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Sigma EX Aspherical DG DF RF 20mm F1.8 Review RSS Feed

Sigma EX Aspherical DG DF RF 20mm F1.8

Sharpness 
 7.3
Aberrations 
 7.2
Bokeh 
 7.8
Handling 
 8.0
Value 
 8.2
Autofocus 
 7.8
Reviews Views Date of last review
16 46,975 Sat September 25, 2021
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Recommended By Average Price Average User Rating
81% of reviewers $374.08 7.19
Sigma EX Aspherical DG DF RF 20mm F1.8
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Description:
Weight: 520g/18.3 oz
Length: 89.5mm/3.5 in
Filter Diameter: 82mm
Min. Focus: 20cm/7.9 in
Max. Magnification: 1:4
Diagonal FOV:
Horizontal FOV: 94.5 degrees
Horizontal FOV on Digital: 63 degrees
Min. Aperture: F22
Optical Construction: 13 Elements in 11 Groups
Buy Lens: Buy the Sigma EX Aspherical DG DF RF 20mm F1.8
Price: $629
Mount Type: Pentax KA
Price History:



Add Review of Sigma EX Aspherical DG DF RF 20mm F1.8 Buy the Sigma EX Aspherical DG DF RF 20mm F1.8
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Pentaxian

Registered: October, 2011
Location: British Columbia
Posts: 377

2 users found this helpful
Review Date: September 25, 2021 Recommended | Price: None indicated | Rating: 8 

 
Pros: close focus ability, sharp from f/2+, sunstars, aperture ring, AF, communicates correctly with camera body, accepts filters
Cons: push/pull focusing collar, wide open performance
Sharpness: 8    Aberrations: 8    Bokeh: 8    Handling: 8    Value: 10    Camera Used: K-1    New Or Used: Used    Autofocus: 8   

By this point, it's pretty clear that there is a great deal of sample variation within this lens. I would only recommend purchasing a copy if you can test it before buying or if the seller can provide sample photos. With that being said, here's my 2¢ on the copy that I own:

Optical Qualities:
•Wide open, this lens isn't anything to write home about. It's a tad soft even in the centre. Bokeh highlights have visible 'soap bubble' bokeh balls. There is noticeable purple CA in high contrast areas. Vignetting is obvious.

•At f/2, which is one 'click' down (~1/3 of a stop), the lens smartens up to have very good sharpness in the centre and the bokeh mellows out somewhat. CA is reduced. Vignetting is still obvious. The lens' IQ is entirely useable at this point.


Chip @ f/2

•At 2.8 through f/8, the lens is a great performer. Sharpness continues to improve and becomes more uniform across the plane of focus. Bokeh smooths out to pleasing levels. CA is negligible (except in the extreme corners), Vignetting is reduced to the point of not being a concern.


Chip @ f/3.5

•A great feature of this lens is its ability to make pleasant sun-stars when stopped down accordingly.

•Veiling flare is well controlled, but ghosting rears its head (unsurprisingly) when shooting directly into the sun. For what it's worth, this lens handles ghosting marginally better than my Samyang/Rokinon 14mm f/2.8.


Sunstar @ f/22

•As an aside, I feel that the colours produced by this lens are nothing short of excellent. The palette renders as fairly neutral, not straying too cool nor too warm. Contrast is also on par with other Pentax primes, which I found to be pleasantly surprising.

Physical Qualities

•The star of the show with this lens is its close focusing abilities. While not a 'macro' lens, you will be able to get up close to your subjects and create some really interesting perspectives with it's wide angle of view. It's a great way to exaggerate your subject while still pulling context from the background into your shot.

•This Sigma 20mm features a pretty common focusing switch from the era it was made: the push/pull focusing collar. While it does work to disengage the focusing ring during autofocus to stop the focusing ring from spinning while autofocus is active, it does not allow for full time manual focus override while the camera is in autofocus mode.

•While using autofocus, I found that both phase detect (through the viewfinder) and contrast detect (liveview mode), yielded identical results in regards to sharpness. Phase detect AF is quicker to attain focus, however. Autofocus is quick, decisive, and accurate in both bright light and dim.

•This lens is very large. On a K-1, it's not an issue as it balances well on the larger heavier body. On smaller bodies, such as any of the Pentax 'letter' series of cameras (Kp, Kx, Kr, Km, etc), I would imagine this lens would not balance well.

•The hood is a normal reversible bayonet type, is made of plastic, and does its job well.

•Distortion, while present, is very well controlled for such a wide lens. I didn't find it noticeable in the majority of photos taken in the field. When it is apparent, such as when taking photos of architecture, I found that it was easily corrected in Lightroom with a few clicks.

In Comparison to other lenses I own/have owned
♣K series 24mm f/2.8 → The sigma is sharper at all overlapping aperture values and has smoother bokeh. It doesn't hold a candle to the flare resistance of the K series lens, however. The sigma has a better minimum focusing distance.

♣Pentax FA-J 18-35mm → The sigma is both sharper and contrastier while also having smoother bokeh and an aperture ring. The sigma is also more than two stops faster at 20mm and has a better MFD.


♣Rokinon/Samyang/Bower/Bell & Howell/Vivitar 14mm f/2.8 → The sigma is not as sharp by a narrow margin at apertures below f/4. However, the Sig handles veiling flare/ghosting slightly better, can mount filters, has autofocus, and features much better controlled distortion.Also, the sigma has a better MFD.
   
New Member

Registered: February, 2014
Posts: 3
Review Date: November 11, 2019 Not Recommended | Price: $350.00 | Rating: 3 

 
Pros: Hard to say...
Cons: Poor resolution & extreme fringing!
Sharpness: 3    Aberrations: 3    Bokeh: 3    Handling: 5    Value: 2    Camera Used: K-1 Mark II    New Or Used: Used    Autofocus: 5   

This is one of the worst lenses I did ever own: The image quality especially in the corners is very bad, it makes the lens unusable for any serious work. Aberration and resolution ("sharpness") are lousy, even stopped down!
I'm aware about the fact that extreme WA lenses always have issues. But the image quality of this Sigma lens is substandard. And I know that Sigma builds some excellent lenses - but unfortunately no longer for Pentax mount.
Maybe my 20 mm EX is a misaligned specimen? In that case I highly recommend to test the respective lens before you buy.
And - by the way - Ricoh, please renew the Pentax FA 20mm! This is much more important than a Fisheye Zoom...
   
Senior Member

Registered: June, 2016
Location: Paris
Posts: 124
Review Date: June 6, 2019 Recommended | Price: $500.00 | Rating: 10 

 
Pros: IQ, bokeh, handling, construction, af
Cons: zero
Sharpness: 10    Aberrations: 8    Bokeh: 10    Handling: 10    Value: 10    Camera Used: K-1    New Or Used: Used    Autofocus: 10   

a marvelous lens.
done great prints with it for fashion editorials and other types of work, commercial and artistic.
tested it for some video too, it's a pure joy too.
was lucky to find it like new.

It's my favourite prime with the FA* 85 1.4 and ltd 31mm 1.8. on the K-1.


just need a long rage zoom aside and there you have the the perfect quator for any situation.
   
Loyal Site Supporter

Registered: December, 2009
Location: Christchurch, New Zealand
Posts: 2,437

1 user found this helpful
Review Date: July 7, 2018 Recommended | Price: $190.00 | Rating: 8 

 
Pros: Close focus, f1.8, build, CA not bad for this wide
Cons: Bigish, sharpness not uniform
Sharpness: 7    Aberrations: 8    Bokeh: 8    Handling: 9    Value: 9    Camera Used: K-1    New Or Used: Used    Autofocus: 8   

Its a pretty good lens on the K-1. Personally I have not had any issues with AF - although it is a screw drive and so not silent.

The only thing is with my copy sharpness is uneven, with the top right corner being a lot softer than the rest of the frame (and so you need to stop down to f8+ to get good across frame sharpness).

However its fast aperture and close focus make it a fun lens to have around! All in all worth having.

Wide open + close focus:
https://www.flickr.com/photos/kiwi_jono/32136933734

Stopped down a bit (f2.8):
https://www.flickr.com/photos/kiwi_jono/33003902750

f9.5
https://www.flickr.com/photos/kiwi_jono/34491097084
   
New Member

Registered: June, 2012
Location: Blue Mountains
Posts: 18
Review Date: January 19, 2017 Recommended | Price: $150.00 | Rating: 9 

 
Pros: Sharp, Cheap, well built.
Cons: not weather proof
Sharpness: 9    Aberrations: 8    Bokeh: 8    Handling: 9    Value: 10    Camera Used: K1    New Or Used: 10   

After reading some of the negative reviews I was very cautious about buying one of these. Having shot with it for a few weeks now, I’ve found nothing at all wrong with it and can’t understand the poor reviews here.

I use it on a K1, mostly shooting landscapes on manual focus, and find the results very pleasing. Sharpness, as Tibi says, when focused with care, is brilliant. The weight is reasonable and getting filters is no problem at all.

At the prices some of these go for it is a great buy and a gem of a lens. Totally recommended.
   
New Member

Registered: July, 2009
Location: Piatra Neamt
Posts: 16

2 users found this helpful
Review Date: December 9, 2014 Recommended | Price: None indicated | Rating: 7 

 
Pros: Excellent in low light, very sharp (if focused properly)
Cons: Imprecise autofocus, difficult to focus manually
Sharpness: 7    Aberrations: 8    Bokeh: 10    Handling: 7    Value: 8    Camera Used: K20D   

This lens can deliver sharp images, if focused correctly! The image quality is extremely sensitive to this and sharpness decreases very quickly at the slightest focusing error.

When focused correctly, the lens is useable even at f/1.8, with only a slight loss of sharpness and a small amount of purple fringing in high contrast, but these are visible only when pixel-peeping; it should be a concern only for very large prints.
At f/2.0, purple fringing is gone and sharpness is improved, at f/2.4 sharpness is already maximum in the center and almost maximum in the corners of APS-C, while from f/2.8 and on it's perfect. I emphasize that these results are valid only for precise focus!

Unfortunately, precise focus is difficult to obtain, especially in low light. On my K20D, autofocus is not precise enough, but since my camera body is equipped with a split-screen and microprism manual focusing aid, I use that instead. With good technique and patience, I can obtain precise focus this way. On more recent bodies, live view could be used as well (live view sucks on the K20D). If you plan to use the distance scale on the lens, you should check it first: on my copy, true infinity is actually about 1-2mm left of the infinity mark.

I use the lens mostly for night shots in the city, for which it is wide enough to be used hand-held even when limiting the ISO to max 1600. I've captured some nice images with it during festivals. Bokeh is also nice for such a wide lens and could be used creatively if you're into this kind of shooting.

In conclusion, for daylight or action shooting, other, more compact and easier to focus 20mm lenses exist.
But in low light situations, I don't know of any other lenses in this FL with a comparable aperture. In my case, this lens saves me the hassle of carrying and setting up a tripod for night shots in the city. If your shooting situation gives you time to properly adjust focus and compose your shots, this lens will not disappoint.

This lens is also suitable for full-frame, although I haven't tested it on film yet. It would be interesting to see how it performs to the edges of FF at wide apertures.
   
Emperor and Senpai

Registered: June, 2010
Location: Mishawaka IN area
Posts: 6,124

1 user found this helpful
Review Date: March 10, 2012 Recommended | Price: $300.00 | Rating: 6 

 
Pros: Good in low light, but....
Cons: Not so sharp unless stopped down past 3.2 or so

This lens is an interesting one. I bought it for use at anime conventions, but found it wasn't quite what I was looking for. Wide open it is soft, and in fact I would not recommend using it wider than something like 2.8 because of that. There's also alot of grain in the pics on my K5 with it at wider apertures. It is pretty nice as a prime when stopped down, but really if you want sharp you need to take it past 4, maybe to 5.6. The weird thing is it reads on the K5 as an f/1.7 lens.

I was torn between recommending this one and not. While it's nice for stuff outside, or in brighter light where you don't have to have it wide open, that is what I bought it for. I'll go ahead and stick with my Sigma 18-50 inside at anime cons until I get a different one to try. I'll go ahead and recommend buying it based on it working well in some circumstances, but not the ones I bought it for. Ohh, and this thing is HUGE!
   
New Member

Registered: February, 2012
Location: Bristol
Posts: 6
Review Date: February 10, 2012 Recommended | Price: $400.00 | Rating: 8 

 
Pros: IQ, speed
Cons: weight, size, delicate

At the time when these came out, they got rave reviews for the IQ. It was also the fastest thing available in this focal length at anything like a sensible price. I like fast primes, so I bit the bullet and bought one.

I used this on film bodies for a while, and was very pleased with the sharpness and overall quality, but then it got knocked off a lowish chair by a passer-by onto a pavement (while inside a padded bag) and the focus became horribly stiff. I was a long way from home (other side of the planet) but a local repairer loosened and re-seated the mount, whereupon something inside went "snick" and settled back into place, after which the focus was smooth and free again, but I think it'd become mis-aligned and de-centred, as it's never been the same since, in terms of IQ - field not so flat and absolute sharpness lacking wide open, whereas when new, it was very sharp, even wide open.

Recently, I got another example secondhand in Nikon fit - while better, it's not as blistering-sharp as the first one was when new. I suspect either manufacturing QC is suspect, or these lenses are just too delicate - the very minor drop that mine had (inside a padded bag!) wouldn't have hurt any of my Pentax or Nikon lenses, I was astonished and apalled that the Sigma was damaged by it.

A protective filter cost loads at this diameter, too. If you can find a truly mint one, going cheap, then it's a lovely fast wideangle, but I wouldn't expect it to stand any rough handling.
   
New Member

Registered: October, 2010
Location: Almaty
Posts: 20
Review Date: February 8, 2012 Recommended | Price: $450.00 | Rating: 9 

 
Pros: Fast, sharp, focus distance, build quality, RF
Cons: Weight, size, filter size

I never considered this lens because of low rating on the internet and especially on pentaxforums, but i've got an opportunity to buy it used and test. It was a big suprise for me that this lens takes very sharp photos wide open (my K-x shows max aperture as f/1.7 instead of f/1.8 as it is)...
Also, a big plus is minimum focus distance + fast aperture = more sharp photos with unbelievable blurred background (OOF)
Cons: size and weight, 82mm filter size.
I'll rate this great lens 9/10 because of max aperture at this focal range.
If you are looking for fast 20mm lens, this is just for you. Otherwise, 21 limited, which is 1,6 stops slower, but smaller, lighter and metal built.

----rate is updated 24.02.2012
   
Pentaxian

Registered: July, 2011
Posts: 2,378
Review Date: December 3, 2011 Not Recommended | Price: None indicated | Rating: 5 

 
Pros: Interesting specs
Cons: Cannot live up to specs

A third party fixed focal length prime lens with large aperture. Just what everybody would love to have. The specs are really pushing the limit here, performance is below average even at smaller apertures. I would not recommend any of the 1.8 Sigma wide angles.
   
Site Supporter

Registered: May, 2008
Location: Melbourne, Florida
Posts: 147
Review Date: October 31, 2011 Recommended | Price: $519.00 | Rating: 10 

 
Pros: Fast, sturdy, great for video and low light
Cons: Size (maybe)

I had read the reviews of this lens prior to purchasing and what I list as pros were key in my decision to buy. It is a charm for me, with it being the most used lens on my K7. To me this is a superior lens and knowing what I was getting, I find no fault.

One thing that may not be to everyone else is the size of this lens. It takes an 82mm filter and appears much bigger when the almost mandatory hood is on. I like it and can handle it.

I can recommend this lens, with the proviso the research is done prior to acquiring. That way you know what you are getting.
   
Forum Member

Registered: May, 2010
Location: Brisbane
Posts: 99

1 user found this helpful
Review Date: February 23, 2011 Recommended | Price: $350.00 | Rating: 6 

 
Pros: Close min focus distance, fast
Cons: soft up until F2.8

F1.8 has a ridiculously thin DOF when focusing at the minimum focus distance which is only a few cm in front on the lens.

I have found very uses for F1.8 at 20mm other then wide MACRO shots.

This was my first lens after the kit lens. I have since purchased several Pentax primes including the FA 43mm and FA 77mm limiteds. The pentax lenses are in a different stratosphere in terms of image and build quality.
   
Site Supporter

Registered: July, 2009
Location: Charlotte, NC United States
Posts: 146
Review Date: August 19, 2010 Recommended | Price: $230.00 | Rating: 9 

 
Pros: Fast, Sharp, Very Useful Indoors without flash
Cons: Cost, a little bulky, odd filter size

I looked at the reviews before I bought my copy and was a little concerned by some of the comments, but the price that I managed to buy it for was so good that I really couldn't pass on it.

After having spent a couple of days with it, I am very happy that I have it. I find it to be sharp, particularly stopped down to 2.8 and above, it's relatively light compared to comments above and handles well. I have so far noticed no problems with vignetting, even wide open and the brokeh is quite pleasant. I don't know if I have a particularly good copy, but I am certainly pleased with it so far. We will be in Italy this fall and I suspect that it will be just perfect for interior shots without flash.

At $670 MSRP or even Street $519, I would probably not buy it, but at what I paid for it, I am jumping up and down. Yes, I certainly would recommend it if the price is right.

12/10/2010: Update to the above review. I wanted to add a couple of comments after out Itay trip this fall. This lens performed magnificently inside the historic buildings that we visited in Tuscany. Images are very sharp, colors a true to nature, and I was able to bring home shots that I simply could not have gotten with other lenses that I have. I actually left my Pentax 50 mm F1.4 at home to save weight and am actually glad that it did; the 20 mm focal length was much more practical in the monuments than the 50 mm would have been. I am very pleased with the results and recommend this highly.
   
New Member

Registered: April, 2010
Location: London, UK
Review Date: July 31, 2010 Not Recommended | Price: $750.00 | Rating: 1 

 
Pros: feels solid and well built
Cons: image quailty very poor with apertures 1.8-2.8

I bought this as I wanted a fast AF 20mm prime. There are not many choices: this sigma, the 21mm DA which is slower by some margin and a used FA 20mm. I went for the sigma due to its speed and FF compatibility.

Although it's sharp at f5.6, below this the sharpness falls off rapidly and i consider it totally unusable at f2.8 and wider. I went thru making adjustments for FF/BF but it was still woefully inadequate. It seems a little pointless to have an f1.8 lens that can only be used at much smaller apertures. Especially since you pay for it in size and weight - this is a big lump of glass.

In the end i returned it the retailer and acquired a used 20mm FA which is a stunner!
   
Veteran Member

Registered: March, 2009
Location: Salt Lake City, UT
Posts: 509
Review Date: July 20, 2010 Recommended | Price: $300.00 | Rating: 7 

 
Pros: 20mm at f1.8, contrasty, well built, and great for video
Cons: Low IQ at f1.8 and quite heavy

It is really hard to evaluate this lens. It is in such a good focal spot and seems to be well suited for urban evenings and indoor shootings. However, I think that Sigma may have pushed the envelop a bit too far and seems to try to accomplish a mission impossible. And the result is this lens.

So where to start?!

Construction: Well done. Like all other EX series lens. Everything fits tightly, even the hood. The quality is above regular DA lenses. Well, I would not say it is flawless, it is close to be that. Focusing is smooth, very fast and accurate. BTW, it is hard to focus a 20mm lens slowly. The paint can be controversial for some users. But I am OK with it.

The front element is fixed and does not MOVE at all. Only its rear elements move back and forth when focusing. Since it is Internal Focusing, you will not feel the torque with focusing. Focusing is also pretty quite, even though it does not have HSM.

The front element is so big that a decent UV filter would cost a fortune (as much as 1/5 of the lens price?), not even mentioning the price of a decent polarizer (as much as 1/2 of the lens price?). Even though the large filter size may be necessary for a full frame lens, it is just too big.

The AF and MF focus switch does not work for Pentax, at least not for me (same issue with Sigma 105mm Macro). It is on the heavy side since it is made for full frame cameras. It fits better with K10D and less so on K-x. The limited lenses are better suited for K-x.

Image quality: could be the trouble spot for some. At best, it is a mixed bag.

I think that Sigma might have stretched too much for a full frame lens like this. I echo many other reviews. f1.8 is hardly usable, IMHO. The image quality is not quite like Pentax 50mm at f1.2. At the largest aperture, the image feels mushy instead of the soft focus feeling or the result of chromatic aberration. While I can tolerate Pentax 50mm at f1.2 and sometimes even enjoy the soft touch, I can not stand the Sigma at f1.8. It gets better with smaller aperture and at f4 IQ becomes quite good. But what is point then to have such a big monster around?! Would it be better to increase ISO instead?

The improvement of IQ is much more gradual than my Pentax 50mm lenses. However, this lens controls Chromatic Aberration and PF better than non DG lens and than some older Pentax lenses.

BUT: this can be a great lens for videographers. It works fantastically for video shooting. It can produce unique scenes that few other lens can.

Bottomline: I used to dislike Sigma lenses in the film era. They were soooo crappy in terms of construction and image quality. I am amazed by the extent to which Sigma has improved. They have made a number of unique lenses in recent years. Some of them can even rival Pentax lenses. This might be an exception. Can I say that in this case Sigma has become a "stretcher" in optical design and lens making?

For the sheer uniqueness, I would recommend this lens. Would I keep one? Probably not.
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