With the introduction of the EOS 5D Mark IV, Canon opened up some new possibilities for image processing with the introduction of Dual Pixel Raw files. Following on from our previous post on the subject, we’ve been investigating how these files can help with autofocus calibration and general camera analysis. The answer is… quite a lot!
A brief introduction
Canon introduced the first Dual Pixel sensor in the EOS 70D camera to bring an unprecedented autofocus system to the live view capability of this camera. By cutting each pixel in half and having the halves look in slightly different directions, the camera can determine how to quickly and accurately focus at almost any point on the sensor.
The Dual Pixel system has been used on the sensors of the 7D Mark II, 80D, 1D X Mark II and now the 5D Mark IV. However, up until this most recent camera the dual-pixel magic was kept solely inside the camera.
With the addition of Dual Pixel Raw capability, Canon now allows the information from both halves of these pixels to be saved in the image files. Canon themselves have demonstrated a few tricks that can be performed with this extra information within their DPP software – micro refocusing of images, shifting of bokeh and reduction of ghosting.
How can FoCal use this information?
The Dual Pixel Raw files contain all the information necessary to determine the focus position at any point on the sensor. Not only can we find out how far from “perfect focus” any point is, but we can also find out which direction the focus is shifted in – front or back. All this information from a single image file!
We can use this information to improve confidence in microadjustment calibration results, to figure out calibration values from the AF Consistency test, and to determine focus shift in the Aperture Sharpness test. There’s more details below…
Fully Auto Calibration
The fully automatic test can use the dual-pixel information to determine the correct AF Microadjustment. This can be used in tandem with the “normal” FoCal analysis results in order to confirm validity of the suggested value.
Here’s an example – the pink area shows the result of the dual-pixel analysis, and the black line is the result of the FoCal analysis. They are both very close here (less than 1 AF Microadjustment until apart) so we can be very confident of the calibration result:
The Front/Back Focus chart (Chart 7) shows how the AF Microadjustment shifts across the range. When enabled, this chart is entirely constructed from the dual-pixel information:
Things get a little more interesting with the AF Consistency test. Running with Dual Pixel Raw files, we can measure the shift to perfect focus for each image… which means we can tell how calibrated the camera is from this test alone!
Here’s an example of a well calibrated run:
And looking at the Absolute Focus Error chart (Chart 4), we can see that the measured focus offset (determined from the dual-pixel data) is very close to perfect. “Perfect focus” has a value of 0 and is indicated with the blue line, and the red line indicates our result which is very close to 0.
Take a look at the Test Results panel too – there’s a new “DPR Suggested AFMA” value, which indicates “-2 (Calibrated)”. This says that the recommended AF Microadjustment calculated from the dual-pixel data is -2, and the camera is currently set very close to this value so can be considered calibrated.
Taking a poorly calibrated example (the camera AF Microadjustment was set to +20 and the test run again):
It’s clear to see that the results are far from the blue line – all the way up around +23 (yes, this value can go above +/- 20 as it’s showing an offset rather than an actual calibration value). This means that the perfect focus is around 23 AF Microadjustment steps away from the current value set on the camera… or an absolute value of -3 as suggested in the “DPR Suggested AFMA” result line. The current value is producing a large back-focus (the text is actually the wrong way round in the window!)
So, with Dual Pixel Raw files, we can run an AF Consistency test and get an AF Microadjustment calibration value!
Again, dual-pixel data can give extra insight into lens performance when running the Aperture Sharpness test.
Here’s a fairly well focused Aperture Sharpness test on the Canon EF 40mm f/2.8 STM lens:
All good – this is a fairly normal profile showing a decent lens. The sharpness doesn’t change much from wide open until diffraction starts affecting the image. But that’s not the whole story…
Take a look at the “DPR Focus Shift” chart:
Here, we can see a couple of interesting things.
Firstly, what appeared to be “perfect focus” when we started the test is actually 1 AF Microadjustment until away (this is the value of the red line at f/2.8).
Secondly, there’s a pretty large focus shift. Remember, the Aperture Sharpness test does not change the focus of the lens. What we’re seeing here is a consequence of the lens design – as the aperture stops down from f/2.8 through to f/4, the focus point of the lens shifts almost 4 AF Microadjustment points away, then starts to return.
What this focus shift means in reality is that if you manually focused the lens at f/4, you’d get a sharper picture than if you used autofocus (this is because normal camera autofocus operates with the lens wide open).
When will Dual Pixel Raw be available in FoCal?
Most of the implementation is done for handling Dual Pixel Raw files. All the necessary conversion and analysis processing is finished, and FoCal has been optimised to handle the Dual Pixel Raw files as fast as possible. However, there’s a bit of tidying needed and testing to make sure things are robust and work in all situations (especially that nothing is broken for non 5D Mark IV users!)
We want to get this functionality out quickly, but there are some other important updates that we need to get into FoCal too. We will get it to you as soon as possible though!