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SMC Pentax-M 20mm F4

Reviews Views Date of last review
27 120,038 Tue November 20, 2018
Recommended By Average Price Average User Rating
96% of reviewers $242.67 8.78
SMC Pentax-M 20mm F4

SMC Pentax-M 20mm F4
SMC Pentax-M 20mm F4

This relatively slow extreme-wide-angle lens was more compact than its K predecessor.

SMC Pentax-M 20mm F4
©, sharable with attribution
Image Format
Full-frame / 35mm film
Lens Mount
Pentax K
Aperture Ring
Yes (no A setting)
Automatic, 5 blades
8 elements, 8 groups
Mount Variant
Check camera compatibility
Max. Aperture
Min. Aperture
Min. Focus
25 cm
Max. Magnification
Filter Size
49 mm
Internal Focus
Field of View (Diag. / Horiz.)

APS-C: 72 ° / 62 °
Full frame: 94 ° / 84 °
Dedicated rectangular clip-on hood
Dedicated hard case
Lens Cap
Plastic clip-on
Weather Sealing
Other Features
Diam x Length
63 x 29.5 mm (2.5 x 1.16 in.)
150 g (5.3 oz.)
Production Years
1977 to 1984
Engraved Name
smc PENTAX-M 1:4 20mm
Product Code
User reviews
Manual FocusAperture RingFull-Frame SupportDiscontinued
Price History:

Add Review of SMC Pentax-M 20mm F4
Sort Reviews by: Date | Author | Rating | Recommendation | Likes (Descending) Showing Reviews 1-15 of 27
Junior Member

Registered: July, 2010
Location: Cardiff, UK
Posts: 47

1 user found this helpful
Lens Review Date: November 20, 2018 I can recommend this lens: Yes | Price: N/A | Rating: 9 

Pros: Compact, sharper one has any right to expect
Cons: Nothing serious
Sharpness: 8    Aberrations: 8    Bokeh: 6    Handling: 9    Value: 9    Camera Used: K3/Sony A7iii   

I have a couple of copies of this lens. One is significantly better than the other. On a Pentax K3 the lens it is not really wide enough to stand out: it works well enough, but a 30mm equivalent f4 is nothing to shout about. On a 24x36mm sensor things are different. Once you stop down to f6.3 or f8 the lens is commendably sharp across the frame (very sharp in the centre) when used on a Sony A7. By f11 it is very good indeed. In fact, given how compact this lens is, the image quality is rather amazing. One can compare this usefully to the faster A series equivalent. The Pentax A 20mm f2.8 might be slightly sharper in the corners at f5.6 (neither is brilliant at this aperture, although the centre is fine on both). However, the f2.8 suffers badly from flare. The M f4 does not and is easy to carry in a pocket. A fine lens.
New Member

Registered: March, 2018
Posts: 8
Lens Review Date: June 28, 2018 I can recommend this lens: Yes | Price: $100.00 | Rating: 7 

Pros: Center sharpness, handling, colour rendition, contrast, flare resistance
Cons: Edge sharpness, distortion
Sharpness: 7    Aberrations: 8    Bokeh: 5    Handling: 10    Value: 8   

I tested this lens on a Sony a7. The center is really sharp and contrasty from 5.6 to 11.
The corners are really weak at all apertures. The corners are the best at 16, probably due to field curvature.
The colours and contrast of this lens is outstanding. I also like the flare en ghost resistance.
Veteran Member

Registered: May, 2009
Location: India
Posts: 380

2 users found this helpful
Lens Review Date: January 24, 2018 I can recommend this lens: Yes | Price: $130.00 | Rating: 9 

Pros: size, built, sharpness
Cons: Nothing, at least for the price
Sharpness: 9    Aberrations: 8    Handling: 10    Value: 10    Camera Used: K1, K5iis, K-01   

When I first used the lens borrowed from a friend, I had a very bad feel about it's image quality. But, I have plans to upgrade to K1 and in need of a small wide-angle prime. When he is selling it for a good price, couldn't say NO and grabbed it. Got an opportunity to loan K1 and used the lens and was simply blown away by the performance of the combo this time. Such an old lens performing superbly on 36mp digital body is great. As many said, it is a well built, compact and typical pentax film era prime lens. What else.. check the pics attached.. weather is pretty hazy when I shot these pics, but I love them

IMGP2155 by Tirumala Nalla, on Flickr

IMGP2247-HDR by Tirumala Nalla, on Flickr

IMGP2342 by Tirumala Nalla, on Flickr
Site Supporter

Registered: April, 2015
Posts: 90
Lens Review Date: August 30, 2017 I can recommend this lens: Yes | Price: $130.00 | Rating: 10 

Pros: Build quality, versatility,Design
Cons: none
Sharpness: 10    Aberrations: 8    Bokeh: 9    Handling: 10    Value: 10    Camera Used: k1 k3   

Great lens, small and easy to focus. well made as are all m lenses. Really no negatives!
New Member

Registered: February, 2016
Location: lake constance
Posts: 1

2 users found this helpful
Lens Review Date: March 10, 2016 I can recommend this lens: Yes | Price: $300.00 | Rating: 10 

Pros: plenty of sharpness, very compact and light, build, coating
Cons: edges could be darkened when using filters
Sharpness: 10    Aberrations: 10    Bokeh: 9    Handling: 10    Value: 10    Camera Used: Sony A7   

For me, the Pentax M 4/20mm is a well built and compact companion for my MX and also Sony A7. To a compact camera belongs a compact and powerful lens. Robust in mechanics and excellent in optics too. That are the ingredients for a lifelong partnership. I bouht this one by a bargain and paid round about 160$ for it. It has some scratches on its body, but this doesn`t disturb me.
Very flexible use in creative landscape photography, because you fit this lens with small and affordable filters. I often use a big stopper with ND3.0 a modified polarizer ( otherwise vignetation possible ).
Additional i use a cokin gradual filter system. An enormous advantage of the M lenses is the fantastic smc coating - perfect in digital Full Frame too.
There are no ghosts by using filters. The M 4/20mm has a short way of focusing, is very light - perfect to handle on trips in landscape and nature. And here it lets combine with my Pentax M 3,5/24-35mm and M 1,4/50mm or M 1,7/50mm. But there is one thing to consider: you better make some aperture stops down to 8-11. In wide angle photography mostly needed. You have got a large depth of field. Then the corner have plenty of sharpness. And even the distorsion is not striking.
May be, the newer A lens 2,8/20mm has a better optical performance, but it is not so compact and is much more expensive. I often put this lens in my pocket of my coat or jacket.
It is impressive to me, how Pentax engeneers put such a know how into a lens in the 1970s. You don`t find any comparable 20mm lens in todays production worldwide. Pentax was unique in manufacturing well built compact lenses in the 1970s and early 1980s. Until today i think, no other brand reached this level of overall performance.
I believe, most of the Pentax users can support me in this opinion. Full Frame recommended in the past with for example the MX, today f.e. with Sony A7.

A morning at lake constance - Insel Reichenau
Site Supporter

Registered: June, 2011
Location: Texas
Posts: 1,261
Lens Review Date: July 15, 2015 I can recommend this lens: Yes | Price: N/A | Rating: 10 

Pros: Size, image quality, handling
Cons: No neon full frame
Sharpness: 8    Aberrations: 8    Bokeh: 8    Handling: 10    Value: 10    Camera Used: MX, Sony A7   

I've stayed with Pentax film bodies where this is truly an ultra wide, and this lens is a winner on them. I didn't bother with it on my K-5. When I wanted wide on digital I used a 21 on my Leica M9 (FF), but the need for an accessory finder is inconvenient. When I got an A7 body to use with my manual Pentax M lenses I was amazed how good this lens is. Yes, there is softness at the extreme edges, but for a FF 20 it is much better than I expected. Recommended!
Site Supporter

Registered: November, 2012
Location: Kansas City, MO
Posts: 2,290
Lens Review Date: March 2, 2015 I can recommend this lens: Yes | Price: $300.00 | Rating: 9 

Pros: Size, weight, sharpness
Cons: Field curvature
Sharpness: 9    Aberrations: 7    Bokeh: 7    Handling: 9    Value: 9   

When I decided to go all manual focus, but before I developed this obsession with collecting lenses, this was on my short list. What's not to like? It's small, light, wide, and had great reviews. The problem was, they were pretty hard to come by. After a few weeks of scanning the boards, I put up a wanted post for an M20, and received an offer to sell a pristine copy of the lens, recently CLA'ed, and even came with the original box.

Physically, the lens is a gem. It's small, light, and solidly built. The lens is slightly larger and heavier than the DA21, and I wonder how much DNA these two lenses share? The M20 is small enough to put in a coat pocket if you just want to carry an extra lens (however, the practicality of this depends on the size of the lens you'd switch out). The lens is only f/4, but fast apertures in my opinion aren't as important for ultra wides since you can still achieve significant DOF control with a modest aperture.

I've used the lens on a Sony NEX-6 with a Lens Turbo, both v1 and v2 where it is a true ultra-wide, and the field of view is awesome. However, with v1, the "blue dot" problem occurs infrequently, and the lens is not compatible with v2 (the rear of the lens contacts the glass inside the LT, preventing the lens from reaching infinity). With that said, this review concentrates only on my use of the lens on APS-C.

The lens is easy to use, with a relatively short focus throw that becomes even shorter if you're using small apertures, because everything's in focus. I did find myself feeling around a bit for the focus ring at times when my eye was at the viewfinder... the focus ring is a bit small, but it has to be because the whole lens is small. Larger hands might have similar problems, although I don't consider my hands large.

The lens has 49mm threads, and an aftermarket metal lens hood does the job well on APS-C format*. That said, I didn't use a hood at all this month, and flare was not a problem.

I find myself seeking out close-ups or scenes with foreground interest, where I think the lens shows it's best optical qualities. Sharpness and color are very good for close subjects, often requiring only light sharpening, even wide open. Objects at infinity (ie, trees, far away buildings) seem a little softer. There is very little chromatic aberration, unlike many of its K and M brethren, and rarely did I need to make a correction in Lightroom. The lens shows some strange distortion when shooting geometric patterns, however I see it rarely in my practical experience.

Overall, I truly enjoyed using the lens this month, and I never got tired of it. It's a fun lens, but sometimes I felt like I wanted to go even wider (it's only a 30mm equivalent on APSC, you know). So, when the FF comes out, I'll be looking forward to seeing what this thing can really do.

*UPDATE, 27SEP2016: The same lens hood that did not cause problems on APSC did vignette significantly on the K-1. I'd like to hear any suggestions for an alternative lens hood that will work on this lens without vignetting!

Registered: October, 2009
Location: North
Posts: 4,242

4 users found this helpful
Lens Review Date: February 2, 2015 I can recommend this lens: Yes | Price: N/A | Rating: 9 

Pros: small; good flare performance; generally good at f11 on FF
Cons: corners can be a bit sharper stopped down on FF
Sharpness: 8    Aberrations: 8    Bokeh: 7    Handling: 9    Value: 9    Camera Used: K30; Sony A7   

This lens made little sense on APS-C so it never passed my mind to get one.
But with use of Pentax lenses on FF, I re-explored what was available and got hold of one.

DSC08446-2400 by jenkwang, on Flickr

This is a real small lens for a 20mm.
Roughly speaking about 2/3 the length of a Canon EF50/1.8 for those more familiar with that lens.
Or the same size as a M28/2.8.
Few makers seem to want to make smaller more portable f4 wide angle primes anymore, which is a pity imo as very often such FL see more use as a f8 and smaller wide view / landscape lens than anything else, so less need for a f2.8 which adds to the weight and size.

DSC08453-2400-bw by jenkwang, on Flickr

DSC08510-2400 by jenkwang, on Flickr

20150131-DSC08642-2400 by jenkwang, on Flickr

It seems that the lens is nicely resistant to flare (as in sample shots)
Field curvature seems real slight (almost none) compared to other legacy faster 20mm which tends to have a significant amount that needs to be taken care of during focus.
f8 is generally good, with some softness only in the corners (tips), perhaps a hint less sharp on the last 1/10 of the frame.
Thats not uncommon for many legacy 20mm (and even quite a few more modern wides).
The latter may also be an artifact of the thick sensor stack of the A7 series cameras.
f11 is probably where I want to set if I want better corners and no issues at all on the edges.

Sharpness up to the edge at f11 is no complaints at all.
Its the far corners that will be soft(er).
I would have given a higher sharpness score if not for the corners (and corners only)

Overall, a great portable option.
Far under rated imo having tried the FA20 and the Canon FD20, especially when one considers encumbrance and actually carrying the lens for days on a trip matters and cost.
Forum Member

Registered: April, 2014
Posts: 60

2 users found this helpful
Lens Review Date: May 1, 2014 I can recommend this lens: Yes | Price: $300.00 | Rating: 10 

Pros: size, sharpness, colurs, contrast, build
Cons: none
Sharpness: 9    Aberrations: 8    Bokeh: 9    Handling: 9    Value: 9    Camera Used: sony a7r   

This is a great lens. Why??
It fits in one's pocket so it's not going to be left at home!!
The colours are so vibrant. The sharpness is great at f8.
Manual focus and the depth of field scale are excellent for control of depth of field and the hyperfocal distance.
Build quality is excellent. The focus ring is great and the aperture ring clicks true every time.
Distortion in landscape isn't noticeable.
Distortion in architecture is easily corrected in LR5.

This is a must lens for landscape photography.
Veteran Member

Registered: April, 2010
Location: Wiltshire/Hampshire
Posts: 1,760
Lens Review Date: May 28, 2013 I can recommend this lens: Yes | Price: N/A | Rating: 7 

Pros: Sharp in the centre, tiny, great FOV on film
Cons: Not great in the corners
Sharpness: 7    Aberrations: 8    Bokeh: 3    Handling: 8    Value: 6    Camera Used: LX, ME, K5   

Not a bad little lens - sharp in the centre, but never gets really sharp in the corners, even on a crop body.

However, its great field of view and tiny size on a film body make it well worth keeping. Not worth getting just for digital though,

Lens test here:
Senior Member

Registered: November, 2011
Location: Los Angeles
Posts: 167

1 user found this helpful
Lens Review Date: April 27, 2012 I can recommend this lens: Yes | Price: $245.00 | Rating: 10 

Pros: Almost a pancake, Bokeh, fun factor X!
Cons: Sharpness
Sharpness: 9    Aberrations: 9    Bokeh: 10    Handling: 10    Value: 10    Camera Used: Pentax ME Super   

I shoot film exclusively, so this is a review of the entire lens. Caveat: I'm a total sucker for small lenses. I like to be able to stick a lens in my jacket pocket, another on my camera, and go shoot. If you feel the same, you'll love this lens too.

The bokeh of the M20/4 is really pleasing. I end up taking a fair amount of "impressionistic" photos that are intentionally out of focus or overexposed. These shots end up being pretty good bokeh "test shots":

It's plenty sharp wide open.

My only gripes are that with a maximum aperture of f/4, it's a little dark in the viewfinder, and that it's not a 24mm lens. But how stupid is it to criticize a lens for being the wrong focal length.

Hope this helps.

Registered: February, 2010
Location: Eureka, CA
Posts: 2,749

7 users found this helpful
Lens Review Date: May 26, 2011 I can recommend this lens: Yes | Price: $285.00 | Rating: 9 

Pros: Compact, good contrast, excellent color rendition
Cons: Some distortion, soft wide open
Sharpness: 8    Aberrations: 7    Bokeh: 7    Handling: 10    Value: 10   

I'm not a huge fan of M series glass. But when I needed a 20mm lens on which I could use my 52mm filters, it was either this M 20 or the DA 21, and as this was cheaper option (by $75 to $150), this is the lens I chose.

While I would not describe the M 20 as a great lens, it's still a very fine piece of glass capable of producing stunning images. Although it's the least sharp of all my wide angle or normal range primes, it's still "sharp enough" when stopped down. Its color rendition is very unique and striking: different from my K glass and DA glass, but well suited for landscape work involving water. It produces especially luminous blues and warm ambers. This is a lens with loads of character, and since acquiring it, it has helped me take some of my very best photos.

I have only two lenses that share a focal length with the M 20: the DA 12-24 and the DA 18-55. The M 20, IQ-wise, stomps all over the optically-challenged DA 18-55 and produces finer image quality. The DA 12-24 presents a stiffer challenge. While the 12-24 is not much sharper at 20mm than the M 20, it features more contrast and has a better overall IQ (though hardly by a huge margin). The M 20 has a bit more character and is significantly smaller. I wouldn't want to be without either lens, as they each have a significant role to play in my landscape workflow.

Veteran Member

Registered: December, 2007
Location: Vancouver, British Columbia
Posts: 334

4 users found this helpful
Lens Review Date: December 20, 2010 I can recommend this lens: Yes | Price: N/A | Rating: 9 

Pros: lovely IQ, good build, compact
Cons: currently expensive, not all that wide on digital

I got this lens very inexpensively - very lucky yard sale find (along with a bunch of other Pentax gear). So I won't say exactly how much it cost

Anyway, I only ever had a chance to run it through its paces with one roll of film (long expired generic KodakMax 400) and also took a few digital test shots.

I must say, on my Pentax ME, it had a certain indefinable quality and produced fantastic shots. I would call it ‘cinematic’ – kind of bold yet mysterious. I never experienced any perspective distortion, which is great considering just how wide this little prime is. The colors and contrast were always punchy and nice, and the sharpness was also excellent to my eye. It was a joy to compose with and to manually focus on the Pentax ME. Together, they made for such a nice little compact package. And the couple of images I got with this lens on digital (my Pentax K-x) also looked nice.

Unfortunately, I was silly enough to sell it, right here on the forum marketplace. Part of me wishes I hadn't. I hope the new owner holds on to it for quite a bit longer.

The reason I sold was I thought that a Sigma 10-20 would be a better and more practical choice for DSLRs. I believed f4 to be a bit too slow and the lack of AF and zoom to be downsides. In reality, for my style of shooting, the lightweight nature or this lens and its image quality outweigh those considerations by far. Oh well, live and learn I guess. Definitely, I'd say the images I got from the Sigma were much more prosaic and less 'cinematic', much less “wow” factor.

Here are a few sample shots from that one roll of cheap expired film on my Pentax ME:

Veteran Member

Registered: February, 2009
Location: Perth, Western Australia
Posts: 2,295

2 users found this helpful
Lens Review Date: December 16, 2010 I can recommend this lens: Yes | Price: N/A | Rating: 9 

Pros: size, handling, optical performance
Cons: wide open performance

I purchased this lens intending to use it for both digital and film cameras, however I was not too sure how it would compare to my existing Pentax-M 28mm f/3.5, which is already very good in terms of sharpness, both on film and digital.
However as soon as I mounted the M20 to a film body, I was in love with the size and handling of the lens. It really is tiny - comparable to some of the small modern limited lenses.

On film the field of view is fantastic. Extremely wide, but I was surprised by how practical it is in use. I had some reservations that this lens would be special purpose only, but really it is very practical in much the same way as the excellent DA15 limited is on digital.

The M20 continues to be a solid performer on digital. I did find that sharpness wide open at f/4 was a bit lacking, however there is quite a dramatic improvement from f/5.6 onwards. Perhaps this is the compromise that the lens designers had to make in order to keep the lens so tiny? In my mind, it is worth it! Even though I have listed this point in the cons list, I would consider it almost a non issue.

Another point that is relevant on both film and digital is that the M20 appears to be relatively resistant to flare when compared to some of my other M series lenses. This is a rather useful attribute, as with such a wide lens it is likely that you will have the sun close by for at least some shots. Keep in mind however that I am comparing it against other M series lenses, not modern DA lenses.

On digital the M20 will act like any other K or M lens - this means stop down metering and using manual mode. However in my opinion this is less of a hassle compared to other K and M series lenses of a longer focal length. Don't let it hold you back from using this lens.

One thing about the M20 on digital is that in my mind it is a bit of a strange focal length. Wide but not really wide (I would consider wide on digital 18mm and shorter). Of course this is not the lenses fault, Just something to think about.
On that note, I really do encourage the more adventurous DSLR user to try this lens out on film. I'm sure you will be impressed as it really does come into it's own. Paired with an MX body, you end up with a very compact kit.

In conclusion, I would rate the M20/4 as a 9 on film and an 8 on digital. Here I am rating it as a 9 due to the surprisingly practical nature and excellent optical performance on the 35mm film format.
I have attached a couple of shots taken with scanned Kodak Ektar 100 film. No complaints with contrast, colour or resolution here... I will endeavour to return at some point with some digital images for comparison.

New Member

Registered: October, 2007
Location: Toronto
Posts: 4

1 user found this helpful
Lens Review Date: May 28, 2010 I can recommend this lens: Yes | Price: $200.00 | Rating: 8 

Pros: Very Sharp, Short Minimum Focus Distance, Nice Colour
Cons: Slow at f4

This was my second copy of this lens. For whatever reason, I sold the first one, and then bought another, but I was very upset when I bought this lens, because the aperture lever broke. Someone took it out so that it could be used on the Canon bodies, and it was GLUED back when it was sold to me. In any case, I have neglected this lens for more than a year until I started using it on the Digital Rebel 550D (this lens does not work on full frame Canon cameras).

I wish I had used this lens sooner. Such a small and lightweight lens is perfect match for the also petite Digital Rebel. I love the fact that anything after 2 meters is consider infinity, so that's where the focus usually stays. When used on non-full frame cameras, corner sharpness issues can be neglected.

The 20mm f4 is very sharp and contrasty. It's now my favourite lens!

Pentax K10D & 20mm f4 at f4:

Canon 20D IR Converted & 20mm f4:

Canon Digital Rebel 550D & 20mm f4:
Add Review of SMC Pentax-M 20mm F4

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