A near-perfect optic: Sony 20mm F1.8 G

A near-perfect optic: Sony 20mm F1.8 G

Sony has just launched its newest fast aperture wide-angle prime for its full-frame Alpha lineup: the FE 20mm F1.8G lens. With two advanced aspheric and three ED elements, Sony promises outstanding corner-to-corner image quality, as well as fast autofocus thanks to XD (extreme dynamic) linear focus actuators.

Spoiler alert: it’s a near perfect optic.

Sharpness and coma

The FE 20mm F1.8 is sharp. Sharp and contrasty. One day we hope to have the MTF90, MTF50, and MTF20 graphs to quantitatively show you just how sharp or contrasty a lens is, but for now have a look below at the whiskers and hair on Allison’s feline (slide left to view a 100% crop):

The strand of hair at top, center, emanating from the eye is a mere 2 pixels wide, yet perfectly resolved. That suggests that even wide open, this lens is capable of resolving detail well when paired with sensors of higher resolution than the 60MP a7R IV. And the lack of any haziness indicates that contrast is very high, even at this aperture and close focus distance. Sharpness does drop off across the frame, but only modestly, and edge sharpen up quickly upon slightly stopping down.

We’ve only informally tested for coma and find it to be well controlled. Meanwhile, our friend and Sony Artisan Colby Brown has been shooting the starry skies in Norway with this lens for his review and has kindly provided a 100% crop1 of one of his images from the upper right extreme of the frame:

There’s little to no sagittal flare, even with the high-resolution 60MP sensor of the a7R IV (pay particular attention to the stars exhibiting less distortion correction or motion blur due to the earth’s rotation).

(Lack of) chromatic aberrations

One of the biggest headaches when it comes to fast prime lenses is longitudinal (axial) chromatic aberration (LoCA), which rears its ugly head as purple and green fringing typically in front of, and behind, the focal plane. It’s not an aberration that you can simply click a checkbox to get rid of, and professional photographers spend hours manually cloning it out. It’s particularly a problem when it comes to wide-angle fast primes, and even Sony’s own 24mm F1.4 GM suffers from some modest LoCA.

This lens has almost no detectable LoCA or fringing to speak of. That’s pretty remarkable. Have a look above, sliding left to view cat hairs at 100%. Typically, backlit hairs will have magenta fringing and green fringing on hairs in front of, and behind, the plane of focus, respectively. But we see nothing of the sort. And the window in the background shows no color fringing around its edges, also indicating the lack of any detectable LoCA.

Lateral chromatic aberration

If you’re worried about lateral chromatic aberration (LaCA), the one that is easy to fix in post-processing software, well, don’t be. There’s not much of it, and Raw converters automatically correct for it (also corrected in JPEG) so as to not leave any trace of it in your final output. Thankfully, lateral CA is well enough controlled that even corrected images don’t show much softening when viewed at 100% in the corners, as you can see below (slide left for the 100% view):

Distortion and vignetting

While distortion correction does impact a large portion of the frame, the correction itself is extremely subtle, meaning that much of the distortion has itself been corrected for optically. When it comes to vignetting, a large portion of the frame displays vignetting-induced darkening at the widest aperture, but the magnitude is fairly modest. While it’s easily corrected in post-processing or in in-camera JPEGs, do note that you might have up to 1.7 EV darkening in corners of the image when you shoot wide open.


Although many of our readers won’t think much about ‘bokeh’ when it comes to such a wide lens, it’s actually very important. Astrophotography and nightscape applications aside, a major reason to pair a fast aperture with this type of lens is to counter the extensive DOF wide-angles exhibit. This allows you to isolate or emphasize your subject against its environment in wide field-of-view compositions. And in such instances, the quality of the background (and foreground) blur certainly matters. Thankfully, due to improvements in grinding and polishing processes, particularly of aspheric element molds, Sony has managed to deliver pleasing bokeh. Have a look below at the rendition of out-of-focus highlights behind, and in front of, the focal plane:

There’s essentially no onion ring patterning to out-of-focus highlights. Furthermore, there aren’t any high-brightness rings to the out-of-focus highlights, indicating that out-of-focus areas should be rendered relatively smoothly without distracting bokeh. We should note that there are slightly brighter rings to foreground vs. background out-of-focus highlights that might indicate the presence of some spherical aberration, but it’s so slight as to be photographically irrelevant. However, we do see some bright-edged highlights as you approach outer edges of the frame, so we’d expect slightly busier bokeh at image extremes, much like the Sony FE 35/1.8 lens.

Finally, the lack of any discernible green – and only a very slight presence of magenta – rings around out-of-focus highlights corroborates our earlier finding that longitudinal chromatic aberration is nearly nonexistent, which is an impressive achievement for a lens of this type. Nine aperture blades help to retain circular out-of-focus highlights as you stop the lens down, and produce pleasing 18-point sunstars (though, admittedly, we’ve seen better).


Fast aperture wide-angle primes in particular are hard to produce without common optical aberrations like longitudinal chromatic aberration and a sharp drop-off in sharpness across the frame, not to mention lateral chromatic aberration, distortion and vignetting. The Sony FE 20mm F1.8 exhibits extremely high sharpness and contrast wide open, even at close focus distances, and a lack of longitudinal CA, seen as purple and green fringing in front of, and behind, the focus plane. Those attributes in and of themselves would have made this lens desirable, but the excellent coma performance, the lack of lateral chromatic aberration (or, more accurately, the lack of a major decrease in sharpness after correction of any lateral CA), and only mild distortion, make it quite exceptional.

Video shooters will appreciate the linear response of the focus ring, as well as the silent focus motor and de-clickable aperture ring. Unfortunately, there is a good degree of focus breathing.

While there’s modest vignetting, performance is better than many other fast wide-angle primes of its type, and vignetting in general is less of a concern given modern sensors. Busier bokeh towards the edges of the frame and ghosting when shooting into sunlight are our only real concerns with this lens. Couple all the optical positives with fast autofocus speeds thank to the ‘XD’ (extreme dynamic) linear motors and you have arguably one of the best ultra-wide fast primes available today.

Blog credit to www.dpreview.co.za (Rishi Sanyal)