- Ed Dozier
MTFMapper 7.29 Adds Chromatic Aberration Measurement
Anybody who has followed my site knows what a fan I am of the MTFMapper program. Frans van den Bergh, the author, has expanded his program’s offerings (version 07.29) to now include lateral chromatic aberration (CA) measurement. His free program can be downloaded from here:
If you’re interested in axial chromatic aberration, this same program can help you measure that, too. Check out this article
What Does Lateral Chromatic Aberration Look Like?
Lateral chromatic aberration rears its ugly head
The shot above shows typical CA. It’s those purple edges you see mostly in the corners of your photos, especially when you have a dark object against a light background.
Notice how those purple edges aren’t the same in the radial/sagittal (think wheel spokes) direction versus the tangential/meridional direction. Most lenses show this CA defect much more in the tangential direction.
The colors of CA can look different, depending upon the lens. The new CA measurement feature gives you a pair of measurement plots that contain Red/Green and Blue/Green analysis. Since the camera sensor has a Bayer grid of red, green, and blue pixels, the program takes advantage of this extra information.
If you’d like to be able to attach useful numbers to this coloring defect, in order to compare one lens to another, or to compare different aperture settings on the same lens, then this program feature is for you.
My absolute worst lens for chromatic aberration: Rokinon 8mm
When it comes to CA, a lens that has an embarrassing abundance is my Rokinon 8mm fisheye. As you can see above, it’s much worse in the blue-green region than the red-green region. The plot above shows typical output from this new program feature, where I configured it to measure CA in “microns”.
Notice in the chart above that CA isn’t perfectly symmetric about the lens center. Most web sites will just give you a single number for this measurement, but it just isn’t that simple in real life.
The plot ‘negative’ measurements certainly don’t mean something like “less than no CA”. Negative measurements just mean the relative shift of one color versus another. With many lenses, this color shift can even transition from negative-to-positive or positive-to-negative. A measurement of zero is still the lens designer’s holy grail.
To get the best accuracy in your measurements, be sure to carefully focus the lens and get the chart perfectly flat and parallel to your camera sensor.
The shot above was done with a D7100, which has 3.92 micron pixels. The worst measurement here (-12) is equivalent to 12/3.92 or 3.06 pixels. There are multiple units that you can select to measure CA, and those ways are discussed next.
I really love the perspective abilities that this fisheye lens gives me (especially after I remove the curved lines in Lightroom) but I don’t love its CA.
Select how to measure CA
You have the option to measure lateral chromatic aberration in units of microns, pixels, or “percent of radial distance from the lens center”. You will find that different web sites will measure CA in different ways, and this gives you the ability to compare your own results against those web sites.
Bear in mind that the “absolute” CA measurement is in microns, since this removes the camera sensor pixel size from consideration. If your goal is to compare different lenses on different cameras, this would be the best choice. You’ll need to remember to enter the correct sensor pixel size in the Preferences dialog, or else your measurements will be garbage.
Set the pixel size for micron measurements
The units of measurement for lateral chromatic aberration are in microns if you tell it the sensor pixel size. If you don’t select “Line pairs/mm units”, you will get measurements in “pixels” instead of “microns”. Note that the “CA display type” doesn’t have a “microns” option in its list.
Preferences to get CA “pixel” measurements
Preferences to get “Radial %” measurements
A word of caution when using new resolution charts
New resolution chart style (with annotations on it)
The chart style above may not produce any chromatic aberration plot output unless you deselect the option to use “Line pairs/mm units” in the Preferences dialog. This means you can still get pixel or radial percent output.
This limitation will probably be eliminated in the next release of MTFMapper. I notified the author about finding this issue.
Frans did fix this issue with the 07.30 version of MTFMapper. The new chart style works just fine, even when using the "Line pairs/mm units".
Preferences dialog for new resolution chart style
The shot above shows how I specifically avoid Line pairs/mm units when using the ‘new’ resolution chart style. With this setting omitted, my CA plots can be successfully created.
The “old” printed chart to use for measurements
The same web site where you download the software also has PDF files of various test charts that you can print out. The chart that I used to make the chromatic aberration measurements looks like the one shown above. To get the most reasonable measurement results, you’ll want to print a large chart. My chart is 36 inches X 48 inches. The chart style shown above works for all of the chromatic aberration plot output options.
As I already mentioned above, I tried shots using a newer-generation resolution chart (it has an hourglass shape in its center) and the software refused to make chromatic aberration measurements. When I stopped selecting the “Line pairs/mm units” option in Preferences, it started working with this chart style. I’m glad that I still have both kinds of charts. You can just multiply any “pixel” units by the number of microns per pixel of your sensor to convert into microns. For instance, the Nikon D850 has 4.35 microns per pixel, so 2 pixels of chromatic aberration would be (2 * 4.35) or 8.7 microns.
To display the chart measurements, click on the “chromatic aberration” option after opening and processing your chart photos.
Remember to use “Raw” format photos, because your camera will probably attempt to remove any chromatic aberration if you shoot in “jpeg” format. I suppose that if you took a shot in both Raw and jpeg (with corrections active), you could evaluate just how well your camera can correct for CA.
You’ll find that lateral chromatic aberration decreases as you stop down your lens aperture. The MTFMapper program will let you now explore how you can control this lens defect with your lens.
Keep in mind that you’ll get different measurements if you shoot with DX versus FX format; FX will look worse than DX on the edges, of course.
Rokinon 8mm f/3.5 chart
A note of caution: I found out that the use of “stereographic” lens distortion correction (in the Preferences dialog), which should normally be used for my 8mm fisheye, caused problems for this version of MTFMapper. If I left the default distortion correction at piecewise quadratic, it could successfully complete the CA plot. The measurement results for this fisheye lens may be suspect, since I was forced to use a non-optimal distortion correction option.
Frans did fix this issue with the 07.30 version of MTFMapper. The "stereographic" lens distortion selection now works for all of the plot types.
The vast majority of lenses work just fine in the analysis (I typically use piecewise-quadratic distortion correction). I just thought that I should mention that extreme-perspective specialty lenses might not be handled properly in this program version. I imagine that this shortcoming will be addressed soon. I have the bad habit of stress-testing programs to look for any weaknesses in them. I always try to notify Frans if I notice anything amiss, so that he can analyze and address any issues; Frans has always been extremely responsive. He is already working on this distortion correction issue, which he thinks is associated with the “Bayer channel” selection in the Preferences dialog.
Much less chromatic aberration when stopped down
If you compare the f/8.0 plot of this Rokinon 8mm with the f/3.5 plot near the top of this article, you can see how much improvement there is when you stop down.
Having the CA measurements at different apertures, in addition the standard resolution measurements at those same apertures, you’ve got a lot of good information to determine the optimum aperture to use on a particular camera/lens combination.
Lateral chromatic aberration measurement is a great addition to the MTFMapper program. Even though photo editors such as Lightroom can mostly remove this lens defect, there is still a slight cost in overall lens resolution to do so. If you’re comparing two lenses that are basically locked in a tie, maybe this CA measurement feature can help to cast the deciding vote.
Frans is now accepting PayPal donations at his website http://mtfmapper.blogspot.com/. I encourage you to show Frans your support for his continuing hard work and excellent product. No, he didn’t ask me to mention this. I just think that it's to everybody's benefit if Frans can keep his program current and make it even more powerful.