Canon’s Ra mirrorless camera, introduced in late 2019, was designed for deep-sky astrophotography and works very well. But after only two years on the market Canon has discontinued it. Unlike the Ra, the newer Canon R6 is not a modified camera (meaning it is not designed to record the deep red light of nebulas).
However, with only 20 megapixels, low by today’s standards, the R6 should be ideal for most low-light astrophotography. I found that it is, but with some caveats and oddities most reviewers might not catch.
NOTE: I tested an R6 purchased in June 2021 and updated in August with firmware v1.4. After completing the testing I updated the camera to firmware version 1.5 that was released on December 2, 2021. Sadly, it did not fix the amp glow issue reported below.
Live View framing
Mirrorless cameras present live views that show much more at night than do most DSLRs. However, the R6 also has a special trick to help frame nightscapes. Switch the Mode dial to Movie, and set the ISO up to 204,800 and the preview image brightens enough to show the Milky Way and dark foreground.
While it’s meant just for aiming and framing, this super-sensitive mode makes the R6 a pleasure to use for framing nightscapes and deep-sky images. You can see the scene and the target. It is similar to the excellent but well-hidden Bright Monitoring mode on Sony alpha cameras.
On the R6’s 20-megapixel sensor, the individual pixels are each 6.6 microns in size, the “pixel pitch.” The larger the pixel pitch, the more photons each pixel can collect in a given amount of time. More photons equals more signal, and therefore a better signal-to-noise ratio.
In noise tests comparing the R6 against the 30-megapixel Canon Ra and 26-megapixel Canon 6D Mark II DSLR, all three cameras showed a similar level of noise at ISO settings from 400 up to 12,800. But the 6D Mark II performed well only when properly exposed. Raising the shadows of underexposed 6D Mark II images reveals significant artifacts.
In nightscapes and deep-sky images the R6 and Ra looked nearly identical at each ISO setting. This was surprising considering the Ra’s smaller pixels, which perhaps attests to the low noise of the astronomical “a” model.
The flaw in many Canon DSLRs (such as the 6D Mark II) is their poor dynamic range due to the lack of an ISO invariant sensor design. The R6, as with Canon’s other R-series mirrorless cameras, has addressed this weakness. The sensor in the R6 appears to be nicely ISO invariant and performs as well as the Nikon and Sony cameras I have tested, models praised for their ISO invariant behavior. (The links go to reviews on my blog of other cameras.)
Where this trait shows itself to advantage is on nightscapes where the starlit foreground is often dark and underexposed. Bringing out detail in the shadows in raw files requires a lot of Shadow Recovery or increasing the Exposure slider. Images from an ISO invariant sensor can withstand the brightening in post-processing far better, with minimal noise increase or degradation such as loss of contrast, added banding, or severe discolorations like those often shown by the 6D Mark II. The R6 performed very well for shadow recovery, making it an excellent nightscape camera.
The R6 did not exhibit any oddities such as oddly colored or green stars, flaws that plague some Sony cameras, and Nikons when using Long Exposure Noise Reduction. I also saw no “star-eating,” a flaw Nikons and Sonys have been accused of over the years, due to aggressive in-camera noise reduction even on raw files, that results in fainter stars being wiped out.
Edge artifacts and amp glow
The R6 did exhibit one serious and annoying flaw in long exposure images – a magenta glow along the edges, especially the right edge and lower right corner.
Whether this is the true cause or not, it looks like “amplifier glow,” an effect caused by heat from circuitry illuminating the sensor with infra-red light. It shows itself when images are boosted in contrast and brightness during processing. It’s the sort of flaw revealed only when testing for the demands of astrophotography.
Amp glow is something I have not seen in Canon cameras for many years. Canon needs to fix this with a firmware update, as Sony did with a similar problem in their a7III.
Turning on Long Exposure Noise Reduction (LENR, which automatically takes, and then subtracts, a “dark frame” from the previous exposure) eliminates the amp glow, as it should. But using LENR is not always practical, such as when taking time-lapses and star trails.
Compared to the factory-modified Ra with its boosted red sensitivity, the R6 did well at recording red nebulosity, but could not record the depth of nebulosity the Ra can, which is to be expected for a stock camera.
In wide-field images of the Milky Way, the R6 picked up a respectable level of red nebulosity, especially when shooting through a broadband light pollution reduction filter, and when carefully processed. However, I would not recommend it for use with narrowband filters, as the R6 has good, but not sufficient, red sensitivity.
The R6 uses a new version of Canon’s standard LP-E6 battery, the LP-E6NH, that supports charging through the USB-C port and has a higher 2130mAh capacity than the 1800mAh LP-E6 batteries. However, the R6 is compatible also with older batteries.
On warm nights, I found the R6 ran fine on one battery for the 3 to 4 hours needed to shoot a time-lapse sequence, with power to spare.
A major selling point for me was the R6’s low-light video capability. It replaces my Sony a7III, which had been my “go to” camera for real-time 4K movies of auroras.
As best I can tell, the R6 performs as well as the Sony. It is able to record good quality (i.e. acceptably noise-free) 4K movies at ISO 25,600 to ISO 102,400. An example is here on Vimeo.
The R6 can shoot at a shutter speed as slow as 1/8-second. That slow shutter speed and a fast f/1.4 to f/2 lens are the keys to shooting movies of the night sky. Unlike the R, Rp and Ra, the R6 records 4K movies from the full width of the sensor, making it possible to capture large swaths of sky with fast wide-angle lenses such as the TTArtisan lenses I tested for AstroGearToday here (11mm) and here (21mm).
The R6, like Canon’s low-end Rp, lacks a top LCD screen for display of camera settings and battery level. Without it, the R6 provides no indication of crucial battery level while a shoot is in progress, for example during a time-lapse. A top screen is also useful for checking ISO and other settings by looking down at the camera, as is usually the case when it is on a tripod or telescope.
We are forced to rely on looking at the brighter rear screen for all information. Yes, it is a flip-out screen, so can be angled up for convenient viewing on a telescope. However, during a Bulb exposure this screen lights up with a readout of the elapsed time. While handy, it is overly bright and it cannot be dimmed or turned off, except by closing the screen, in which case the electronic viewfinder turns on to provide that same display.
Either way, the timer display draws power needlessly when you most need to conserve it. It should be possible to completely turn off all displays. I’m happy to report that as of v1.5 firmware hitting the Info button now does turn off the Bulb Timer display.
The bright live view image, low noise, and ISO invariant sensor of the R6 make it superb for nightscapes, apart from the nagging amp glow. That glow will also add an annoying edge gradient to deep-sky images, best dealt with when shooting by the use of LENR in camera or applying dark frames in processing. The other main deficiencies are the lack of a top screen and the overly bright rear screen, both inconveniences for astrophotography.
For low-light movies the R6 is Canon’s answer to the Sony alphas. No other Canon camera can shoot night sky movies as well as the R6. For me, it was the prime feature that made the R6 the camera of choice to complement the Ra.
And no, I have not tested a 45-megapixel Canon R5 to know from first-hand experience how it compares!
NOTE: This review is an edited version of a full-length and even more detailed review of the R6 that appears on the author’s blog at AmazingSky.net.