Colour and power benefits of Sony 20 megapixel sensor
Sony has now released full details of the Alpha 58. Although I don’t think the camera is a game-changer or a vital upgrade for owners of Alpha 55 and 57 (the 55 will leave me only when it expires, with its useful GPS, 6fps/10fps and fully articulated reversible rear screen) there are hidden bonuses for anyone investing in the 58.
Firstly, the new OLED finder – probably a step better visually – is a league better in power consumption. The penalty for using the EVF instead of the rear LCD on the Alpha 77 and is siblings has been a sharp reduction in the battery stamina for shots, 470 versus 530 official figures for the 77 as an example. The new finder on the 58 gives a reduction for 700 to 690 – not just an overall improvement, but a minimal difference you can ignore. The smaller, non-reversile tilting rear 2.7″ LCD screen may also be less power-hungry than 3″ types.
Secondly, the camera supports an extended TriLuminos colour gamut. The colour gamut of existing Sony DSLRs and SLTs (and NEX) equipped with HDMI output does not need to exceed AdobeRGB (52.1% of the recognised visual gamut for a ‘Standard Observer’, CIE 1931 vintage). That’s because regular HDTV throws away a stack of this colour, showing only 35.9% of the gamut. That’s why it looks so colourful and bright. The less gamut you show, the brighter and more saturated colours look, for the capabilities of any given display. That may sound the reverse of what you would believe to be the case, until you apply a bit of thought to it.
TriLuminos gamut is the larger triangle, regular HDTV is the smaller (similar to sRGB) while AdobeRGB falls between the two. One colour space you can use when processing raw files – ProPhotoRGB – is so large is exceeds part of the CIE 1931 colour space.
The TriLuminos gamut is massive. Unlike HDTV, it’s bigger than AdobeRGB and much bigger than regular sRGB (what most computer screens can show). It is 75.8% of the CIE 1931 colour space. That, by the way, is simply a standard based on what a bunch of test subjects could perceive back in 1931 and it’s been criticised for failing to include a wide enough range of genetic backgrounds and learned visual abilities. We all see colour differently (men notably with far less accuracy and discrimination than women, young better than old). If you’re a teenage girl you’ll love the TriLuminos displays. If you’re an old bloke you may not notice…
Sony claims that the A58 can output colours to the TriLuminos TV sets which show “a dramatically expanded palette of vivid, ultra-realistic colours when videos and still images (are played back)”. In theory since AdobeRGB (offered by all Sony models to date) would already show an expanded palette, this might not mean any big change in the sensor. But TriLuminos uses a colour space which requires 12-bit depth and it can’t be used effectively unless the sensor itself is going beyond the range of AdobeRGB. You can’t get out what you do not put in. Then again, if you’re using a normal printer or computer, you can’t get it out anyway. The camera captures colours you can’t see on its own rear screen, in its viewfinder, on your computer screen or in a print.
We can therefore deduce that the Bayer filter colours on the new 20 megapixel sensor may be changed, along with the BIONZ processing and the JPEG colour management and compression (after all, the JPEGs will still be 8-bit and going beyond AdobeRGB risks significant banding in smooth graded colours such as skyn blues). Sony say this is the first ever A-mount camera to offer this colour ability. Will DxO Mark have to change their colour measurements to cope with it?
It is possible the sensor has no colour gamut benefits and that all Sony is doing is expanding AdobeRGB (or the native gamut, which is close enough to AdobeRGB) to fill the wider space of the TriLuminos TV screens, making certain colours appear dramatic in the process, but not realistic. Obviously what we should all hope for is that this improvement starts with the sensor itself.
Since the NEX-3n (possibly not the camera rumoured by Nippon Camera as NEX-F3R) also offers TriLuminos extended gamut but has a regular 16 megapixel sensor, I’m going to have to wait to see what the real colour science experts at DxO, and our various friends in Russia with special knowledge of this field, find. We do have a resident colour scientist but sadly none of the gear needed to analyse this properly.
Whatever the case, we appear to be getting a camera whose new 20 megapixel sensor will have significantly better power consumption which almost certainly also means lower heat generation, in turn meaning lower noise and longer ‘safe’ durations for video. Sony is gearing up for the next phase of HDTV – 4K – and the UHDTV beyond this going to 8K. They will eventually need to produce 39 megapixel sensors for uninterpolated 8K, and this will be the target for both APS-C/Super35 and full-frame between now and 2015 when the industry expects to see the first 8K UHDTV retail sales (those in the UK, don’t hold your breath, we’re likely only to get 4K and may not see that become the standard until 2020).
Nikon has stolen an interesting march by enabling a 1.3X, 15 megapixel crop for 7fps shooting in the new 24 megapixel D7100 – a very useful size almost equal to a 2X crop from full frame. Sony has an unspecified ‘tele-zoom’ feature in the A58 to achieve 8fps. But no-one has so far been able to reveal what the tele-zoom crop is; Sony’s ‘technical specifications’ so far released for the A58 are minimal.
If the same 24.1 megapixel, AA-filter-less sensor is used in an A78 (as some rumour sites think likely) then perhaps sub-frame readout aka tele-zoom will be implemented on that too.
The A58 has a new 18-55mm SAM lens with improved build quality and a redesign to the rear element configuration. Sony says this is to avoid ghosting. We’d be surprised if it was not also to change the exit pupil geometry slightly, in order to work better with current and future phase-detection on sensor models.
– DK