Sony's A33 uses new translucent mirror technology in a digital camera for the first time. Is this the beginning of the end for DSLR cameras as we know them? That What Digital Camera Sony Alpha A33 review...
Design – What Is ‘SLT’ & How Does It Work?
Sony Alpha SLT-A33 – Design
Without the pentaprism as a fundamental part of the design the A33 is smaller than a usual DSLR – but only ever so slightly in terms of height, to the point it’s barely noticeable. Most of the hand will comfortably fit around the grip, though your little finger may stray off the bottom of the body and that can be uncomfortable for long periods of use.
Elsewhere the design is as expected from a usual DSLR – the mode dial to the top left can be quickly adjusted, while the opposite side has one-touch buttons for each of the Finder/LCD, D-Range, Movie, Exposure Compensation and AEL functions. The rear has a standard D-Pad and an Fn (Function) button above this for quick access to the most commonly used settings.
The main menu, accessed for the button behind the main mode dial, is the same as current Sony Alpha DSLR cameras. Divided into a number of sections with up to seven settings per page it avoids being too cramped and, as there’s no scrolling to located options, it’s fairly easy to find those more detailed settings. It’s really with the quick menu by using the Fn button that most control will be had, however, and this offers a comprehensive and easily navigable list of options.
What Is ‘SLT’ & How Does It Work?
The best way to explain how a translucent mirror system works is to compare it side by side to a DSLR system’s operation.
In a DSLR camera the light enters from the lens and bounces off the camera’s mirror into an AF sensor for fast phase-detection autofocus. Simultaneously light also bounces into a pentaprism that in turn feeds a preview image to the eye via an optical viewfinder (the pentaprism is there to flip the image from upside down and back to front to the usual way we see). Once the image is framed and the focus set a full shutter press will flip the mirror upwards and out of the way before the shutter itself fires to expose the image sensor (for the time period the shutter is set at) for the final image capture.
An ‘SLT’ camera, as Sony is naming the A33, also has a mirror construction – but, as this is translucent, 70% of light can permanently pass through, thus eradicating the need for the mirror to move at all during an exposure. Without the need for this slow mechanical movement it’s only the shutter that fires to make an exposure – and this can happen much more frequently, hence the A33’s super-fast 7fps shooting. The 30% if light that bounces off the mirror is still available to reach the AF sensor for continuous phase-detection autofocus during live view mode – the first time this has been possible.
However the construction does not lend itself to an optical viewfinder due to the lack of available light that could reach a hypothetical optical viewfinder – this is why the A33 features a built-in electronic viewfinder in its place.