Electronic front-curtain shutter

As well as its high pixel count and 60fps 1080 output, the latest EXMOR CMOS sensor also has the ability to act as the first curtain of the shutter. It's something we've seen on Canon DSLRs dating back to the EOS 40D - the idea that you start with the physical shutter fully open, and begin the exposure by activating the sensor electronically, one line at a time. The exposure is then ended when the physical shutter travels down across the sensor, blocking off the light.

This video shows the A77's shutter's behavior with first curtain shutter turned off, then turned on. There are four exposures in total, two taken in each shutter mode.

The advantage of such a system is that you don't need to cock the shutter or wait for it to mechanically trigger, which improves the shutter response time. This ability is made use of across the latest NEX and SLT cameras, but offers a more obvious benefit in the mirrorless models. The NEX-5N, for example, is noticeably more responsive with electronic first curtain shutter activated, but the difference in responsiveness between the A77 and last-generation A700 is measured in milliseconds, well below anything you're ever likely to notice (and well below anything we can practically measure).

The video above demonstrates the difference as you experience it - with first-curtain shutter turned off, you hear the shutter make two movements, and with first-curtain shutter turned on, the shutter only makes one movement, to end the exposure. In our shooting, we see no reason not to keep front curtain shutter enabled at all times. It makes no difference to image quality and we prefer the more responsive feel of the single shutter movement.

Full-time phase-detection AF

The advantage of the SLT design - and the reason why Sony uses it - is that it always redirects light to a DSLR-style phase-detection AF sensor. Phase detection has two great advantages over contrast detection systems - firstly, it is able to determine exactly how to move the lens to achieve focus from a single reading (contrast-detect AF has to move the lens through its focus range to find focus) but, more importantly, phase detection technology is more refined - around 40 years of research and development has been applied to PDAF in SLRs.

This (Sony supplied) diagram shows very clearly how the SLT-A77's imaging and focussing systems are implemented. Light coming in through the lens is split by the fixed, semi-transparent mirror - a portion feeds the phase-detection AF sensor, and the majority hits the main imaging sensor for live view and image capture.

As with the implementation in previous SLT models, there are some limitations placed on the amount of exposure control you can take with the A77 if AF is to be maintained in the 12fps and video modes. Most significantly, the A77's AF module does not work at apertures smaller than f/5.6, so in video recording, if you want AF, you must sacrifice aperture control.

Although the A77 offers aperture-priority, shutter-priority and manual exposure modes for video recording, these are not compatible with AF. You can use full-time (continuous) AF during video shooting but only in program AE mode. If you initiate movie recording in any of the conventional PASM modes from the exposure mode dial, the A77 will switch to program AE regardless of your current exposure settings.

The A77 is free of the overheating concerns during movie shooting that affected the A55 (recording times could drop as low as 6 min at 30°C with SteadyShot switched on). Sony estimates the A77 can shoot for its full 29 minutes at 30°C, dropping to 13 minutes only when the ambient temperature rises to 40°C. In our shooting we've seen no problems at all.

Object Tracking

Pressing the joystick button once brings up this screen. Pressing it again sets the target to the object in the center of the frame. The target then dances around the screen, attempting to track the subject. When you try to focus the camera will use the AF point nearest the subject, lighting the target box up green when it's in focus.

When Object Tracking is engaged, you press the central joystick button to enter the 'Object Setting' mode, then again to select the object you wish to track. Interestingly, the Sony system appears to use the live view information as the basis of its subject tracking (something the company has plenty of experience of, from its compact cameras), it then uses this information to select the nearest AF point.

In use, Object Tracking AF is (usually) capable of following a subject around the frame, assuming that it is of a significantly different color/tonal makeup to the background, but it isn't capable of keeping track of fast or erratic subjects. Object Tracking AF is not available in the A77's 12fps shooting mode. For more information about all aspects of the A77's AF performance, head over to the performance page of this review.

Continuous Advance Priority AE

The SLT-A77 has three continuous shooting modes, 'Lo','Hi' and 'Continuous Advance Priority AE'. In continuous 'Lo' mode the A77's nominal framerate is 3fps, which increases to 8fps in 'Hi' and 12fps in 'Continuous Advance Priority AE' mode. Live view is maintained at continuous 'Lo' but is not available in the 'Hi' and Continuous Advance Priority AE modes.

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This is what 12fps looks like - 12 frames, taken over the course of a single second. Even if you don't shoot action or sports, you shouldn't dismiss a fast continuous framerate - it can come in very useful for portraiture and social photography.

In-camera GPS

The SLT-A77V (the variant sold in most territories) features built-in GPS, which can append location information into the exif data of still and movie files. This data can be read by an increasing number of software platforms and photo-sharing websites, including Sony's Picture Motion Browser, which is bundled with the camera. In our experience of using the A77's GPS, we've found that it works very well. The first time the camera achieves a satellite 'lock' can take around 3-4 minutes but after that, if you head indoors or power the camera down then turn it on again, it picks up a signal within a few seconds.

This icon in the A77's status display designates that GPS is switched on, and the camera is recieving gelocation information. GPS is turned on in the A77's custom menus. From here you can also activate automatic time correction, which lets the camera maintain accurate time/date information using GPS. To speed up GPS data aquisition you can download 'GPS Assist Data' via Picture Motion Browser and upload it to the camera via direct USB connection or from a memory card.
When GPS is turned on, geolocation data is apended into the metadata of still and video files. Here's how the data is displayed in Adobe Bridge CS5... ...and here's how Sony's Picture Motion Browser uses it to tag images by the location in which they were taken. In our experience, accuracy is within a block, assuming that you're not 'tunnelled' by surrounding buildings.