The Canon 50mm F1.4 USM gives a somewhat mixed performance on 35mm full-frame [although one that will come as no surprise to anyone experienced at shooting film]. Performance is unremarkable at wider apertures where the new Sigma 50mm F1.4 EX DG HSM does much better, however image quality increases rapidly on stopping down, giving excellent results between about F4.5 and F16.
Sharpness At F1.4, the lens is quite sharp in the centre but distinctly soft across much of the rest of the frame. However it improves rapidly on stopping down, with the centre excellent by F2, and the corners catching up at F4. Optimum results are obtained between F5.6 and F11, where it gives superb sharpness right across the frame; stopping down further gives a gradual reduction in sharpness due to diffraction.Chromatic Aberration Lateral CA is extremely low, and even on the EOS-1Ds Mark III there's practically no fringing visible. However as on APS-C, the non-zero CA figures towards the centre at wide apertures betray a more problematic issue, high levels of mainly blue 'colour blur' due to axial chromatic aberration, which essentially disappear on stopping down to F2.8.Falloff We consider falloff to become perceptible when the corner illumination falls to more than 1 stop less than the centre. Falloff is about average for its class on full frame at 2.7 stops wide open, falling to below 1 stop on stopping down to F2.8; this will certainly be high enough to cause concern for some users.Distortion Distortion on full frame is about 1.3% barrel; this is towards the high end for a 50mm standard prime, and has the potential to be visible in real-world shots.
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 distinctly 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 1.6x larger, MTF50 should therefore be 1.6x higher.
In practice this is an oversimplification; our tests measure system MTF rather than purely lens MTF, and at higher frequencies the camera's anti-aliasing filter will have a significant effect in attenuating the measured MTF50. 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 data.
Macro Focus
Barrel distortion has become more visible, but again chromatic aberration is notably absent, and the lens is sharp right across the frame at F8.
Macro - 251 x 167 mm coverage Distortion: Moderate barrel Corner softness: Moderate Focal length: 50mm
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 on both APS-C and full-frame bodies, ranging from the EOS 450D to the EOS-1Ds Mark III.
Flare
Possibly a reflection on its pre-digital design, the EF 50mm F1.4 USM is distinctly susceptible to flare with a strong light source in or just outside the frame. With the sun in the corner of the frame, we see strong flare patterns ranging from red 'doughnuts' at wide apertures, to multicoloured octagons on stopping right down, and at the more commonly used intermediate apertures [~F4 to F8], the effect is of an overall reduction of contrast from broad swathes of veiling flare.
These problems persist when the light source is moved slightly outside the frame, with again an overall loss of contrast and detail. However to be fair the lens regains its poise quickly as the light source is moved further off-axis, and is generally resistant to flare problems in normal shooting situations.
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, generally when using a long focal length and large aperture. The 50mm F1.4 can be made to blur even relatively close backgrounds into oblivion at wide apertures, a huge advantage for portrait shooting especially on APS-C.
The EF 50mm F1.4 USM produces reasonably attractive bokeh in most situations, however it can look distinctly 'busy' at wider apertures. In particular, distant point light sources can show up as distinctly hard-edged [as shown in the second sample], and under high-contrast conditions, bokeh chromatic aberration can also be quite marked [see below].
Chromatic aberration
Lateral chromatic aberration is negligible in our studio tests, and is equally near-impossible to find in real-world shots; quite simply it's not an issue when using this lens. What can be problematic, though, is the presence of quite strong bokeh chromatic aberration, which is most visible at wide apertures, resulting in strong fringing artifacts in high contrast out-of-focus regions. This tends to show up as green fringing around edges behind the field of focus, which can be accompanied by a magenta 'fill'.
Optical performance at wide apertures
Our studio tests show that this lens is not at its best at wide apertures on full-frame; central resolution is relatively low, and the corners extremely soft and subject to significant darkening through vignetting. However in this regard it's also important to appreciate that with the extremely small depth of field afforded by a 50mm F1.4 lens, and assuming a reasonably centrally-placed subject, the likelihood of any object in the corners of the frame being remotely in focus is in fact minimal, and corner resolution therefore near-irrelevant.
The images below illustrate the lens's performance for those inclined to shoot flat subjects at wide apertures [here using the 21Mp EOS-1Ds Mark III]; at F1.4 the centre of the frame is simply soft, and in the corner little detail is visible at all [although it must also be stressed that in this case, depth of field effects are also reducing sharpness, even for an almost planar subject]. However at F4, the corners have nearly caught up with the centre for sharpness, and vignetting has become insignificant.
F1.4
F4
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