|SMC||Multi-layer anti-reflection coating is applied to the surfaces of lens elements. This anti-reflection coating boosts light transmission, ensures sharp and high contrast images, minimizes ghosting and flares. Learn more|
|M||A manual focus lens with automatic maximum aperture indexing, which is a mechanical system for coupling the lens to the camera's exposure system. More compact and lighter than the standard K-series counterpart, but with no sacrifice in quality of construction, resolution, durability or contrast.|
|■Asahi Super-TAKUMAR 20mm F/4.5 ||A||11 - 10||0.2m||⌀77||1968 ●|
|■Asahi Super-Multi-Coated TAKUMAR 20mm F/4.5  [Mod. M42]||A||11 - 10||0.2m||⌀77||1971 ●|
|■smc Pentax 20mm F/4||A||12 - 10||0.25m||⌀58||1975 ●|
|■smc Pentax-M 20mm F/4||A||8 - 8||0.25m||⌀49||1978 ●|
■ Production details
|Production status:||● Discontinued|
|Original name:||ASAHI OPT. CO. SMC PENTAX-M 1:4 20mm|
|System:||Pentax K (1975)|
■ Optical design
|Maximum format:||35mm full frame|
|Mount and Flange focal distance:||Pentax K [45.5mm]|
|Diagonal angle of view:||94.5°|
|Lens construction:||8 elements in 8 groups|
■ Diaphragm mechanism
|Aperture control:||Aperture ring (Manual settings only)|
|Number of blades:||5 (five)|
■ On Pentax K APS-C [1.53x] cameras
|35mm equivalent focal length:||30.6mm (in terms of field of view)|
|35mm equivalent speed:||F/6.1 (in terms of depth of field)|
|Diagonal angle of view:||70.5°|
|Closest focusing distance:||0.25m|
|Maximum magnification:||1:8.33 at the closest focusing distance|
|Focusing modes:||Manual focus only|
|Manual focus control:||Focusing ring|
■ Physical characteristics
|Maximum diameter x Length:||⌀63×29.5mm|
|Super Protect (SP) coating:||-|
|Lens hood:||PH-S49 - Clamp-on rectangular|
■ Sources of data
|1. Manufacturer's technical data.|
|2. Asahi Pentax series lenses and accessories booklet (PUB. 06771, 60 pages).|
|3. smc Pentax-A lenses operating manual.|
|4. Pentax LX booklet (PUB. 06511).|
|5. Asahi Pentax MX booklet (PUB. 06241).|
|6. Asahi Pentax MX booklet (PUB. 06241) (March 1981).|
|7. Pentax MV booklet (PUB. 06021).|
|8. Pentax MV1 booklet (PUB. 06036).|
|9. Pentax MG booklet (PUB. 06221) (July 1983).|
|10. Pentax LX booklet (PUB. 06506) (April 1981).|
|11. The many eyes of Pentax booklet (PUB. 06071).|
|12. Pentax cameras booklet (PUB. 06091, 32 pages).|
|13. Asahi Pentax ME booklet (PUB. 06006).|
|14. Pentax ME Super booklet (PUB. 06731).|
|15. Asahi Pentax smc Pentax lenses booklet.|
|16. Pentax lenses and accessories booklet (PUB. 06771) (April 1983).|
|17. Pentax lenses and accessories booklet (PUB. 08051).|
|18. Pentax lenses & accessories - A Comprehensive and Versatile Photographic System booklet.|
A very compact ultra-wide-angle lens approximately 50% lighter than the standard 20mm f/4 wide-angle lens. Useful for almost every type of wide-angle photography, especially when photographing sweeping landscapes and towering structures, or when desiring extensive depth of field. 49mm screw-in filter diameter.
The 20mm wide-angle lens is useful for a variety of wide-angle applications. It is capable of engulfing towering structures and huge interiors at close range. And, its extreme depth of field makes it useful for fast moving subjects and quick candid shots. Moreover, precise computer calculations assure optimum performance at all apertures, while, thanks to miniaturized M-type optics, it has been designed an unbelievable 50% lighter than the previous 20mm f/4 lens.
Slow full-frame ultra-wide angle prime lens • Professional model
Genres or subjects of photography (5):
Landscapes • Cityscapes • Buildings • Interiors • Travel photography
Adaptation to digital SLR cameras:
Canon EOS SLRs • Sigma SD SLRs • Sony SLRs/SLTs • Nikon SLRs • More information
In order to adapt the lens, the flange focal distance (FFD) of the lens mount must be equal to or greater than the FFD of the camera mount. This lens has the Pentax K mount with a FFD of 45.5mm. This is even shorter than the FFD of Canon EOS digital SLR cameras, which have the shortest FFD of 44mm of any modern digital SLR cameras. Therefore, this lens cannot be adapted to any digital SLR camera.
Recommended slowest shutter speed when shooting static subjects handheld:
1/20th of a second
Sorted by focal length and speed, in ascending order
|smc Pentax 15mm F/3.5||A||13 - 12||0.3m||--||1975 ●|
|smc Pentax-A 15mm F/3.5||A||13 - 12||0.3m||--||1984 ●|
|smc Pentax 18mm F/3.5||A||12 - 11||0.25m||--||1975 ●|
|smc Pentax 20mm F/4||A||12 - 10||0.25m||⌀58||1975 ●|
|smc Pentax-A 20mm F/2.8||A||10 - 9||0.25m||⌀67||1985 ●|
Sorted by manufacturer name
|RMC Tokina 17mm F/3.5 II (SL)||A||11 - 9||0.25m||⌀67||●|
|RMC Tokina 17mm F/3.5 Wide-Auto||A||11 - 8||0.22m||⌀72||●|
|Cosina Voigtlander Color-SKOPAR 20mm F/3.5 Aspherical SL II||A||9 - 6||0.2m||⌀52||2009 ●|
|Cosina 20mm F/3.8 MC||A||9 - 8||0.2m||⌀62||●|
|Irix 21mm F/1.4||A||15 - 11||0.3m||--||2022 ●|
|Panagor 21mm F/4 Auto||A||9 - 7||0.16m||⌀82||●|
|Samyang 20mm F/1.8 ED AS UMC (Bower, Rokinon, Walimex Pro)||13 - 12||0.2m||⌀77||2016 ●|
|Sigma MF 18mm F/3.5 ZEN||A||10 - 9||0.25m||⌀72||1992 ●|
|Sigma MF 18mm F/2.8 II||A||10 - 9||0.25m||⌀72||1984 ●|
|Sigma MF 18mm F/2.8 (XR Rikenon)||A||11 - 9||0.25m||⌀67||1980 ●|
|Sigma MF 18mm F/2.8 Filtermatic||A||? - ?||0.23m||⌀77||1978 ●|
|Spiratone 18mm F/3.5 Pluracoat [I] (Soligor Wide-Auto)||A||11 - 8||0.22m||⌀72||1979 ●|
|Spiratone 18mm F/3.5 Pluracoat [II]||A||11 - 9||0.25m||⌀67||●|
|Spiratone 20mm F/2.8 Plura-Coat (Cambron MC, MC Soligor C/D Wide-Auto)||A||9 - 7||0.2m||⌀58||1983 ●|
|Vivitar 20mm F/3.8 Auto (s/n 22xxxxxx)||A||9 - 7||0.15m||⌀82||1969 ●|
|Vivitar 17mm F/3.5 Auto (s/n 37xxxxxx)||A||11 - 8||0.22m||⌀72||1976 ●|
|Vivitar 17mm F/3.5 MC (s/n 37xxxxxx)||A||11 - 9||0.25m||⌀67||●|
|Vivitar 19mm F/3.8 MC (s/n 9xxxxxxx)||A||9 - 8||0.2m||⌀62||●|
|Carl Zeiss Classic Distagon T* 18mm F/3.5 ZE / ZF / ZF.2 / ZK||A||13 - 11||0.3m||E82||2008 ●|
|Carl Zeiss Classic Distagon T* 21mm F/2.8 ZE / ZF / ZF.2 / ZK||A||16 - 13||0.22m||E82||2008 ●|
|Sigma[-XQ] [Widerama] MF 18mm F/3.2 (Spiratone) [YS]||A||12 - 10||0.17m||⌀72||1972 ●|
|Soligor Wide-Auto 21mm F/3.8 [T-4]||A||9 - 8||0.35m||⌀72||1970 ●|
|Tamron SP 17mm F/3.5 51B [Adaptall-2]||A||12 - 10||0.25m||--||1979 ●|
|Tamron SP 17mm F/3.5 151B [Adaptall-2]||A||12 - 10||0.25m||--||1984 ●|
|Auto Tamron 21mm F/4.5 [Adapt-A-Matic]||A||8 - 6||0.25m||⌀82||1970 ●|
|Vivitar 21mm F/3.8 Auto [T-4]||A||9 - 8||0.35m||⌀72||●|
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Cannot compare the lens to itself.
A technology used for reducing or even eliminating the effects of camera shake. Gyro sensors inside the lens detect camera shake and pass the data to a microcomputer. Then an image stabilization group of elements controlled by the microcomputer moves inside the lens and compensates camera shake in order to keep the image static on the imaging sensor or film.
The technology allows to increase the shutter speed by several stops and shoot handheld in such lighting conditions and at such focal lengths where without image stabilizer you have to use tripod, decrease the shutter speed and/or increase the ISO setting which can lead to blurry and noisy images.
Lens name as indicated on the lens barrel (usually on the front ring). With lenses from film era, may vary slightly from batch to batch.
Format refers to the shape and size of film or image sensor.
35mm is the common name of the 36x24mm film format or image sensor format. It has an aspect ratio of 3:2, and a diagonal measurement of approximately 43mm. The name originates with the total width of the 135 film which was the primary medium of the format prior to the invention of the full frame digital SLR. Historically the 35mm format was sometimes called small format to distinguish it from the medium and large formats.
APS-C is an image sensor format approximately equivalent in size to the film negatives of 25.1x16.7mm with an aspect ratio of 3:2.
Medium format is a film format or image sensor format larger than 36x24mm (35mm) but smaller than 4x5in (large format).
Angle of view describes the angular extent of a given scene that is imaged by a camera. It is used interchangeably with the more general term field of view.
As the focal length changes, the angle of view also changes. The shorter the focal length (eg 18mm), the wider the angle of view. Conversely, the longer the focal length (eg 55mm), the smaller the angle of view.
A camera's angle of view depends not only on the lens, but also on the sensor. Imaging sensors are sometimes smaller than 35mm film frame, and this causes the lens to have a narrower angle of view than with 35mm film, by a certain factor for each sensor (called the crop factor).
This website does not use the angles of view provided by lens manufacturers, but calculates them automatically by the following formula: 114.6 * arctan (21.622 / CF * FL),
CF – crop-factor of a sensor,
FL – focal length of a lens.
A lens mount is an interface — mechanical and often also electrical — between a camera body and a lens.
A lens mount may be a screw-threaded type, a bayonet-type, or a breech-lock type. Modern camera lens mounts are of the bayonet type, because the bayonet mechanism precisely aligns mechanical and electrical features between lens and body, unlike screw-threaded mounts.
Lens mounts of competing manufacturers (Canon, Nikon, Pentax, Sony etc.) are always incompatible. In addition to the mechanical and electrical interface variations, the flange focal distance can also be different.
The flange focal distance (FFD) is the distance from the mechanical rear end surface of the lens mount to the focal plane.
Lens construction – a specific arrangement of elements and groups that make up the optical design, including type and size of elements, type of used materials etc.
Element - an individual piece of glass which makes up one component of a photographic lens. Photographic lenses are nearly always built up of multiple such elements.
Group – a cemented together pieces of glass which form a single unit or an individual piece of glass. The advantage is that there is no glass-air surfaces between cemented together pieces of glass, which reduces reflections.
The focal length is the factor that determines the size of the image reproduced on the focal plane, picture angle which covers the area of the subject to be photographed, depth of field, etc.
The largest opening or stop at which a lens can be used is referred to as the speed of the lens. The larger the maximum aperture is, the faster the lens is considered to be. Lenses that offer a large maximum aperture are commonly referred to as fast lenses, and lenses with smaller maximum aperture are regarded as slow.
In low-light situations, having a wider maximum aperture means that you can shoot at a faster shutter speed or work at a lower ISO, or both.
The minimum distance from the focal plane (film or sensor) to the subject where the lens is still able to focus.
The distance from the front edge of the lens to the subject at the maximum magnification.
Determines how large the subject will appear in the final image. For example, a magnification ratio of 1:1 means that the image of the subject formed on the film or sensor will be the same size as the subject in real life. For this reason, a 1:1 ratio is often called "life-size".
The diaphragm must be stopped down manually by rotating the detent aperture ring.
The lens has two rings, one is for pre-setting, while the other is for normal diaphragm adjustment. The first ring must be set at the desired aperture, the second ring then should be fully opened for focusing, and turned back for stop down to the pre-set value.
The lens features spring mechanism in the diaphragm, triggered by the shutter release, which stops down the diaphragm to the pre-set value. The spring needs to be reset manually after each exposure to re-open diaphragm to its maximum value.
The camera automatically closes the diaphragm down during the shutter operation. On completion of the exposure, the diaphragm re-opens to its maximum value.
The aperture setting is fixed at F/4 on this lens, and cannot be adjusted.
As a general rule, the more blades that are used to create the aperture opening in the lens, the rounder the out-of-focus highlights will be.
Some lenses are designed with curved diaphragm blades, so the roundness of the aperture comes not from the number of blades, but from their shape. However, the fewer blades the diaphragm has, the more difficult it is to form a circle, regardless of rounded edges.
At maximum aperture, the opening will be circular regardless of the number of blades.
Excluding case or pouch, caps and other detachable accessories (lens hood, close-up adapter, tripod adapter etc.).
Excluding case or pouch, caps and other detachable accessories (lens hood, close-up adapter, tripod adapter etc.).
For lenses with collapsible design, the length is indicated for the working (retracted) state.
A rubber material which is inserted in between each externally exposed part (manual focus and zoom rings, buttons, switch panels etc.) to ensure it is properly sealed against dust and moisture.
Lenses that accept front mounted filters typically do not have gaskets behind the filter mount. It is recommended to use a filter for complete weather resistance when desired.
Helps keep lenses clean by reducing the possibility of dust and dirt adhering to the lens and by facilitating cleaning should the need arise. Applied to the outer surface of the front and/or rear lens elements over multi-coatings.
Lens filters are accessories that can protect lenses from dirt and damage, enhance colors, minimize glare and reflections, and add creative effects to images.
A lens hood or lens shade is a device used on the end of a lens to block the sun or other light source in order to prevent glare and lens flare. Flare occurs when stray light strikes the front element of a lens and then bounces around within the lens. This stray light often comes from very bright light sources, such as the sun, bright studio lights, or a bright white background.
The geometry of the lens hood can vary from a plain cylindrical or conical section to a more complex shape, sometimes called a petal, tulip, or flower hood. This allows the lens hood to block stray light with the higher portions of the lens hood, while allowing more light into the corners of the image through the lowered portions of the hood.
Lens hoods are more prominent in long focus lenses because they have a smaller viewing angle than that of wide-angle lenses. For wide angle lenses, the length of the hood cannot be as long as those for telephoto lenses, as a longer hood would enter the wider field of view of the lens.
Lens hoods are often designed to fit onto the matching lens facing either forward, for normal use, or backwards, so that the hood may be stored with the lens without occupying much additional space. In addition, lens hoods can offer some degree of physical protection for the lens due to the hood extending farther than the lens itself.
Teleconverters increase the effective focal length of lenses. They also usually maintain the closest focusing distance of lenses, thus increasing the magnification significantly. A lens combined with a teleconverter is normally smaller, lighter and cheaper than a "direct" telephoto lens of the same focal length and speed.
Teleconverters are a convenient way of enhancing telephoto capability, but it comes at a cost − reduced maximum aperture. Also, since teleconverters magnify every detail in the image, they logically also magnify residual aberrations of the lens.
Scratched lens surfaces can spoil the definition and contrast of even the finest lenses. Lens covers are the best and most inexpensive protection available against dust, moisture and abrasion. Safeguard lens elements - both front and rear - whenever the lens is not in use.