worth it for the extra post-shot options available to you.

One comment

  • Interesting…

    The 40D manual section on capturing dust-deletion data adds another two tips:

    Focus on infinity, even though the plan object you’re photographing is about 30cm away
    Use the longest lens you’ve got

    The idea there is obviously to ensure that the subject is so out-of-focus as to appear uniform even if it has minor defects. For the 40D they say f/22 is the aperture to use; I suspect because that’s twice the diffraction limit so anything slower would buy you nothing and just wastes a bit of time. If that’s the logic, you probably need f/32 or so on a 350D. On my 300D, the sensor gets noticeably noisier on the side which is closer to the imaging silicon chips’ heaton long exposures, so cutting exposure time can only help.

    Diffraction limits are discussed here: http://www.cambridgeincolour.com/tutorials/diffraction-photography.htm