Sony α7S mirrorless interchangeable lens camera
A worthy challenger in the DSLR-dominated camera market See an interactive 360° model of this camera here.
On paper, the specifications of the Sony α7S may make you sit up and pay attention, especially as many of its capabilities also look good for astrophotography. The α7S is a category of camera known as a MILC: a mirrorless interchangeable lens camera. MILCs have been around since 2004, and are similar to DSLRs in that you can change their lenses and directly attach them to telescopes with an appropriate adaptor. The biggest difference is that they don’t have a reflex flip mirror for viewing what’s coming through the lens. Instead, they monitor what the sensor is seeing and display this either on the camera’s rear screen or in an electronic viewfinder.
The lack of flip mirror removes a fair amount of bulk from the camera’s design. Compared to its DSLR rivals, the α7S’s dust- and damp-resistant body looks rather petite, distinguished and, dare we say it, elegant. Inside, the α7S packs a full frame (35mm) sensor containing a modest 12.2 megapixels.
Many high-end cameras break the 20-megapixel barrier and this may cause you to think the α7S is tame by comparison. But for astrophotography, this is a good thing. The lower pixel density means that individual light collecting photosites are larger. For a given amount of light, larger photosites individually collect more photons than smaller ones. More photons mean a stronger signal and an improved signal-to-noise ratio. The content of each photosite is read and amplified according to the gain, or ISO setting. As both signal and noise are amplified together, keeping the signal-to-noise ratio high helps make noise less intrusive.
Amplification is a headline act of the α7S, the camera boasting an incredible ISO range of 50 to 409600. Top-end values aren’t known for their finesse and the α7S doesn’t break that rule, but the mid-levels are interesting. The technical implementation of ISO within a camera produces more noise and smaller tonal range at high ISO values. It’s the job of the camera’s image processor, called BIONZ X, to clean the images as best it can without losing or adding detail. Our tests with low to mid-range ISOs were very positive, with plenty of strong detail being seen with generally low noise. We stacked an image of comet C/2014 Q2 Lovejoy, which was on display during the review period, and the camera managed to pull out the comet’s colour and faint, delicately structured tail with ease.
Single shots of Orion’s Sword at progressively higher ISOs seemed to suggest that ISO 6400 or 12800 was perfectly usable. Stretching an image, background noise starts to become noticeable higher than this, but acceptance is down to personal taste. Image quality above ISO 51200 seemed drop off quite rapidly. We were very impressed with the α7S’s capabilities for astrophotography: it promises great things when it comes to meteor showers, aurorae and wide-field Milky Way imaging. But bear in mind that this is still a general purpose camera, not one optimised for astro imaging. Consequently, the sensor’s infrared-blocking filter cuts out some of the red hydrogen-alpha light emitted by many deep-sky objects, rendering them slightly too magenta or blue. Sadly, for astronomical purposes, this lost the camera a point for imaging quality. If Sony were to bring out an astro-modified version, an equivalent to Canon’s 60Da, it would be a different matter altogether. While performing our tests, we also noted that the rear LCD screen glows when a long exposure is taking place. We could see no purpose to this and couldn’t find any way to turn it off.
The camera can connect to other devices via its in-built Wi-Fi or, if you have one, a near-field communications device. This would allow you, for example, to control the camera and review any pictures you take via a smartphone. It’s worth noting that you can also record videos in either full HD or even 4K with additional kit. The ISO range for video work is 200 to 409600. This means that you could, for example, wax lyrical in a piece to camera under the stars, with many of them making an appearance in the video too. What we really liked about the α7S was the way it got our creative side working. It was exciting to experiment with and overall great fun to use. It’s also good to see another serious contender enter the DSLR-dominated astrophotography field – even if it’s not strictly a DSLR.
Scaling the heights of iso
The α7S’s extensive ISO range of 50 to 409600 is vast. The highest values create images of understandably questionable quality, but the middle values are good. A test sequence of Orion’s Sword showed that you could use shots up to ISO 51200; great news if you are using a fixed tripod or don’t have the best mount tracking accuracy. At high ISO, exposures can be kept short to avoid trailing. However, it must be stated that best results are had using lower ISO values with accurately tracked long exposures.
Higher ISOs really come into their own for focusing a region of sky devoid of bright stars; the camera’s live view is simply superb at high ISO. The higher mid-range ISOs are also ideally suited to capturing fast moving events such as quickly shifting displays of the aurora and the rapid changes that occur at the critical stages of a total solar eclipse. They are also perfect for killer untrailed shots of the Milky Way over beautiful foreground scenery.
Lenses and lens adaptors - Sony produce a range of lenses and lens adaptors compatible with the a7s. These include (left to right) the 70-400mm G SSM II, Vario-Tessar T* FE 16-35mm ZA OSS, Vario-Sonnar T* 24-70mm ZA SSM and LA-EA4 35mm full-frame A-mount adaptor.
Control cluster - The α7S’s controls are clustered on one side of the camera. Three primary dials give access to ISO, f/number and exposure settings quickly and efficiently. However, the shutter button lacks a tactile half-press feedback. It is too easy to think you’re half-pressing to get an exposure reading, only to find you’re actually taking a shot.
Articulating LCD screen - This 3.0-inch TFT LCD screen provides the main interface to the camera’s functions. Fine detail is beautifully presented on this 912,000-pixel display. The screen can tilt up and down but not from side to side. Move your eye to the viewfinder and the LCD turns off, switching the display to the internal XGA OLED screen.
Mirrorless design - The α7S uses a uses an electronically controlled, vertical-traverse, focal plane type shutter (1/8000-30 seconds, plus bulb). This can be disabled, turning control over to a purely electronic shutter (silent shooting). The lack of a reflex mirror in the MILC design also removes the issue of mirror-shake, which can sometimes occur when taking a shot with a DSLR.
Sony E-Mount - The a7S natively uses E-mount lenses, but commercial adaptors are available that allow other manufacturer’s lenses to be used with the camera. An inexpensive E-mount to T-thread adaptor is required for telescope coupling.
Wi-Fi - A built-in Wi-Fi and near-field communications option allows you to control the camera with a smartphone or similar. Paid-for apps can also be downloaded to the camera to add functionality: current offerings include apps to automate star-trail production, give timelapse facilities and auto-upload to a Flickr account.
This review originally appeared in the April 2015 issue of BBC Sky at Night Magazine.