Reikan FoCal Dust Analysis Test
The FoCal Dust Analysis Test is a quick and simple way to find out if your camera’s sensor needs a clean. It automatically analyses the sensor for dust, telling you not only where any dust may be, but also how much of an impact it’s likely to have on your images and at what apertures.
Dust in your camera!
Unlike a the film in a film camera, the sensor in a Digital SLR camera is not replaced, and over time dust and dirt can end up on the sensor and start to become noticeable on the images you take. The internal components of the camera (mirror box and sensor chamber) are not environmentally sealed and through general use small particles of dirt can end up inside the camera – typically through changing lenses and even zooming a lens that’s attached as air can be pumped through the lens into the camera. When you take a shot, the shutter opens and the sensor is exposed, allowing light and dust to hit the sensor.
Modern DSLRs have sensor vibration systems built in which are designed to remove loose particles from the sensor and typically activate when the camera is turned on or off, but these will not remove stubborn particles. You can clean your sensor, but it’s not without risks and it’s better to only do it when you need to.
Seeing the Dust
In order to see the dust on your sensor, you can take a test shot of a plain surface. You should use the smallest aperture of the lens to best highlight the dust and make sure the lens is defocused (you’re looking for dust on the sensor so you want to ensure there’s no other distractions in the image). The shot below is at f/22, and it’s not very interesting…
One way of seeing all the dust is to open the file in an image editor (e.g. Photoshop) and use an Auto Levels operation:
The problem with this method is that it scares you half to death. So you set off cleaning your sensor, and try again… only to find you’ve just moved the dirt around or made it worse!
The issue is that there’s no context. Auto-levelling will enhance the dirt or defects on even the cleanest sensor – in fact, the cleaner the sensor, the worse it will look when auto-levelled. But most of the highlighted dirt and marks won’t affect an image at all because it’s so feint as to be almost transparent.
And that’s where FoCal’s Dust Analysis test comes in. It’s designed to highlight problem spots across the aperture range, and you can then make an informed decision as to whether to clean the sensor or not.
Setting Up the Dust Analysis Test
Setting up the test is pretty simple. You need to start FoCal and choose the Dust Analysis Test (on the main window for Mac or in the Tools panel for Windows). You’ll be presented with a big window which has a large white area (all the example in this post are from the Windows version of FoCal, but the operation is exactly the same for the Mac version):
The large white area on the window is intended for you to point your camera at so the dust can be easily spotted during analysis:
The camera should be fairly close to the screen – typically less than 1 foot although the actual distance is not critical as long as the whole view through the viewfinder is even white. You should focus the lens to infinity so the screen image is totally out of focus:
Checking the Setup
You can then click the Check Setup button. This will open a small Dust Analysis Setup window – click the Start button and the camera’s Live View will start up. You’ll see the image in the Dust Analysis Setup window along with a red cross or a green tick to show whether the setup is acceptable.
The following shows an unacceptable setup – there’s a big black area on the top-left of the Live View image:
Using the Live View image, the camera can be slightly adjusted to give a plain white* image, and the green tick appears showing the setup is acceptable:
Once the setup has been checked and is OK, you can click the Stop button on the Dust Analysis Setup tool and close the window.
* I’ve used the term “plain white” throughout the details above, but it’s often grey or even slightly colour tinted from the monitor. This doesn’t matter – “white” from the monitor will contains plenty of red, green and blue light so all pixels on the sensor will be analysed, and FoCal controls the camera’s metering in order to properly expose the shots for the test.
Running the Test
Click the Start button on the main test window to begin the test. FoCal will control the camera and start at the narrowest aperture, taking shots all the way to the widest aperture. The results will be plotted on the small graph at the bottom of the test window as it runs:
Real-Time Results
With the set up described above, the dust spot markup cannot be shown as the test progresses because the large white area on the test window needs to stay solid white. However, if you use a different plain white test target (e.g. a second monitor) to aim the camera at, you can click the Show Real Time Results tick box before running the test. You’ll see the following warning:
As the test runs, you’ll see the dust spots indicated in the main section of the window:
Saving the Images
You can also choose to save the images taken by FoCal as the test runs by clicking the Save Captured Files tick box. You then choose a directory and the full size images captured from the camera are saved for you to review later.
Saving the Report
When the test has finished, you can save a PDF report which contains the markup for all tested apertures, along with information about the camera and environment (such as shutter count, temperature, focal length etc). The report for the test shown in this blog post is available for download here.
Interpreting the results
When the test has completed (or during the Real Time Update mode), you can click any point on the chart to see a markup of the dust spots. But what do all the blobs mean on the test window?
Small Blue Dots – these are the dust spots themselves. The blue dot shows a circle over the area of the dust spot. The spot itself may not be circular, but it will fit completely within the blue circle shown on the markup.
Large Coloured Circles – these are proportional to the area of the dust spot, but are much enlarged so you can easily see small dots. The colour of the circles indicates the opacity (blackness) of the dust spot. They are coloured from red (very opaque/black) through yellow to green (very transparent – almost unnoticeable).
Red Dots with Blue Circles – there aren’t any blue circles on the screen shot above, but if shown these highlight potential hot pixels on the sensor.
Looking at the example results
If we take the result at f/22 as shown in the markup screenshot above, in Photoshop we can autolevel the areas of highlighted areas and you can see the spots. They all look pretty serious!
But FoCal is showing the large circles for the spots in yellow and green, meaning they’re not very opaque – more smudges on the sensor rather than big black dirt blobs. If we take the captured file from the shot at f/22 and look at the actual image (straight out the camera) for two top-right most spots, we can see they’re actually not very noticeable at all and certainly not likely to cause a problem in the image unless there’s a clear blue sky or something similar underneath them:
When should I clean the sensor?
The answer to this question depends on how and what you shoot. In the example above you can see that by f/14 there’s no noticeable dust – you can tell this because the graph shows 0 dust spots at f/14 and wider, and the information panel on the left of the window tells you the Largest Dust Free Aperture. So unless you regularly shoot stopped down to smaller than f/14 then you don’t need to do any cleaning.
Even then, the markup shows yellow and green circles around the dust spots, meaning that what little dust is present on the sensor is not really significant as it’s very pale. With the results above, I’d say you don’t need to touch the sensor unless you shoot a lot of landscapes with plain blue skies at smaller than f/14.
Conversely, if the chart had a lot more red circles which persisted until a much wider aperture, it’s probably time to clean your sensor.
Using the Dust Analysis results to help you clean
The chart can help you pinpoint the dust spots on your sensor, but it’s important to understand how to map from chart position to the sensor position.
Within the camera, the image falls on the sensor upside down and swapped from left to right, and the camera processor turns this around before displaying the image on the LCD and saving the file. FoCal shows the results as they appear on the saved image.
When you turn the camera round to clean the sensor (as shown in the picture below), you have just swapped the image from left-to-right. Therefore, in order to translate from the FoCal chart to the image sensor position, you need to flip the position vertically as shown:
So, a dust spot on the top left of the FoCal chart will be on the bottom left of the sensor. And dust spot in the middle on the bottom of the FoCal chart will be top middle of the sensor.
How do I get the Dust Analysis Test?
The Dust Analysis test is one of the many tests available in FoCal Pro. If you already have this, you can use the test straight away. If you’re currently using FoCal Standard or FoCal Plus, you can upgrade easily from within the License Management System.