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Here's a brief look at the Sony 20mm F/2.8 lens.  Scroll down for the review.

Lens

SAL-20F28  Sony 20mm F/2.8 

Box contents

Front and rear caps, carrying pouch, hood and a users manual.

Cost

$748 retail

Build quality

Good

Additional information

Re-badged Minolta lens from the 1990s, which dates back to the mid 1980s.  No distance encoding.

Specifications below

 

Optical configuration

10 elements in 9 groups

Angle of view

94° full frame, 70° APS-C.

Aperture

7 blades, circular

Full frame and APS-C

Yes, made for full frame.   APS-C equivalent, 30mm

Depth of field and focus scales?

Yes and yes

Minimum focus, image plane to subject

9.6"  (244mm)

Minimum focus, end of lens barrel to subject

5.6"  (142mm)

Hard stop at infinity focus?

Yes

Length changes when focusing?

No

Focus ring turns in AF?

Yes

Filter size

72mm

Filter ring rotates?

No

Distance encoder?

No

Max magnification

0.13x

Min. F/stop

F/22

Sony teleconverter compatible?

No

Dimensions W x L (my measurements)

3.1" x 2.1"   78mm x 54mm 

Maximum  extended length (my measurements)

3.1"  (78mm)                                                      

Weight bare (my scale)

9.9oz  (280g)  11oz (312g) with caps


Requisite product shots.

october08/20mmft2a.jpg
Bulging front element
october08/20mmkita.jpg
Box contents with no box
october08/20mmfta.jpg
Front again
october08/20mmbka.jpg
Backside
october08/20mma700.jpg
Mounted on Sony A700.
2012/s20mmmtf.jpg
X-ray view, Sony MTF chart


All testing done with the Sony A700, and Sony A900.  For full frame results, go to the bottom of the page.  For a better understanding of my review methods and terminology, go here.                 
 
The Sony 20mm F/2.8 (made in Japan) is overall, a compact lens, with the body about as small as the Minolta 28mm F/2.8, and a front element the size of the Minolta 28-70mm F/2.8 G.  It looks cartoonish, that's why I put the side view mounted on the A700 in the last product shot above.  This lens was designed back in the mid 1980s by Minolta, and was re-styled in the early 1990s, adding a better manual focusing ring and a circular aperture.  Sony stamped their name on it in 2006, but it looks identical to the Minolta re-styled version, and probably performs the same, though I haven't had the chance to review that lens.  Also, this is one of very few lenses from Sony's current lineup where distance encoding is not used, typical of a carry-over from the 1980s.  Sony claims no aspheric elements are used in the construction of the lens.  Box contents include a hood, and a cloth type carrying pouch, which is the only one I've seen that isn't made of black vinyl.
 
Build quality is good.  Fit and finish are the same as other Sony re-badged lenses.  It has a satin black finish with rubber inserts around most of the circumference.  It has a focus distance window with ft and m in different colors along with DOF hash marks.
 
Focusing.  This lens has a short focus throw, and tends to want to "snap" when you turn the camera on or off depending on where the focus is at.  As you can imagine it focuses very quickly, Though I did have a few occasions where it focused at infinity when it should have been slightly inside that, even so, it didn't cause a blown shot, and isn't noticeable at normal viewing sizes.  I've found this is a common occurrence with all super wide angle lenses I've reviewed so far.  There's a little slop on the focus ring if you wiggle it by hand when engaged, and none in actual MF use.  Manually, the ring is very easy to manipulate with a finger and thumb--almost too easy.  About 1/5 turn gets you from close focus to infinity.  The rubberized focus ring turns in auto-focus mode.
 
The lens is multi-coated and resists flare and ghosting with mostly average results, and similar to other wide angle lenses reviewed here.  It comes with a petal type hood, ($35 to replace, so don't lose it) and as usual with wide angle lenses, it doesn't do a very good job at blocking light and preventing flare.  Use your hand to help minimize problems when the sun is near the image.  Veiling glare is controlled fairly well.  See examples below.
 
Filter size is 72mm.  Sharing this size in the Sony lineup are the 135mm F/2.8 STF, Carl Zeiss 24mm F/2, and the Carl Zeiss 85mm F/1.4.
 
Normal filters, not thin type, cause slight Corner darkening at F/2.8 with full frame coverage, but none on APS-C cameras.
 
Coma is very strong at F/2.8, but lightens considerably at F/4, where you wouldn't notice it in regular shots.  Full frame results are different, see those at the bottom of the page.
 
Color looks the same as other Sony lenses. 
 
Bokeh is rather harsh at F/2.8.  At F/4 it looks better in my opinion.  See crops below.
 
Lateral color fringing is present, but not very apparent unless you shoot bright subjects flanked by dark areas.  I mostly came upon red and cyan, though red seems more noticeable.  Stopping down won't help with this type of CA.  This lens is average in this department.
 
Random shots below.
 

Sun centered, F/5.6

Sun in shot, F/5.6

october08/20mmsunctr.jpg

october08/20mmffsunretro.jpg

Bokeh, F/2.8

Bokeh, F/4

october08/20mmbok28.jpg

october08/20mmbok40.jpg

Coma, F/2.8

Coma, F/4

october08/20mmcma28.jpg

october08/20mmcma40.jpg

 

The top left shot shows how the lens handles the sun when it's smack dab in the middle of the image, and it does a good job, no rings or color blobs.  The right shot shows the sun at an angle, with an orange ghost below, which shows up in most shots, regardless of angle.  The upper blue ghost is very dependant on angle, and mostly shows up using a full frame camera. 

 

The middle  crops are bokeh at F/2.8, and F/4.  Busy and harsh at F/2.8.  I like the look of F/4 better, which is slightly smoother in my opinion.

 

The last row are the results of coma, which looks rough in this extreme corner sample at F/2.8.  One stop down at F/4 and things look good.  The points of light are supposed to be round and very small.  This wouldn't really show up in a normal picture, so don't worry about it if you're a fair weather photographer.  

 

Distortion below.

 

october08/20mmdis700.jpg
Barrel distortion.

 

Distortion is moderate to light, and corrects easily with standard lens correction tools in your photo imaging software.

 

Light fall-off.

 

F/2.8

F/4

october08/20mmvig28.jpg

october08/20mmvig40.jpg


Light fall-off or corner darkening is mild to moderate at F/2.8.  It blends well into the center of the image so it doesn't show in real life.  By F/4 it's gone
 

october08/20mmover.jpg

 

Shot at F/2.8, no adjustments.  Light fall-off in real images is not noticeable on a cropped sensor camera.

 

I wonder how sharp the corners are?

 

         F/2.8

          F/4.0

october08/20mmcn28.jpg

october08/20mmcn40.jpg

         F/5.6

          F/8

october08/20mmcn56.jpg
october08/20mmcn80.jpg

 

These crops are from the extreme bottom left corner.  Things look rough here at F/2.8, and the corners definitely respond to stopping down.  F/5.6 is a little better, and I think the sharpest comes at F/8 or possibly F/11.  Exposure differences are from light fall-off at F/2.8

 

How sharp are the centers if the image is enlarged to a staggering size?

 

          F/2.8

          F/4.0

october08/20mmctr28c.jpg

october08/20mmctr40c.jpg


Center sample results.

The center sample crops above show F/2.8 and F/4 to be very similar is sharpness, though it appears the lens is at its sharpest at F/4.0, (due to contrast loss or spherical aberration) but not by much.  Look along the wall on the extreme bottom of the image.  There isn't a whole lot of change as you can ascertain, and it's hardly noticeable cropped and displayed side-by-side.  This lens shows center sharpness improvements by short focusing.  If you want to "short focus" for sharper shots with this lens, check out my article which explains this focusing method.
 
Comparison to Sony kit lens below.
 

Sony 20mm,  @ F/2.8

Sony 18-70mm, 20mm @  F/4.0

20mm28ctr.jpg

1870ctr4.jpg

 
The comparison above shows the center sharpness gain (at maximum aperture) you'll get by using this lens, as opposed to using the Sony 18-70mm F/3.5-5.6 kit lens.  The kit lens is soft at its maximum aperture at 20mm, which is F/4.  The 20mm lens is sharp at F/2.8.  If you really need light, use this lens instead of the kit lens, if you're not really in to low-light shots, use the kit lens or other less expensive lenses starting at 18mm.  The kit lens sharpens up nicely at F/4.5, where it's sharper than the 20mm is at F/2.8.
 
Close focus sample.
 
Below, click for larger image.  Check out the close focus shot, a 100% cropped portion of the full image.  The sample shot was taken with the Sony A 700 12.2mp camera.  The subject is a standard US stamp, 1"x 3/4" or 25.4mm x 19mm.  Also, note the macro shot was taken as close to the subject as focusing allowed; In this case, 5.6" or 142mm, measured from the front of the lens barrel to the subject.
 

october08/20mm41mac.jpg
Close focus, click for larger size, F/4


This maximum magnification shot is sharp, but it's small.  If you're using this lens properly, you won't care about macro type shots.  On a minor note; this lens looked nearly as good at F/2.8 as it did at F/4, with only a small amount of contrast loss visible wide open, and is one of the sharpest lenses at F/2.8 I've ever tested at close focus.
 
 

Full frame results using the Sony A900 below.

 

Check out the differences when using a film or full frame camera below.  I'm only pointing out the noticeable issues as compared to the APS-C bodies, so if I don't show it here, the results are not significantly different enough to warrant posting an additional set of images in this section.

 

Light fall-off 

 

         F/2.8

          F/4

20mmffvig28.jpg

20mmffvig40.jpg

         F/5.6

          F/8

20mmffvig56.jpg

20mmffvig80.jpg

 

Light fall-off is worse than the APS-C crops shown earlier.  At F/2.8 it's heavy, and noticeable in real shots, but one stop down and things change dramatically for the better.  It doesn't get much better by stopping down past F/5.6.

 

Full image from A900 below.

 

october08/20mmffover.jpg

 

The dark corners are not very noticeable here, it all depends on subject placement and background. This shot was F2.8, ISO 200, 1/2500sec.  As always, try and avoid shooting normal daylight scenes at F/2.8.

 

Corner samples next.

 

         F/2.8

          F/4

october08/20mmcnff28.jpg

october08/20mmcnff40.jpg

         F/5.6

          F/8

october08/20mmcnff56.jpg
october08/20mmcnff80.jpg

         F/11

          F/16

october08/20mmcnff11.jpg
october08/20mmcnff16.jpg

 

The corners are softer than the APS-C crops show, by about one stop.   Don't concern yourself with this as normal daylight shots would be F/5.6 or smaller, and low light shots would probably hide soft corners depending on subject matter.  The corners are their absolute best around F/11, but not that much better than the F/8 crops.  The exposure differences are from light fall-off.

 

Distortion.

 

october08/20mmffdis.jpg
Barrel distortion on A900

 

The distortion amount is about the same as the APS-C crops, but full frame coverage results in a complex signature.  There's a gentle rise in the middle, then it drops and flattens out towards the outer central area, and finally points up at the edges of the frame.  This is tough to correct in post processing, but isn't noticeable unless straight lines are near the frame edges.

 

Coma results with full frame.

 

F/2.8

F/4

20mmcmaff28.jpg

20mmcmaff40.jpg

F/5.6

F/8

20mmcmaff56.jpg

20mmcmaff80.jpg

 

This is coma on the A900.  It's very heavy at F/2.8, and creeps well into the sides and middle regions of the image, even on a small photo.  It lightens up at F/4, and stopping down to F/5.6, coma is barely noticeable.  This lens handles coma about the same as the Minolta AF 28mm F/2.  Keep in mind the samples above are 100% cropped portions of the original image, if you printed the whole image out as you might see it on your computer screen, it would measure 65" (1.9m) wide using the A900.

 

 
 
My final thoughts. 
 
The Sony 20mm F/2.8 is a specialty lens.  For most people, 20mm is wide enough, for some, like me, it isn't wide enough.  So where does it fall?  For APS-C users; the only reason to buy this lens (as opposed to the "DT" lenses starting at 16-18mm and F/3.5) is if you really need the extra stop of light.  The Sony 20mm is very sharp in the centers at F/2.8, where the "DT" lenses, such as the kit lens, 18-200mm, and 18-250mm are soft from F/3.5-4.  Distortion, light fall-off and color fringing are all about the same when the zoom lenses are set at 20mm.  This lens has limited use on a cropped sensor camera.  Do you work in low-light, and need a sharp wide angle lens at F/2.8, which equals 30mm on an APS-C camera?  If so, I'd recommend this lens.  
 
For full frame users; The Sony 20mm F/2.8 makes a very good, and relatively low cost super-wide angle lens.  For most people, it's wide enough and fast enough.  It's sharp in the centers wide open as stated above, (and really sharp at smaller apertures by doing this) though the corners will need an extra stop to sharpen up as compared to the APS-C camera, but that's only when viewing your images at large sizes.  Light fall-off is noticeable wide open, but is nearly gone at F/4.  Coma is bad at F/2.8, but looks better at F/4.  Watch your night time street scenes at F/2.8, especially Christmas light type pictures.  This lens has a fairly hefty price tag of $748 brand new from most legitimate dealers, but also consider the Minolta branded versions for much less money.  If you want a factory super-wide angle lens for your full frame camera, and can live at 20mm, this little lens would be a much less-expensive alternative to the $2000 Sony 16-35mm F/2.8