Studio Tests - 35mm full frame
The 24mm F2 turns in a truly excellent performance on full frame. Technically its only slight fault is corner softness wide open (which is rarely a problem in practical use), and overall it's at least a match for the highly regarded Canon EF 24mm F1.4 L II.
|Sharpness||Sharpness is impressively high across most of the frame wide open - only the extreme corners look soft. These sharpen up progressively on stopping down - by F5.6 practically the whole frame is critically sharp. Diffraction progressively softens the image at smaller apertures, but as usual for full frame F16 is eminently useable, although contrast visibly starts to suffer at F22.|
|Chromatic Aberration||As on APS-C, lateral chromatic is kept impressively low - there's a hint of green/magenta fringing towards the extreme corners, but you have to look very closely to see it.|
|Falloff||We consider falloff to become perceptible when the corner illumination falls to more than 1 stop less than the center. Here we see a maximum of two stops at F2, which decreases gradually on stopping down - it's essentially gone by F4.|
|Distortion||Distortion remains very low on full frame; indeed thanks to some subtle recorrection towards the corners it measures even lower than on APS-C, at a mere 0.4% barrel. Nothing to be concerned about here at all.|
Full-frame compared to APS-C
Eagle-eyed viewers will no doubt have noticed that the MTF50 sharpness data at any particular focal length/aperture combination is higher on full-frame when compared to APS-C. This may at first sight appear unexpected, but in fact is an inevitable consequence of our presentation of the sharpness data in terms of line pairs per picture height (and thus independent of format size).
Quite simply, at any given focal length and aperture, the lens will have a fixed MTF50 profile when expressed in terms of line pairs per millimeter. In order to convert to lp/ph, we have to multiply by the sensor height (in mm); as the full-frame sensor is 1.5x larger, MTF50 should therefore be 1.5x higher.
In practice this is an oversimplification; our tests measure system MTF rather than purely lens MTF, and the camera's anti-aliasing filter will also have a significant effect in attenuating the measured MTF50, especially at higher frequencies. In addition, our testing procedure involves shooting a chart of fixed size, which therefore requires a closer shooting distance on full frame, and this will also have some influence on the MTF50 results.
Specific image quality issues
As always, our studio tests are backed up by taking hundreds of photographs with the lens across a range of subjects, and examining them in detail. This allows us to confirm our studio observations, and identify any other issues which don't show up in the tests. We tested the lens primarily on the highly demanding 24Mp full-frame Alpha 850, alongside the 12Mp APS-C Alpha 700.
The 24mm F2 does exceptionally well in studio testing, but point it towards a bright light source and its Achilles' heel becomes apparent - it's not always great at suppressing flare. This is most problematic on full frame: in particular, if you place the sun towards the corner of (or just outside) the frame then a large, bright red flare spot appears opposite, which gets ever more defined and intense on stopping down. Unfortunately you won't necessarily spot this in the viewfinder until it's too late (as the red spot is quite diffuse at F2).
Sun in corner of frame
Sun outside frame
|F16, Sony Alpha 850||F8, Sony Alpha 850|
Lateral chromatic aberration is exceptionally low in our studio tests, and likewise turns out to have little impact in normal use. It's scarcely visible at all on the Alpha 700's 12Mp APS-C sensor, but can become a bit more obvious on the 24Mp full frame Alpha 850 and 900, giving green/magenta fringing towards the corners of the frame. However it's trivial to correct in post-processing if necessary, and naturally it's best to do this from RAW. The examples below show results with and without CA correction using Adobe Camera Raw 6.2, with 'Fix Red/Cyan Fringe' set to -15 and 'Fix Blue/Yellow Fringe' to 15.
APS-C (Sony Alpha 700)
Full frame (Sony Alpha 850)
|F7.1, RAW + ACR||F11, RAW + ACR|
|100% crop, left of frame||100% crop, top left corner|
|100% crop, CA corrected||100% crop, CA corrected|
Fast primes often show some 'bokeh' chromatic aberration (colored fringing in out-of-focus regions of the frame), and the 24mm F2 is no exception. However it's not particularly objectionable, and does of course disappear on stopping down. The examples below show how this looks in practice: green / magenta fringing is clearly visible throughout the distant background, most obviously around high contrast edges. When the background is closer, or the edge contrast lower, this fringing becomes rather less obvious.
|F2, Sony Alpha 850||100% crop|
|100% crop, behind plane of focus|
|F2, Sony Alpha 850||100% crop, in front of focus plane|
Background blur ('bokeh')
One genuinely desirable, but difficult to measure aspect of a lens's performance is the ability to deliver smoothly blurred out-of-focus regions when trying to isolate a subject from the background. With its large maximum aperture the 24mm F2 can give a perhaps unexpected degree of background blur, and on the whole it does so in an attractive fashion. Distant backgrounds are generally rendered convincingly, with recognizable structure but without harsh bright edges to out-of-focus highlights. Transitions to out-of-focus regions when shooting close up are also handled well.
|F2, Sony Alpha 850||F2, Sony Alpha 700|
|50% crop, upper right||50% crop, upper left|