Choosing lenses for astrophotography
Establishing some standards
Your camera’s sensor is crooked
Adjusting the camera
Adjusting the lenses
A better Giant Ruler
Yet more alignment pitfalls
Possibly another method
In December 2017, it was first suggested to me by my friend and esteemed colleague, Joseph Holmes, that my Sony α7R II’s (or more commonly written as a7R II) image sensor might be crooked relative to its bayonet mount. Don’t laugh yet, it’s not a problem exclusive to Sony. I had previously known about the same problem with digital medium format backs from precisely the same person, who put out two extensive articles on the topic over a decade ago, here and here. Neither of us wanted to believe this initially. One would certainly like to assume that 35mm format camera manufacturers, making single-unit camera bodies rather than modular system digital backs, could hold the sensors to tight enough tolerances with today’s technology. Alas, we discovered that it was not quite good enough for what we wanted from our cameras.
A diffuse and complex story unfurls, necessary for the explanation of small details in great detail. For anyone wanting the very best out of their modern, wide-aperture, breathtakingly-sharp lenses, on equally modern, high-resolution (read: highly revealing of alignment errors) digital cameras, photographing the most optically demanding subject matter in the known universe—the starry night sky—you will want to sit up and pay attention. It’s entirely fine if one does not care about subtle differences between what’s in-focus and out-of-focus. I would not soon be forgetting how many failed to appreciate the camera-shake induced blurring of the Sony a7R’s violent shutter mechanism. If one only wants photos displayed no larger than Instagram thumbnails, none of this is pertinent and you should stop reading now. Alright then, some good news! We have fortunately found reasonable solutions to the worst problems. I share them below freely.
I’ve held for some time a passing interest in astrophotography. I don’t live in a country where there are dark enough skies for a great picture but the rapidly rising popularity among landscape photographers meant it often filled my social media feeds. It’s hard to ignore so compelling a thing—our view of the universe, as it were—and the humbling smallness of our existence within the grand scale of all else.
I am drawn in particular to what is called astro-landscape photography, which is typically a wide-field view of the night sky paired with some kind of interesting foreground landscape. Pictures of faraway things like planets, nebulas and galaxies are known as deep-sky astrophotography, requiring long telephotos or telescopes to magnify the tiny portion of sky they subtend, affixed to a tracker to counter the movement of the stars, which is actually the rotation of Planet Earth itself (stars also move relative to each other but not on a time-scale that matters to photography). To make many of the otherworldly pictures like what NASA publishes, folks usually have to sit around all night, sometimes for many nights, making hundreds or more exposures to gather infinitesimally small numbers of photons coming from these faraway celestial beings. They can usually afford to use smaller apertures and wait around a long time for good seeing conditions, thus avoiding most of the pitfalls I cannot.
Making astro-landscape pictures may sound the same at first but is actually quite different, and arguably a lot more challenging. Most photographs you’ll find on the web are not made using the process detailed below, rather they are single-frame wide-angle images at an extremely high ISO setting with the camera usually on a tripod with no tracking device used. The 500-rule is nowhere near stringent enough to avoid trailing and even the 200-rule is not quite enough, not to mention the latter forces one to use relatively short shutter speeds, making noise worse. Some may do some simple exposure stacking to reduce noise, but still without the use of a tracker. These look OK at Instagram sizes but upon enlargement, betray many quality issues. Often, the stars have noticeable trailing, the roundness of stars off-axis are distorted from spherical aberration, coma and astigmatism among the various higher-order aberrations and colour-fringed with violet from longitudinal chromatic aberration. The brightest stars are usually overexposed and clipped, appearing harsh. Meanwhile, the entire image is grainy due to high levels of noise, which is worse in the light-starved image corners from strong lens vignetting. Finally, the pixel resolution is never quite enough for a large print. I can sometimes notice these issues even at Instagram sizes. None of this is satisfying, to us at least.
To get the kind of pictures we are after, firstly, one has to find some interesting foreground landscape that faces the portion of the night sky of interest, at the right time of the year. Ideally, no sources of intense light pollution will be on the map for at least 100 miles in all directions, though something like 500 miles or more is best to avoid the unmistakable glow on the horizon which the camera will see, even if our eyes cannot. Access to such locations at night may be challenging, perhaps requiring camping in wilderness or hours of trail hiking in the dark. We also like to make single-row or multi-row stitches to be free of the aspect ratio of the camera but mostly to gain significantly more pixel resolution for beautiful large format prints. To eliminate streaking by UFOs (satellites, planes and other space junk zipping around the Earth) and also to increase the signal to noise ratio, we generally aim to make ~10 exposures per camera angle to be aligned, stacked and averaged later. Each of these exposures is typically 32 seconds long. To avoid trailing stars we mount our setup on a tracker (equatorial mount). We prefer to make exposures of the sky and land in the same session, so for the sake of expediency to gather enough light, we need to expose with an aperture of at least f/2.8. The camera should offer sufficiently high resolution natively so we can avoid standing around for too many additional hours, collecting more frames for stitching to achieve high levels of detail. The presence of any moving clouds, changing airglow or an aurora make it exponentially more challenging. It is a dignified rush to catch as many of these rare photons as quickly as we can.
Usually, a three-hour session is about all one can tolerate, as the stars would have shifted so much relative to the landscape it would be impossible to marry the two thereafter. The foreground landscape is photographed separately, with no tracking, obviously. I have no interest in making elaborate fakes by using skies from a completely different time and place than the landscape, yet it is impossible to achieve the level of refinement we want from a single night time exposure either, so some compositing is necessary to reveal the elusive beauty of the night. I try to recreate something close to what I experienced, within reason. It goes without saying that the editing process to make such a thing is by no means straightforward. After 15 years of study of digital imaging processes, astro-imaging is easily the hardest thing I’ve ever attempted. Add to that the complication of travel in an unfamiliar foreign country, and the hunt for worthwhile landscapes during the day, is already hard enough on the planning and preparation. All of the above sounded way too complicated in the beginning to be enticing.
In late summer of 2016, Joseph made his first serious attempt at such a picture in Yosemite. I was really turned on when I saw it. Soon after, I reasoned the possibility to seriously tackle this kind of photography and make special images of uncommon quality. We began discussing star trackers and shopping for lenses, his enthusiasm fuelling mine, him doing many lens tests early on by buying multiple copies of the same lens, returning the duds. My situation and local availability prevented me from doing the same. This is a necessary process to objectively pick the best examples of each lens we would like to have. While we wish modern precision manufacturing guarantees two of the same thing to be very much alike, the reality is that the pixels on our also-modern high-resolution cameras are something like 3-5 microns across and incredibly good at revealing minuscule differences between lenses. Every lens we have looked at is usually more than a little different from each other. I’ve not yet encountered a single design where I could not tell apart the optical behaviour of individual copies with the right kind of test subject.
Because so many lenses were evaluated simultaneously, Joseph eventually noticed that their plane of focus tended to be tilted in the same direction, meaning the plane of focus was falling nearer on one side of the frame and further away on the other. Ah ha—a trend, revealing a problem of a different nature. Maybe the camera’s sensor is crooked relative to its bayonet! It’s referred to as ‘swing’ in the horizontal direction, and ‘tilt’ in the vertical direction, but as the skew can happen at any degree of rotation not orthogonal to the sensor’s rectangle, I often refer to it as tilt unless speaking specifically about the horizontal and vertical direction. Try not to be confused. Guess what, both the lenses and cameras are crooked, but how on earth can we isolate the error contributed by each, and what can we do about it?
These are some of the issues which I will address:
1. Optical deficiencies of the lens (aberrations, variance, vignetting, field curvature, field tilt)
2. Bayonet mount machining imprecision of both the lens and camera body, causing the mating of the two to be affected
3. Parallelism of the camera’s sensor relative to its bayonet mount
A few words on perfection and straightness and flatness and parallelism: I’m speaking from my own perspective of the kind of pictures I’m interested in making, which may not be the kind of thing you’re after. I’m generally aiming for about 100 million pixels in the final result, though what I’ve seen from ~300 million pixels to make really large prints that are virtually flawless when inspected from 6 inches away is highly alluring, like the one above. I’m currently looking at a 40×50 inch print of it. Non-photographers with no particular interest in astronomy who have seen it, understand how satisfying the effect is. I’ve surprised even myself with it. This approach, of course, precludes many kinds of creative compositions but I’m not bothered by that for the time being. Such requirements place extreme stress on all system components to perform optimally, sometimes exceeding what they are designed for.
Need I say it again? Never, ever assume your camera (or lens) is straight, no matter who makes it.
Imaging the night sky can essentially be thought of as a wide-field point spread function (PSF) test, and to rub salt in the wound, it is in continual motion due to the rotation of Planet Earth. What is PSF, you might ask? Well, if you are familiar with the MTF testing published by the folks at Lensrentals using OLAF, this is what they are doing, minus the motion. Stars are essentially perfectly round, tiny points of light, of widely varying brightness, set against a very dark background, which stresses the camera sensor’s dynamic range too. There’s no lens that can render these perfect points perfectly across its imaging circle, as OLAF at Lensrentals demonstrates. I don’t know of any lens remotely close to perfect at f/1.4. This is the most severe and revealing test of the way a lens draws light.
Also, stars are effectively all at an infinite distance to a lens on Earth. Thus the field curvature needs to be as flat as can be, something most lenses do not exhibit. Add the need for optical perfection at very wide apertures, it is like hunting for a fantasy lens. I knew going in that this was going to be challenging but never imagined it would be quite this formidable.
It’s certainly possible to image the Milky Way over the course of several months and stitch together a giant reference image (Wei-Hao Wang has made an impressive one), which one can handily drop into any random landscape. It would be easier. I can stop down to shoot – less demanding on the optics and perfect alignment. I don’t have to worry about seeing conditions. But this is pushing it too far for me, not to mention one misses the novelty of having any pretty clouds or airglow or even auroras that might occur. I would much rather be out under real stars than on the computer cooking up fake scenery.
The recent development of many fine wide-aperture lenses, probably accelerated by the extraordinary progress in high-resolution image sensors, was encouraging for what we wanted. Few of the older lens designs were worth considering. Not so long ago, a lens that gave you sharp corners at f/11 to f/16 was considered decent, and sharp by f/8 was very good indeed. You would never dare to dream of one that is great at f/5.6, let alone f/2.8. It would have been too bulky and too expensive to be practical, and they simply didn’t exist anyway. But now, several designs appear to reach their peak sharpness on-axis at f/2.8 and in the corners by f/4 to f/5.6 without breaking the bank or your back. Truly incredible.
For a time, I considered Zeiss Otus lenses (BTW they are made by Cosina in Japan, not by Zeiss in Germany) to be reference lenses. But after a lot of looking, I determined none were good enough to justify their physical heft and cost. The 55mm suffers from too much variation and together with the 28mm, are unable to render PSFs as points in the corners. Their wide-open resolving power has also been surpassed by more recent designs. The Otus’ falloff performance isn’t exemplary either, despite their massive size. Disappointing. We would generally like the vignetting at the outer 25% of the image edge to be under half a stop and preferably under 1 stop in the extreme corners, otherwise, the gradient of noise across a stitch might be too obvious.
Perhaps, you argue, having good corner performance doesn’t matter if one is stitching. Well, yes, but only to a degree. In a single row stitch, the central portion of the short edge of the sensor is used in the final result, which is more taxing on the lens as it is further off-axis than the central part of the long edge of the sensor. Also, the distortion of points don’t only occur at the extreme edges and suddenly magically disappear away from the extreme corner. It’s a gradual effect, less but still present and you can actually see the orientation of the distortions depending on their radial position, which could cause a stitching seam to become visible—oh the horrors! Fewer messes are always welcome. Furthermore, if one has to depend on a single-frame result, you would be thankful for a good lens.
Generally, one assumes they want as wide a focal length as possible for a general-use wide-field night sky lens but I have yet to find one under 35mm that I really like. The web is awash with zillions of lens reviews and people will invariably find their own favourites. Just remember to also test your lenses at infinity or near-infinity focus, if you want to use it to shoot stars. A lens can surprise you by looking OK when shooting a chart on a wall 6 feet away but not at 1000 feet. Faraway city lights are another great test subject too. They are awfully similar to a PSF test and unlike stars, they don’t tend to move!
The Sony GM 24mm f/1.4, Sony GM 20mm f/1.8, Voigtlander Nokton 21mm F1.4 and Zeiss Loxia 25mm f/2.4 all seemed to be good contenders for a Sony system, though each has its own irritating issues. I didn’t pick any in the end. The new Nikon Z f/1.8 primes seem to be pretty good too, judging by the praise Marianne Oelund lavishes on them and her nifty coma illustrations on the DPReview forums. We have typically regarded Canon quite highly when it comes to variation control though I’ve had unusually bad luck with their lenses. Unfortunately, their sensors have not been as good as we would like in recent years. Sigma’s chief lens designer, an astro-enthusiast himself, has been the main driver for many of their new large-aperture, astro-orientated primes. They make many lenses of interest but getting good copies have proved surprisingly hard despite their claims of testing the MTF of every lens leaving the factory. Samyang/Rokinon makes some surprisingly good designs for astro work at unbelievable prices, but good luck finding a good copy. Something’s got to give at such low prices (Lensrentals has torn down their lenses and found questionable hardware). The same goes for the many Chinese lensmakers that have flooded the market in recent years.
All the mirrorless native AF lenses use fly-by-wire electronic manual focus systems. Very few feel good to me. They are usually not sensitive enough to small and slow input, the rotational ratios are not intuitive and many feel non-linear and thus unpredictable and imprecise. The Nikon Z lenses I’ve tried are dreadful, so are the early Sony E-mount lenses. Zeiss Batis lenses are somewhat better in this regard but still not super good. Recently, I played around with a few Canon RF lenses and they left a very good impression, so hope springs eternal. One could also make a case for the advantages of a true infinity focus stop, found on some manual focus lenses but never on these fly-by-wire types, given how challenging it can be to focus accurately on stars. Beware the wide range of temperatures one may have to operate in. Consequently, due to expansion or contraction, the infinity stop won’t necessarily be infinity.
After some estimates of the various angles of view of potential compositions and a lot of reading of Roger Clark’s articles, I arrived at a similar conclusion. He uses a Sigma 35mm f/1.4 Art for most of his astro-landscape ‘mosaics’, stitching as much as needed to get a wider field of view, taking advantage of a potentially sharper lens (longer lenses tend to be better off-axis) with potentially less vignetting (wider lenses tend to have more), getting higher pixel resolution in the final result to boot. I initially preferred the Canon EF 35mm f/1.4L II USM over the Sigma in my own tests. Alas, after evaluating multiple copies (if memory serves, I inspected 7) and purchasing two, I never found one that was sufficiently optically centred. Plus inherent to the lens design is a complex M-shaped field curvature not nearly flat enough by f/2.8 and worst of all, every copy had a focus field that was tilted by some non-trivial amount. It had to go.
This article will be 10x longer than it already is if I go into any more detail about what didn’t work. Let’s talk about the three successes I’ve got for the moment: I eventually replaced my Canon EF 35mm f/1.4L II USM with the Sigma 35mm f/1.2 DG DN Art, though I cannot wait for the day Cosina makes a Voigtlander 35mm f/2 APO-Lanthar (they since have, praise be!). The Sigma is outstanding but it is much too large and heavy to travel with, and I still don’t really like the fly-by-wire manual focus feel. The Sigma has low coma and astigmatism in the corners, minimal falloff, high sharpness across the image circle and relatively well-behaved field curvature which is largely overcome by using compromise focus.
I also considered the highly-regarded Tamron 35mm f/1.4 SP Di USD and tested a copy, but it did not perform as well as my Sigma despite the lavish