Panasonic Lumix G 20mm F1.7 ASPH Review
The Panasonic 20mm F1.7 pancake does rather well in our studio tests. Distortion is being corrected in software, as is lateral chromatic aberration when used on Panasonic camera bodies (although not on the Olympus E-P1), and this is reflected in the results for these tests. Compared to the Olympus M Zuiko Digital 17mm F2.8, the Panasonic simply shines - it's much sharper (to the extent of being better at F1.7 than the Olympus is at F2.8), and has lower CA (even before software correction), less falloff and lower distortion (both 'native' and corrected). Overall very impressive results for a lens this small.
|Sharpness||Sharpness results are extremely good; the lens is very sharp in the center even wide open, and while the corners are somewhat soft at F1.7, they sharpen up rapidly on stopping down. Optimum results are obtained between F2.8 and F6.3; stopping down further sees sharpness progressively fall due to diffraction (mercifully Panasonic doesn't provide apertures smaller than F16).
Comparison of results obtained using the Panasonic G1 and Olympus E-P1 show slightly better results on the former - the gradual attenuation of MTFs by the E-P1 at higher frequencies indicates a slightly stronger low-pass filter.
|Chromatic Aberration||When used on Panasonic bodies, lateral chromatic aberration is corrected in software and is effectively removed. However this isn't the case using the Olympus EP-1, and on this camera fringing is visible (although not hugely objectionable).|
|Falloff||We consider falloff to start becoming a potential problem when the corner illumination falls to more than 1 stop below the center. We measure 1.7 stops falloff wide open, which disappears on stopping down to around F2.8 - this is unlikely to be noticeable in normal use.|
|Distortion||Distortion is being corrected in software, with absolutely neutral results returned on both the G1 and E-P1.|
Software correction of distortion
As expected, the 20mm F1.7 pancake takes advantage of the Micro Four Thirds system's ability to correct geometric distortion in software. For most users this is completely transparent - the camera corrects both the viewfinder image 'on the fly', and the JPEG files it records. Also, both the software supplied with the camera (Olympus Master or SilkyPix) and industry-standard alternatives such as Adobe Camera Raw convert raw files correctly too. However users who like to experiment with more obscure raw developers which are unable to apply the requisite corrections will find themselves with distorted images.
To investigate the lens's 'native' distortion prior to correction, we shot our standard test in raw and converted the image using dcraw, which doesn't apply any corrections. The comparison below illustrates the transformation from the barrel-distorted raw image to the properly-corrected final version. Notably the 20mm has lower native distortion than the other MFT lenses we've tested so far, so the corrections required aren't as extreme. In context many superzoom lenses for DSLRs show more barrel distortion than this, albeit at wideangle settings.
|Uncorrected (+ 2.6%)||Corrected (+ 0.0%)|
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. The 20mm F1.7 produces near-flawless results in everyday shooting, with no problems worth mentioning.
The 20mm F1.7, despite its fast maximum aperture, excels in its handling of flare. It's possible to shoot directly into the sun with little ill-effect, and during extended use in bright late summer sun we saw little evidence for any images being adversely affected by flare, no matter how hard we tried to stress the lens.
The examples below show this - even with the sun directly in the frame, the lens shows few flare effects at all (just a small patch of reduced contrast diametrically opposite the sun). Shooting directly towards the sun on a very bright day, and there's little evidence of any significant image degradation from flare (although the E-P1 has, left to its own devices, somewhat overexposed the shot). Normally we're concerned when a lens isn't supplied with a hood, but in this case it doesn't seem to be a major cause for complaint.
|E-P1, 1/2000 sec F16||E-P1, 1/160 sec F8, strong backlight|
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 F1.7 aperture allows quite a useful degree of control over depth of field and blurring of backgrounds - and the rendition of those backgrounds is very pleasant for a fast 'normal' prime too. The examples below illustrate this - in the closeup shot, the transition to the out-of-focus regions is handled perfectly smoothly. Distant backgrounds are equally rendered in a fairly neutral fashion.
|DMC-G1, 1/100 sec F1.7, ISO 100||50% crop, top right corner|
|DMC-G1, 1/320 sec F1.7, ISO 100||50% crop, bottom left corner|
These samples also demonstrate that, while the 20mm F1.7 gives a fair degree of background blur (and will undoubtedly delight users coming from compacts with near-infinite depth of field), it struggles to match SLR systems. This is because background blur and depth of field are fundamentally dependant upon the size of the entrance pupil (the apparent size of the aperture as seen through the front element), which is about 11.8mm compared to 27.8mm for a 50mm F1.8. Ultimately the 20mm F1.7 gives similar results in this regard to a 27mm F2.2 lens on APS-C, or a 40mm F3.5 on full frame (of course it still does better than more directly comparable lenses such as the Olympus 17mm F2.8 pancake, or the 24mm F2.8 of the Sigma DP2).
Lateral chromatic aberration is automatically corrected when the lens is used on Panasonic bodies, but Olympus E-P1 owners won't get this benefit, regardless of whether they use out-of-camera jpegs or raw conversions (at least from Olympus Master or Adobe Camera Raw / Lightroom. This isn't necessarily a disaster - the 20mm F1.7 doesn't show particularly objectionable levels of fringing anyway.
The samples below show pretty much the worst case scenario - high contrast black/white transitions in the extreme corner of the frame. There's a narrow green / magenta fringe, with a yellow / blue one extending beyond that. The three processing methods (camera JPEG, Adobe Camera RAW and Olympus Master 2) all give identical results, both in terms of CA and distortion correction.
|Olympus E-P1, F4, JPEG||100% crop, bottom right of frame|
|100% crop, Adobe Camera Raw 5.5||100% crop, Olympus Master 2.2|
Longitudinal chromatic aberration can sometimes be seen when shooting at larger apertures. It takes the form of colour fringing around bright highlights, which is blue-magenta at in-focus high-contrast edges, and green in out-of-focus areas. Unlike lateral CA this fringing isn't removed automatically by Panasonic bodies. Overall, while it's there, we found it not to be a major concern in normal use (and had to look quite hard to find this example).
|Olympus E-P1, F2, JPEG||100% crop|