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SMC Pentax-M 20mm F4 Review RSS Feed

SMC Pentax-M 20mm F4

Reviews Views Date of last review
32 164,941 Tue April 20, 2021
Recommended By Average Price Average User Rating
97% of reviewers $243.84 8.84
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 32
New Member

Registered: April, 2020
Location: Lincolnshire
Posts: 4
Review Date: April 20, 2021 Recommended | Rating: 9 

Pros: size
Cons: only f4
Sharpness: 9    Aberrations: 8    Bokeh: 8    Handling: 9    Value: 9    Camera Used: LX MX   

I've owned this lens for 9 year and I only use on film bodies on which it works very well producing usable prints upto 12" x 16". There is a level of distortion at the edge of the frame which is to be expected with a 20mm lens of this age of design.

New Member

Registered: April, 2021
Posts: 16

5 users found this helpful
Review Date: April 15, 2021 Recommended | Price: $250.00 | Rating: 9 

Pros: sharp (up to 10% of the border on FF). Excellent for APS-C where is scores 9
Cons: Soft borders 10% and corners on FF
Sharpness: 9    Aberrations: 8    Bokeh: 8    Handling: 8    Value: 9    Camera Used: K1 and K3   

I have just acquired the FA 20mm f2.8 for my K1. I already owned the M 20mm f4.

I am reviewing (both these) because I was unable to find an adequate comparison anywhere of the FA 20 f2.8 and the M 20 f4. Hopefully this review will help other folk choose between these two lenses.

My results are based on taking the same landscape with buildings detail and winter trees in the distance in the corners on a full frame K1, using both the FA 20 f2.8 and the M 20 f4, from f2.8 to f11 focused at infinity. I checked that both lenses focus correctly at infinity at all apertures before commencing; there is no obvious focus shift, which is good. Observations were made in Photoshop at 100%. (I am not posting images, because unless they are full size, which is impractical, there isn't any point.)

First the M 20 f4:
f4 soft all over. Significant light fall-off in corners.
f5.6 sharp in the centre but soft borders. Light fall-off improving.
f8 very sharp, until 10% of the borders. Within this boundary, the f4 is similarly sharp as the f2.8. To give an objective measure: A car number plate at 55 metres (on a 20mm lens the plate’s length is a mere 0.6% of the width of the frame which really is small) is just readable. Light fall-off towards the corners on the f4 is more pronounced at all apertures than the f2.8 but only slight at f8 and easily edited.

If I crop the 10% soft border on the 4, this increases the effective focal length from 20 to 24mm and the K1 file size reduces from 36 to 25mp. This is OK and excellent on my K3, but I wanted 20mm and the full K1 file size.

The FA 20 f2.8 on the K1 does not disappoint:
f2.8 Sharp border to border; softness in corners. Noticeable light fall-off towards corners but less than the M 20 f4
f3.5 Sharp border to border; mild softness in corners.
f4.5 Sharp border to border; very mild softness in only in very tips of corners
f5.6 - 8 Sharp corner to corner.
f11 just begins to soften from diffraction.

Light fall-off diminishes progressively when stopped down and is negligible at f8. CA and aberration are good.

Happily my f2.8 came with a hood. Being 20mm this doesn't look like it does much but as well helping with glare I think it actually helps generally to keep contrast up. As I will only use this stopped down for landscape, I can't comment on bokeh or distortion, but there is nothing obvious on the buildings in my images.

Finally, I should mention that my copy had a slight decentering problem and diagonal corners were not sharp when I acquired it (used); simply disassembling the rear group and re-assembling miraculously completely resolved this - either rotation of the group or simply reseating it must have achieved this.

In conclusion, excepting the border softness of the F4 on full frame (but does not affect APS-C users), these two lenses perform similarly and very well. The obvious advantage of the FA over M is it is fully automatic (and it seems to autofocus well). I don't mind manual for landscape but auto is obviously more versatile. It is still compact and light.

Sweeping statements that the f4 is completely inferior to the f2.8 need to be contextualised: the f4 is not up to full frame (K1) but is outstanding on APS-C (e.g. K3) where it is capable of a very sharp A2 print corner to corner; the f2.8 on a K1 is capable of a sharp corner to corner A1. Price and what you want it for will probably determine your choice.
New Member

Registered: February, 2021
Posts: 1
Review Date: February 5, 2021 Recommended | Price: $400.00 | Rating: 9 

Pros: Sharp, wide angle, small form factor
Cons: fringing on corners
Sharpness: 10    Aberrations: 9    Bokeh: 6    Handling: 9    Value: 10    Camera Used: Sony A7R   

Love the lens overall, its my must bring for any travelling or trips since its so light and easy to use.
Veteran Member

Registered: September, 2017
Location: Medellín
Posts: 1,314
Review Date: December 23, 2019 Recommended | Price: $168.00 | Rating: 10 

Pros: Small, light, pancake-like, 49mm filters, handling, build
Sharpness: 8    Aberrations: 9    Bokeh: 9    Handling: 10    Value: 10    Camera Used: Film cameras, K-5   

This lens is a dream. Smallest and lightest FF 20mm I've ever handled (the Oly Zuiko 20/3.5 is smaller and lighter, but just for half frame; I've never tried the OM 21/3.5). Mine weighs just 148 g (without caps)! This is a lens you'll never leave at home.
The five aperture blade diaphragm makes nice ten point sunstars. If it had seven, it would have the mythical status of the smc DA15/4 which for the most part is its later APS-C equivalent. Ricoh couldn't go wrong with an updated version of this lens with the added characteristics of the aforementioned lens plus WR.
Senior Member

Registered: June, 2013
Location: Utrecht
Posts: 190

1 user found this helpful
Review Date: July 4, 2019 Recommended | Price: $175.00 | Rating: 9 

Pros: Compact, build quality, flare resistance, excellent performance on APS-C
Cons: Corners suffer sharpness and fringing at wider apertures
Sharpness: 9    Aberrations: 8    Bokeh: 8    Handling: 10    Value: 10    Camera Used: K1-II and K-01   

This is my standard walk around lens on my white K-01. Together with the M 28 F 2.8 this 20 mm performs extremely well without any serious flaws. Both are very sharp om APS-C, show contrasty colorfull rendering and have great flare resistance. They look cool on the K-01 either with those purple/blue coatings. On Full Frame you have to stop down to F8 or F11 to get good corner performance, at wider apertures the corners suffer a bit. I would rate this a 10 for APS-C and a 8 on Full Frame.

See also my quick & dirty comparrison with other ultra wides at

Forum Member

Registered: July, 2010
Location: Cardiff, UK
Posts: 65

3 users found this helpful
Review Date: November 20, 2018 Recommended | Rating: 9 

Pros: Compact, sharper than 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 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
Review Date: June 28, 2018 Recommended | 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: 384

3 users found this helpful
Review Date: January 24, 2018 Recommended | 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
Senior Member

Registered: April, 2015
Posts: 130
Review Date: August 30, 2017 Recommended | 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
Review Date: March 10, 2016 Recommended | 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
Loyal Site Supporter

Registered: June, 2011
Location: Texas
Posts: 1,426

1 user found this helpful
Review Date: July 15, 2015 Recommended | 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!

Registered: November, 2012
Location: Ingelheim am Rhein, Germany
Posts: 4,981
Review Date: March 2, 2015 Recommended | 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: 4,658

8 users found this helpful
Review Date: February 2, 2015 Recommended | 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: 61

3 users found this helpful
Review Date: May 1, 2014 Recommended | 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
Review Date: May 28, 2013 Recommended | 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:
Add Review of SMC Pentax-M 20mm F4

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