The FE 20mm f/1.8 G Lens was another Sony instant hit, becoming a #1 seller immediately upon availability. Some of the reasons for this instant success include a desirable wide-angle focal length and a very wide aperture in a well-built package with a compact form factor and a low price. The overall excellent performance of this lens adds greatly to the demand being seen.
Make focal length selection a priority when choosing a lens because focal length matters. While focal length determines the working distance and therefore perspective, very-wide-angle focal lengths are a lot about making foreground subjects large in relation to the background subjects and about including a lot of background in the frame. This angle of view is notably able to give the viewer a sense of the presence in the images captured by it.
The 20mm angle of view makes it a great "scapes" focal length. It is useful for photographing most scenes that "scapes" can be naturally appended to, including landscapes, nightscapes, cityscapes, buildingscapes, roomscapes, etc.
Include peoplescapes in that list, with environmental photos of individuals and groups captured at a wide range of locations from scenic landscapes to birthday parties in small rooms being a 20mm capability. Note that if multiple people are in the 20mm frame, their distance from the camera should not vary by a significant amount, else those in front will appear larger than those in the back. Avoid getting too close to people for additional perspective issues.
Weddings are a great use for this lens. Think of a bride getting ready with her attendants surrounding her. Think of the first dance at the wedding reception, with this lens capturing the bride and groom large in the frame with the guests encircling them in the background.
Photographing architecture? This is a great lens choice for that pursuit.
While the 20mm angle of view is rather wide for use as a general-purpose lens without at least one additional focal length available, there are general-purpose uses for 20mm. I've used 20mm to capture the entire day of family holiday festivities. This is a very fun lens to carry around with a creative purpose in mind.
Videographers will find a host of uses for 20mm.
I like to look at focal lengths in comparison format and, since this lens has only one focal length, I'll borrow a comparison from the Canon EF 16-35mm f/4L IS USM Lens review.
On an ASP-C/1.5x sensor format body, the 20mm focal length provides an angle of view similar to a 30mm lens on a full-frame sensor format body with an angle of view just slightly narrower than the 28mm example shared above. The 30mm angle of view is only moderately wide and just wider than the ultra-popular and very useful 35mm focal length. While there is some overlap in usage between the 20mm and 30mm focal lengths, they are rather different with 30mm having more general-purpose appeal and uses that better align with the 35mm focal length.
The angle of view provided by the 30mm focal length is a great choice for capturing a natural perspective. It is wide enough to capture the big scene but not so wide that people and other subjects are readily distorted by the close perspective invited by ultra-wide angles.
The 30mm angle of view is a great choice for photojournalistic uses. Wedding and portrait photographers like 30mm, especially for full to mid-body portraits and for group portraits. Landscape photographers have plenty of use for the 30mm angle of view. 30mm is also very popular with videographers, especially for creating documentaries. Many medium and large products can be captured at the 30mm angle of view. I'm always happy when a lens with the same or similar angle of view (or a zoom covering this angle of view) comes across my desk because I know that I can assign it around-the-house use.
The full list of 30mm angle of view uses is very long and I've only scratched the surface here.
Following is a small set of results captured while walking around for a couple of hours with only 20mm available.
No, post-processing was not used to create that perfectly-placed shadow. Showing up at the right place at the right time meant that a field house shaded all but the first lane on this university track. Also aiding in emphasizing the "1" was the perspective. With the 20mm lens positioned closer to the "1" than the other numbers, the "1" becomes the largest in the frame and therefore the most prominent. Everyone loves number "1" and there are far more uses for an emphasized "1" than any other number.
Once the shadow crept over the "1", I found the scene less interesting and moved on.
On the football field inside the track, the "20" yard line number seemed appropriate to photograph at this focal length though finding a good camera position proved challenging. Shooting straight on the number was not creating an especially interesting composition but at an angle seemed to work better. The arrow beside the 20 did not fit well into the frame and in the end, I opted to include part of it with the other visible numbers fully contained. The sun was low and bright, making a shadow selfie a requirement from this position.
Moving to a position farther down the track made another shadow selfie as easy as "1 2 3".
An administration building had caught my eye and photographing it was part of this evening's to-do list. During the blue hour is a great time to photograph architecture and starting with a shooting direction away from the sunset provides the earliest brightness balance between the building lights and the sky.
This camera position required an upward angle to fit the building in the 20mm angle of view and the leaning walls reveal this. As the sky darkened, the light balance on the other side of the building, looking toward the sunset (brighter sky), improved.
It was winter and there was little flora to work with but the leading lines of a curving sidewalk help draw a viewer's eye into the frame.
The camera direction for the last image of the evening was also toward the now fully set sun with a darker sky included in the frame.
To get a level camera for this perspective required fully extending the Really Right Stuff TVC-24L Mk2 Carbon Fiber Tripod legs and positioning the feet as close together as possible without risking stability. The camera was well above head height but the tilt LCD enabled proper leveling and composition.
I often use f/16 to photograph architecture at this time of the day to create large sunstars from the lights in the frame. Unfortunately, these particular LED lights (lots of little lights) were not responding well in this regard and f/8 created very noticeably sharper Sony a7R IV images.
Proved on this evening was something I already knew — that the 20mm focal length is very fun to walk around with.
For the 20mm focal length, the f/1.8 max aperture is really wide. As of review time, Sigma is the only interchangeable lens manufacturer offering a wider 20mm aperture, opening up to f/1.4. Wider apertures allow more light to reach the imaging sensor, allowing action (subject and camera) stopping shutter speeds in very low light levels. The wide focal length and wide aperture combination facilitate handholding this lens, especially on a camera with IBIS (In-Body Image Stabilization), in very low light levels.
Another advantage of a wide aperture lens is the background blur it can create. F/1.8 with a close subject creates a very shallow DOF (Depth of Field), drawing the viewer's eye to the in-focus subject. Wide-angle lenses make background details smaller and that makes it harder to diffusely blur the background, but an f/1.8 aperture can do that. Add artistic capabilities to this lens' list of highly-desired features.
Here is a look at a set of apertures.
Compare the widest aperture you have in your 20mm lens with f/1.8.
The primary downsides to wide-aperture lenses, due to the increased diameter of lens elements required, are increased size, increased weight, and increased price. This lens side-steps those issues.
Sony has been featuring an aperture ring on some of their prime lenses including this one, permitting a manually-chosen aperture to be selected. With the ring in the A (Auto) position, the camera controls the aperture setting. All other settings force the aperture to the selected opening and a 2-position switch on the bottom right side of the lens toggles the aperture ring between 1/3-stop clicks and smooth, quiet, non-clicked adjustments, ideal for video recording.
Aside from a slightly more complicated design, perhaps the primary disadvantage of an aperture ring is that inadvertent aperture changes are made available. Making the A click stop firm enough to reduce the chances of this occurrence eliminates much of that concern.
This lens is not optically stabilized, but Sony generally takes care of that omission with Steady Shot or IBIS (In-Body Image Stabilization). On a traditional DSLR with an optical viewfinder, IBIS results in an unstabilized view, meaning that stabilization was not helpful for composition or for providing a still subject to the camera's AF system. With EVFs being prevalent in Sony's lineup, the viewfinder image is being read from the imaging sensor and that is stabilized. Therefore, the viewfinder image is very nicely stabilized and sensor-based AF takes advantage of the stabilized view for improved accuracy.
Of all of a lens's characteristics, most of us care most about what we refer to as sharpness, a combination of contrast and resolution. In that regard, this lens rocks.
Over most of the full-frame imaging circle, this lens is very sharp wide open and impressively razor-sharp by f/2.8. Extreme full-frame f/1.8 corners have good sharpness, especially for a 20mm lens, and gradually improve through f/5.6.
Below you will find sets of 100% resolution center of the frame crops captured in uncompressed RAW format using a Sony a7R III (trees and grasses) and a Sony a7R IV (brick wall). The images were processed in Capture One using the Natural Clarity method with the sharpening amount set to only "30" on a 0-1000 scale. Note that images from most cameras require some level of sharpening but too-high sharpness settings are destructive to image details and hide the deficiencies of a lens.
Yeah, you will be happy with results like these.
Focus shift, the plane of sharp focus moving forward or backward as the aperture is narrowed (residual spherical aberration, or RSA), is not an issue with this lens (many modern lenses automatically correct for it).
Next up is a comparison showing 100% extreme-top-left-corner crops captured and processed identically to the above center-of-the-frame images. These images were manually focused in the corner of the frame. Again, the trees were captured with an a7R III and the bricks were captured with an a7R IV.
The f/1.8 corners look good with peripheral shading reducing contrast. By f/5.6, the corners are looking exceptionally nice.
Corner sharpness does not always matter but it does matter for many disciplines including landscape photography. When I'm photographing landscapes with corner sharpness being desired, I'm probably using f/8 or f/11 to obtain enough depth of field for in-focus corner details and this lens works beautifully for this purpose at these apertures. When shooting at wide apertures, the corners are most often out of focus and not supposed to be sharp. Videos captured at normal wide-aspect ratios also avoid the use of corners.
As mentioned and as with all other wide-angle, wide-aperture prime lenses, peripheral shading darkens image corners at wide apertures. At f/1.8, expect a noticeable but not unusual 3.5 stops of shading. More unusual is that the reduction in shading is not especially substantial when stopping down with about 2 stops of vignetting remaining at f/5.6 and narrower apertures.
APS-C format cameras using lenses projecting a full-frame-sized image circle avoid most vignetting problems. In this case, a just-visible about-1.5-stops of shading shows at f/1.8.
One stop of shading is the amount often used as the visibility number, though subject details provide a widely-varying amount of vignetting discernibility. Vignetting can be corrected during post-processing with increased noise in the brightened areas being the penalty or it can be embraced, using the effect to draw the viewer's eye to the center of the frame. Study the pattern showing in our vignetting test tool to determine if your subject will be darkened or if it will be emphasized by the darker periphery.
The effect of different colors of the spectrum being magnified differently is referred to as lateral (or transverse) CA (Chromatic Aberration). Lateral CA shows as color fringing along lines of strong contrast running tangential (meridional, right angles to radii) with the mid and especially the periphery of the image circle showing the greatest amount as this is where the greatest difference in the magnification of wavelengths typically exists.
With the right lens profile and software, lateral CA is often easily correctable (often in the camera) by radially shifting the colors to coincide though it is always better to not have the problem in the first place. Any color misalignment present can easily be seen in the site's image quality tool, but let's also look at a worst-case example, a 100% crop from the extreme top left corner of an a7R III frame showing diagonal black and white lines.
There should be only black and white colors in these images with the additional colors seen showing the modest presence of lateral CA.
A relatively common lens aberration is axial (longitudinal, bokeh) CA, which causes non-coinciding focal planes of the various wavelengths of light, or more simply, different colors of light are focused to different depths. Spherical aberration along with spherochromatism, or a change in the amount of spherical aberration with respect to color (looks quite similar to axial chromatic aberration but is hazier) are other common lens aberrations to look for. Axial CA remains at least somewhat persistent when stopping down with the color misalignment effect increasing with defocusing while the spherical aberration color halo shows little size change as the lens is defocused and stopping down one to two stops generally removes this aberration.
In the real world, lens defects do not exist in isolation with spherical aberration and spherochromatism generally found, at least to some degree, along with axial CA. These combine to create a less sharp, hazy-appearing image quality at the widest apertures.
In the examples below, look at the fringing colors in the out of focus specular highlights created by the neutrally-colored subjects. Any color difference is being introduced by the lens.
That result looks excellent.
Sony utilizes Nano AR Coating on this lens to reduce flare and ghosting. In our standard flare testing utilizing the sun in the corner of the frame (it is easy to get the sun in the 20mm angle of view), this lens shows practically no flaring at f/1.8. As usual, flare effects increase in appearance as the aperture narrows but the f/16 results show only modest flaring.
Flare effects can be embraced, avoided, or removal can be attempted. Removal is sometimes very challenging and, in some cases, flare effects can be quite destructive to image quality.
Two lens aberrations that are particularly evident when shooting images of stars, mainly because bright points of light against a dark background make them easier to see. Coma occurs when light rays from a point of light spread out from that point, instead of being refocused as a point on the sensor. Coma is absent in the center of the frame, gets worse toward the edges/corners, and generally appears as a comet-like or triangular tail of light which can be oriented either away from the center of the frame (external coma), or toward the center of the frame (internal coma). Coma clears as the aperture is narrowed. Astigmatism is seen as points of light spreading into a line, either meridional (radiating from the center of the image) or sagittal (perpendicular to meridional). Remember that lateral CA is another aberration apparent in the corners.
The image below is a 100% crop taken from the top-left corner of an a7R III frame.
Those stars are certainly not round, though they are not unusually distorted for a wide-aperture, wide-angle lens.
Bring on the straight lines in your subjects because this lens will keep them straight. Aside from a touch of pincushion distortion slipping in at the extreme corners, this lens has an excellent geometric profile.
The amount of blur a lens can produce is easy to show (and was shown earlier in the review). Assessing the quality is a much harder challenge due in part to the infinite number of variables present in all available scenes. I'll share some 100% crop f/8 (for aperture blade interaction) examples below.
In the first example, the shapes are nicely rounded and very smoothly filled. In the second example, we see an anomaly, dark blobs, that effects a very small portion of the frame in the review lens. The third example shows a crop from an outdoor scene.
Except for a small number of specialty lenses, the wide aperture bokeh in the corner of the frame does not produce round defocused highlights with these effects taking on a cat's eye shape due to a form of mechanical vignetting. If you look through a tube at an angle, similar to the light reaching the corner of the frame, the shape is not round and that is the shape seen here. The image below is a reduced top-left quarter of the frame example.
While not perfectly round, the corner circles are not very distorted. As the aperture narrows, the entrance pupil size is reduced and the mechanical vignetting absolves with the shapes becoming round.
With a 9-blade count aperture, point light sources captured with a narrow aperture setting and showing a sunstar effect will have 18 points. Wide aperture lenses tend to have an advantage in this regard and this lens is capable of producing gorgeous stars as illustrated below.
This lens design incorporates two AA (Advanced Aspherical) elements  and three ED (Extra-low Dispersion) glass elements  as seen below.
This lens has rather strong peripheral shading at narrow apertures and extreme full-frame corner performance is not perfect at wide apertures. Most are going to overlook those issues in light of the balance of the image quality this lens produces including the impressive sharpness.
"Two XD (eXtreme Dynamic) Linear Motors easily keep up with the high-speed capabilities of today's most advanced camera bodies, taking advantage of their full autofocus speed, precision, and tracking potential to capture the most dynamic subjects. New control algorithms make it possible to drive the lens's large focus group smoothly and without delays for responsive, silent, low-vibration autofocus." [Sony]
The Sony 20mm f/1.8 G Lens internally focuses smoothly, quietly, accurately, and the speed is quite good. This lens mounted to an a7R IV focuses in very low light scenarios (though focus slows) when adequate contrast is provided.
Sony provides an AF hold button on this lens. While in continuous focus mode, this button can be pressed to lock focus at the currently selected focus distance, permitting a focus and recompose technique. This button also acts as a custom button (C5) and can be programmed to another function using the camera's menu (note that not all models support this).
FTM (Full Time Manual) focusing is supported via Sony's DMF (Direct Manual Focus) AF mode.
The ribbed-rubber-coated manual focus ring is nicely sized. It turns very easily, slightly more easily than I prefer, and it has a small amount of front-to-back play. This ring has decent smoothness and the 80° of Linear Response MF rotation is just right for precise manual focusing at all distances. This is a nicely-implemented focus-by-wire design with adjustments being reasonably-smoothly made.
Normal is for the scene to change size in the frame (sometimes significantly) as focus is pulled from one extent to the other, referred to as focus breathing, a change in focal length resulting from a change in focus distance. This lens shows a modest change in subject size as full extent focus adjustments are made.
The Sony FE 20mm f/1.8 G Lens, in manual focus mode, has a 7.1" (180mm) minimum focus distance that generates a 0.22x maximum magnification for very decent close focus performance. Switch to AF mode and those numbers deteriorate slightly to 7.5" (190mm) and 0.20x.
|Canon EF 20mm f/2.8 USM Lens||9.8"||(250mm)||0.14x|
|Nikon 20mm f/1.8G AF-S Lens||7.9"||(201mm)||0.23x|
|Nikon 20mm f/2.8D AF Lens||10.2"||(259mm)||0.12x|
|Rokinon (Samyang) 20mm f/1.8 ED AS UMC Lens||7.9"||(200mm)|
|Sigma 20mm f/1.4 DG HSM Art Lens||10.9"||(277mm)||0.14x|
|Sigma 20mm f/1.8 EX DG Lens||7.9"||(201mm)||0.25x|
|Sony FE 20mm f/1.8 G Lens||7.1"||(180mm)||0.22x|
|Sony FE 24mm f/1.4 GM Lens||9.4"||(240mm)||0.17x|
|Tamron 20mm f/2.8 Di III OSD Lens||4.3"||(109mm)||0.50x|
|Zeiss 21mm f/2.8 Milvus Lens||8.7"||(220mm)||0.20x|
|Zeiss 21mm f/2.8 Classic Lens||8.7"||(220mm)||0.20x|
A subject measuring approximately 5.5 x 3.7" (140 x 93mm) will fill the frame of a full-frame camera at the minimum focus distance.
Need a shorter minimum focus distance and greater magnification? An extension tube mounted behind this lens should provide a very significant decrease and increase respectively. Extension tubes are hollow lens barrels that shift a lens farther from the camera, which permits shorter focusing distances at the expense of long-distance focusing. Electronic connections in extension tubes permit the lens and camera to communicate and otherwise function as normal. Sony does not publish extension tube specs nor do they manufacture these items, but third-party Sony extension tubes are available.
This lens is not compatible with Sony teleconverters.
From a design standpoint, the Sony FE 20mm f/1.8 G Lens is a slightly-size-reduced Sony FE 24mm f/1.4 GM Lens. Here is that comparison:
These are nicely-built lenses — no concerns here.
The AF/MF switch is recessed, making it hard to inadvertently change and making a bit more effort required to intentionally change it, especially with gloves on. As mentioned before, the focus ring has some play, mostly front-to-back.
"A dust- and moisture-resistant design provides the reliability needed for outdoor use in challenging conditions" though "Not guaranteed to be 100% dust- and moisture-proof." [Sony]
The dust and moisture-resistant design, including a gasketed mount, can save an outdoor day (and sometimes an indoor day).
"The front lens element features a fluorine coating that repels water, oil, and other contaminants, while making it easier to wipe off any contaminants or fingerprints that do become attached to the lens surface." [Sony]
The Sony FE 20mm f/1.8 G Lens is very small and light, especially for its class, making it a pleasure to carry and use for extended periods, including on a gimbal when recording movies.
|Model||Weight oz(g)||Dimensions w/o Hood "(mm)||Filter||Year|
|Canon EF 20mm f/2.8 USM Lens||14.3||(405)||3.1 x 2.8||(78.0 x 71.0)||72||1992|
|Nikon 20mm f/1.8G AF-S Lens||12.6||(357)||3.2 x 3.1||(81.3 x 78.7)||77||2014|
|Nikon 20mm f/2.8D AF Lens||9.5||(270)||2.7 x 1.7||(69.0 x 42.5)||62||1994|
|Rokinon (Samyang) 20mm f/1.8 ED AS UMC Lens||17.5||(497)||3.3 x 3.5||(83.0 x 88.4)||77||2016|
|Sigma 20mm f/1.4 DG HSM Art Lens||33.5||(950)||3.6 x 5.1||(90.7 x 129.8)||2015|
|Sigma 20mm f/1.8 EX DG Lens||18.4||(520)||3.5 x 3.4||(89.0 x 87.0)||82|
|Sony FE 20mm f/1.8 G Lens||13.2||(373)||2.9 x 3.3||(73.5 x 84.7)||67||2020|
|Sony FE 24mm f/1.4 GM Lens||15.7||(445)||3.0 x 3.6||(75.4 x 92.4)||67||2018|
|Tamron 20mm f/2.8 Di III OSD Lens||7.8||(221)||2.9 x 2.5||(73.0 x 63.5)||67||2019|
|Zeiss 21mm f/2.8 Milvus Lens||30||(851)||3.8 x 3.7||(95.5 x 95.0)||82||2015|
|Zeiss 21mm f/2.8 Classic Lens||21.2||(600)||3.4 x 4.3||(87.0 x 109.0)||82||2010|
For many more comparisons, review the complete Sony FE 20mm f/1.8 G Lens Specifications using the site's lens specifications tool.
I have frequently complained about Sony's Alpha series camera having inadequate space for my medium-large-sized hand's fingers to fit between the camera and many Sony lenses. The good news is that this one clears my fingers even when the camera is solidly gripped.
A visual comparison adds perspective:
Positioned above from left to right are the following lenses:
The same lenses are shown below with their hoods in place.
Use the site's product image comparison tool to visually compare the Sony FE 20mm f/1.8 G Lens to other lenses.
Notice that the Sony lens appears to be positioned higher than those around it in the above comparisons? The lenses are vertically aligned on their camera mounts and with a shallow lens mount cap, Sony saves some space below the line. While the smaller lens cap size does not affect the lens' in-use size, is does modestly impact storage space requirements when unmounted, such as in a backpack.
The Sony FE 20mm f/1.8 G Lens' narrow width allows it to use common, mid-sized, and affordable 67mm filters. A standard thickness circular polarizer filter will noticeably increase peripheral shading. A slim model such as the Breakthrough Photography X4 is highly recommended.
Sony includes a rigid, petal-shaped plastic hood in the box. This hood has a matte interior finish (not flocked) and it lacks a push-button release that makes bayonet mounts easier to use. This hood offers the front lens element reasonable protection from contrast-robbing, flare-inducing light and from impact including from light rain.
Sony includes a thin, fleece-lined lens pouch in the box. Only the bottom is padded. Consider a Lowepro Lens Case or Think Tank Photo Lens Case Duo for a quality, affordable single-lens storage, transport, and carry solution.
As an "FE" lens, the Sony FE 20mm f/1.8 GM Lens is compatible with all Sony E-mount cameras, including both full-frame and APS-C sensor format models. Sony provides a 1-year limited warranty.
The reviewed Sony FE 20mm f/1.8 G Lens was online-retail sourced.
I admit having to pause when considering which lenses to compare for you here. While there are many alternatives, there are not many direct equivalents to the Sony FE 20mm f/1.8 G Lens.
I'll start in the Sony family with the Sony FE 24mm f/1.4 GM Lens, arguably the best 24mm f/1.4 lens produced to date and a lens I currently own. The 24mm angle of view is modestly narrower than 20mm (a 10° difference) and the f/1.4 aperture is 2/3 stop wider than f/1.8, both noticeable differences. In the image quality comparison, the two lenses perform similarly with the 24mm lens taking advantage of its wider aperture, rendering slightly sharper results at equivalent wide aperture settings. The 24mm lens has modestly less vignetting and shows fewer flare effects at narrow apertures in our test. The 20mm lens has slightly less geometric distortion, slightly less lateral CA, and less color blur.
Looking at the specs and measurements, the Sony FE 20mm f/1.8 G Lens vs. Sony FE 24mm f/1.4 GM Lens comparison shows the 20mm lens being slightly smaller (as visually illustrated earlier in the review) and slightly lighter. The 24mm lens has 11 aperture blades vs. 9. The 20mm lens's AF is driven by a Dual XD Linear Motor vs. the 24mm lens's Direct Drive Super Sonic wave Motor. The 20mm lens has a slightly higher maximum magnification of 0.20x vs. 0.17x. The price difference will encourage those requiring a 24mm angle of view to crop the 20mm lens's results accordingly.
Sigma used to produce a 20mm f/1.8 lens but it has long been discontinued. Having the same focal length but a wider aperture is the Sigma 20mm f/1.4 DG HSM Art Lens. In the image quality comparison, the Sigma lens competes well in the center of the frame at equivalent apertures, but the Sony lens rules the periphery even stopped down a couple of stops. The Sigma lens has less vignetting and more linear distortion.
Looking at the specs and measurements, the Sony FE 20mm f/1.8 G Lens vs. Sigma 20mm f/1.4 DG HSM Art Lens comparison shows the Sony lens being dramatically lighter and significantly smaller. The Sigma lens does not accept threaded front filters. The Sony lens has a higher maximum magnification, 0.20x vs. 0.14x. With the two lenses having price equality, those requiring the f/1.4 aperture will be the group choosing the Sigma lens in this comparison.
I recently reviewed the Tamron 20mm f/2.8 Di III OSD Lens. As made clear in the name, this lens has an f/2.8 maximum aperture and that is significantly smaller than f/1.8.
In the image quality comparison at f/2.8, the Sony lens, especially assisted from being stopped down, turns in sharper results. The Sony lens has less lateral CA and less peripheral shading at f/2.8. The Tamron lens has severe barrel distortion.
Looking at the specs and measurements, the Sony FE 20mm f/1.8 G Lens vs. Tamron 20mm f/2.8 Di III OSD Lens comparison shows the Tamron lens being smaller and much lighter. The Sony lens has 9 aperture blades vs. 7. The Tamron lens has a much higher maximum magnification, 0.50x vs. 0.20x. The Tamron lens's primary advantage is a significantly lower price.
Use the site's comparison tools to create other comparisons.
Aside from a small number of nit-picks (vignetting at narrow apertures, imperfect full-frame corners at f/1.8, and some wobble in the focus ring), this lens offers very little to complain about. The 20mm focal length is fun and useful. The small size and light weight are very convenient and comfortable. The AF performance is good. Quality is there. The price is reasonable and sealing the deal is that the Sony FE 20mm f/1.8 G Lens delivers impressive image quality.
Bringing you this site is my full-time job (typically 60-80 hours per week). Thus, I depend solely on the commissions received from you using the links on this site to make any purchase. I am grateful for your support! - Bryan