Full format for the masses?
Having taken number three spot in the DSLR market with a well balanced range of cameras, Sony has launched its first full-frame model, the A900. Anders Uschold put it through his latest test regime, together with an older Sony 50mm f/2.8 lens and the latest 24-70mm f/2.8 from Zeiss
Three weeks ago in Edinburgh, Sony's chief developer proudly introduced the company's new top-of-the range digital SLR, the full frame Alpha A900. He was accompanied by Sony's European president, who talked market share (now up to 20%) and future strategy, alongside a Scottish photographer who presented sport and landscape pictures taken with the new camera. But there was one significant omission; not one of them mentioned the word 'professional'.
The A900 may have a 24 million pixel resolution, a rugged chassis, wide ISO range and an extremely bright, high class viewfinder, but despite its £2200 price, Sony is targeting advanced amateurs and 'semi-professionals'. It was explained later that rather than compare the A900 with Canon and Nikon's top-range cameras, the EOS 1Ds MkIII and the D3, the new Sony camera will be pitched against the 5D MkII and D700. Perhaps Sony is being a little modest in its ambitions?
Right choice
Sony has used a pixel pitch similar to the A700 camera, so the signal processing, noise and dynamic ranges should behave similarly, but the increase in resolution presents is a real challenge for lenses.
Many will be interested to see how Minolta lenses designed for film capture will fare with the A900. I expect an old Minolta 28-85mm to have similar limitations to those a Canon user would face with an EF 35-105mm on a Mark III body. However, Minolta's G series was just about the finest in AF lens technology, and several of the lenses, such as the 80-200mm f/2.8 or the 17-35mm f/3.5, outperformed Canon and Nikon equivalents. Sony made a wise purchase when it bought Konica Minolta's SLR camera business, and many of the KM engineers in Osaka moved together with their products.
Before discussing the results, however, I want to point to a change in my DCTau lab tests. Version 5.0 is the result of the steady technological progress that has let camera makers manufacturers implement more and more sophisticated signal processing. This is done primarily to improve image quality, but it's also used to 'cheat' standardised test methods. I have subsequently adjusted my marking and applied stricter criteria to noise and input dynamic range, so please don't compare the two marks directly to former tests.
To reveal the negative influence of excessive shadow noise compensation, DCTau now calculates the relative suppression of shadow signals and the accumulated amount of deleted shadow information. High marks indicate that a manufacturer is trying to gain good noise marks that don't necessarily comply to the real image representation or high input dynamic range, which is based on crucial deletion of shadow details. The second new function is corner noise. It is cheap and therefore popular to build a simple lens and compensate the corner shading electronically. Now I can determine whether a company really spends on glass and costs to ensure high quality from image centre to corner or not.
Performance
The 24 million pixel resolution creates a lot of information and manufacturers are understandably weary about the corresponding file sizes. The A900's JPEG compression is well balanced to deal with this, available in three different modes of 24, 13 and 6.5 million pixels. Each resolution mode provides three JPEG quality modes with shrinking ratios of 1:5, 1:13 and 1:17-20. These ratios represent quite large files at the highest settings, but they also employ a very high quality preservation strategy. During an advanced examination of hue and saturation channel loss, I found the camera preserves a maximum of pictorial information in these critical aspects.
Tonal reproduction follows Sony's determination to focus on semi-professionals and advanced amateurs. The shape of the opto-electronic conversion function (OECF) is very inverse-s shaped; a strategy designed to enhance the perceived visual quality of an image. Mid-tones seem more brilliant, while shadows and highlights are softer. This benefits those that need immediately usable results over photographers concerned with absolute accuracy, or who want to work a lot on their photographs in post-production.
At ISO100 the highlights are not that soft, but the mid-range shows enhanced contrast. This is a typical strategy for overcoming saturation of the sensor. Despite the common opinion in analogue photography that lower ISO speeds always lead to better quality, for best results this speed should be avoided.
The fine detail processing strategy is ambivalent. On one hand the contrast of fine structures isn't enhanced, and it follows the classical principle of a flat Modulation Transfer Function. On the other hand the sensor's low-pass filter is thin and allows high optical frequency to pass through. This will lead to lower total resolution marks, even when fine details are discriminated. In combination with lenses of high optical modulation, slanted structures show visible luminance Moire and structures of diagonal orientation are affected by chromatic Moire from demosaicing. Luckily, the camera doesn't eliminate fine detail chrominance - another cheating function to pretend there is low chromatic Moire.
The A900 works visibly on noise. Average noise is quite stable from ISO100 to 800, and provides good to moderate marks. From ISO1600 on the marks rise to excellent, due to additional compensation. The character of the noise is salt-and-pepper formations, which are isolated dark bright pixels caused by relevant internal noise compensation. From ISO100 to 400 luminance noise is mainly visible and is at its highest at mid-range and slightly brighter regions. This touches the more sensitive topics of skin and sky parts. From ISO400 luminance turns more and more into chrominance noise, with dominance at dark to mid-range regions.
Noise distribution shows its maximum between brightness of 150 at ISO200 and 76 at ISO6400. From ISO100 to 800 it is strongly reduced. The accumulated degree of noise signal elimination is -9.1%. At this point of time we have limited reference results to determine how high to mark the evaluated amount of signal elimination to a final degree is.
Fine detail noise shows quite constant results from ISO100 to 400. Neither contrast or noise seem to be affected to any relevance; only some salt-and-pepper pixels can be seen in the non-resolved region. At ISO800 fine, graphic structures turn slightly chromatic, an effect that rises visibly at ISO1600 and 3200. Nevertheless, fine lines remain well modulated. At ISO 6400 specks of chrominance noise affect fine structures and cause visible resolution loss from low frequency chrominance noise.
At ISO100 input dynamic range is good to moderate, a possible consequence of sensor over-saturation. At ISO 200 and at ISO 400 the marks are good and increase to a good to very good input dynamic range mark at ISO 800. From ISO 1600 to ISO 6400 additional noise compensation takes effect and input dynamic range marks increase to excellent.
Output signal range is good at ISO 100, good to moderate at ISO 200 and moderate from ISO 400 to ISO 1600. At ISO 3200 and 6400 it is moderate to restricted. These output signal range marks are a consequence of the enhanced shadow level. Images lose their blackness, and especially at high ISO settings, may gain benefit from some shadow enhancing post-processing.
Edge reproduction is well designed and not too aggressive. Edge symmetry is moderate to good. While the bright side of edges is sharp and clear, the dark side is softer and shows some flare. Thus sharpening is more dominant in bright image parts. Sharpening is excellently stable at any orientation, however, a benefit to more homogenous scene reproduction.
Old and new
The flat Modulation Transfer Function mentioned highly affects resolution marks in DCTau. While tests based on the ISO standard require a detail contrast of only 10% only, DCTau asks for more brilliant structures in the range of 60% contrast, which is much more important to visual perception. So the flat fine detail representation directly leads to a moderate to restricted mean coefficient of resolution.
Despite its age, the Sony 50mm f/2.8 Macro can be used on the
demanding sensor easily: only at open aperture does resolution show a visible restriction. Stopping down two stops leads to good resolution performance. Diffraction starts faintly from aperture f/11.
The preferable aperture range and practical flexibility are good. Centre-to-corner resolution shows a normal fall-off in the outer corner at open aperture only.
Noise compensation and resolution correlate slightly. Resolution is very stable from ISO100 to 800. Then it applies enhanced noise compensation, which results in slight loss from ISO1600. Nevertheless, the degree of loss is quite low, and you only see further decrease at ISO6400.
At open aperture corner shading is strong, but quite homogenous. Stopped down two stops it improves a lot and provides quite good performance for a full-format camera. An increase of noise from image centre to corner is visible at maximum aperture, and the curved shape indicates that the camera applies a quite complex corner shading compensation function to. Hats off to that. Stopped down one stop some noise increase occurs in the outermost corner. Stopped down furthermore the image provides quite constant results.
Optical distortion is excellently low and well suited to the most sophisticated applications.
The impressive Zeiss lens proves to have a very well balanced resolution performance: only at 40mm does resolution show some open aperture restrictions. Stopping down one stop provides maximum performance. Diffraction shows its first influence from f/11. Image centre to corner resolution shows a normal fall-off at 24 mm f/2.8. At 40mm the loss from image centre to corner is more visible from f/2.8 to f/4. At 70mm the lens shows slight to normal corner loss over the full tested aperture range. Regarding the wide angle of view and the full format, the quite visible barrel distortion at 24mm is still a good mark. At 40mm distortion shows almost no practical relevance, and at 70mm pincushion distortion is visible to a normal degree.
As is usual for a very fast full-format lens, the widest angle corner shading is very visible. Stopping down improves it a little. At the outermost image corner some sudden vignetting occurs at maximum aperture, and to a lower degree when stopped down two stops. At 40mm and 70mm open aperture corner shading is visible and shows a sudden rise. Stopped down the marks are very good.
At 24mm corner noise rises visible from 50% image height, and at 40mm it increases from 70% image height.
At 70mm the corner noise is also a little strange, as you only see a rise at apertures f/2.8 and f/11 in the outer corner.
The degree of corner shading is visible at 24mm and 40mm open aperture only. Thus, the total corner shading compensation does not affect image quality significantly.
Conclusions
Sony's first full-format camera is designed to meet the needs of ambitious amateurs and semi-professionals. JPEG compression is perfectly balanced, and input dynamic range at high ISO speeds is excellent. Tonal reproduction focuses on instant image appeal (rather than something more neutral, which you see with more pro oriented cameras), and the sharpening is OK.
Noise is a little unnatural, which was to be expected by this enormous pixel count. The very fine salt-and-pepper character of noise will often virtually vanish, as just a few pictures will need to be enlarged to 100% or more. This should be kept in mind when these results are put side-by-side against a test of a 12 million pixel camera, for example.
The Zeiss zoom provides a very high optical modulation, but due to the reluctant detail contrast enhancement, resolution marks are moderate. Given the full format and fast maximum aperture, corner shading shows some expected restrictions. Optical distortion is quite good over the full zoom range.
Overall the Sony 50mm macro lens performs quite well with such a demanding sensor. Only at open aperture do resolution and corner shading drop a little, though not dramatically. Stopped down, the total performance is well balanced. The low total resolution mark is more a result of the camera's reluctant fine detail contrast strategy, because the lens' modulation is fine.
SONY A900
Input dynamic range
At ISO100 input dynamic range is good to moderate. At ISO200 and 400 the marks are good and increase to a good to very good input dynamic range mark at ISO800. From ISO1600 to 6400 marks increase to excellent.
Shadow noise compensation
At ISO 100 and 200 shadow noise compensation is high - which is a usual mark. The higher the ISO speed is, the less weaker noise compensation gets.
A good and sincere strategy.
Opto-electronic conversion function (OECF)
The OECF is very inverse-s shaped. Mid-tones are more brilliant, shadows and highlights are softer. At ISO100 the highlights are not that soft and mid-range shows enhanced contrast - a typical sign of sensor over-saturation.
Elimination of shadows signal information
The highest signal elimination from noise compensation occurs at ISO100. The amount of eliminated data decreases the higher the ISO speed is. This is a good and sincere strategy between mark optimisation and image quality preservation.
SONY 50MM F/2.8 MACRO
Optical distortion
Optical distortion is excellently low and well suited to the most sophisticated applications.
Corner shading, loss in stops
Open aperture corner shading is strong, but quite homogenous. Stopped down two stops corner shading improves a lot and provides quite good performance for a full-format camera.
Increase of corner noise
An increase of noise from image centre to corner is visible at maximum aperture, and the curved shape indicates that the camera applies a quite complex corner shading compensation function too.
ZEISS 24-70MM
Optical distortion
The quite visible barrel distortion at 24mm is still a good mark for this angle of view. At 40mm distortion shows almost no practical relevance, and at 70mm pincushion distortion is visible to a normal degree.
Corner shading, loss in stops
Widest angle corner shading is very visible. Stopping down improves a little. At the outermost image corner some sudden vignetting occurs at maximum aperture, and to a lower degree when stopped down two stops. At 40mm and 70mm open aperture corner shading is visible and shows a sudden rise. Stopped down the marks are very good.
Increase of corner noise
At 24mm corner noise rises visibly from 50% image height, and at 40mm it increases from 70% image height. At 70mm the corner noise is a little strange; you see a rise at apertures f/2.8 and f/11 in the outer corner.
SONY A900 - Fine image details and artefacts
Fine details show quite constant results from ISO100 to 400, with just some salt-and-pepper pixels visible. At ISO800 fine graphical structures turn slightly chromatic, which rises visibly at 1600 and 3200. Fine lines remain well modulated. At ISO6400 specks of chrominance noise affect fine structures and cause visible resolution loss from low frequency chrominance noise.
Noise
In some mid-range to bright parts, the character of the noise is salt-and-pepper formations, which are isolated dark bright pixels caused by relevant internal noise compensation. From ISO100 to 400 luminance noise is mainly visible and is at its highest at mid-range and slightly brighter regions. This touches the more sensitive topics of skin and sky parts. From ISO400 luminance turns more and more into chrominance noise, with dominance at dark to mid-range regions.
