The smc Pentax-DA 17-70mm F/4 AL [IF] SDM lens belongs to the budget series of Pentax lenses and is designed for Pentax APS-C digital SLR cameras. The effective focal range of the lens (when taking into consideration K-5 IIs’ crop-factor of 1.52x) is 26-106 mm so the 17-70/4 should be considered as a standard zoom suitable for shooting in a wide range of photography genres — from architecture and landscapes to portraits (in theory). The lens has constant maximum aperture 4 on all range of the focal lengths.
Design and Features
Despite the fact that like most of the other Pentax lenses this lens is made in Vietnam, its build quality is very good. The body is made of high quality plastic and the mount is made of metal. The lens has pretty moderate weight of 485 g, maximum diameter of 75 mm and the length of 94 mm.
The zoom ring is located in the central part of the lens, well dumped, rotates smoothly and with proper resistance. Thereby the front element of the lens does not move forward spontaneously under its own weight when the lens is tilted down. The front group of lenses moves forward almost at the distance of 42-43 mm when zooming from 17 to 70 mm.
The manual focusing ring is narrow (13 mm only), located in the frontmost part of the housing, well damped and rotates very smoothly. The angle of rotation is very small (45 degrees only) but it’s well enought for manual focus adjustments through Quick-Shift Focus System. The focusing is internal (as indicated by IF acronym in the lens’ name) so the front element does not move and does not rotate during focusing and you may conveniently use gradient and polarized filters if you want. The diameter of the filter thread is 67 mm.
Optically the lens is quit complex and consists of 17 elements in 12 groups including 2 aspherical. Aspherical elements provide crisp images.
The Pentax company did not publish not only the MTF charts but even the optical schemes of its Pentax K optics but after its acquisition by Ricoh Imaging now it is possible to visualize the scheme:
The optical elements of the lens have proprietary smc Pentax coating which improves the transmission of the optical system, improves the color rendering and overall contrast of the image, eliminates ghosting and flares. The smc coating is used in all Pentax lenses for K, 645 and 6×7 mounts and also in Asahi Takumar M42 lenses since 1971 and is constantly updated. A special coating is used on the front lens for repelling water and oils. This enables any water and oils on the lens to be easily removed for effective protection of the lens surface.
The diaphragm is circular and has 7 blades. The minimum value of the aperture is F/22. The aperture ring is absent (as with all Pentax lenses designed for digital SLR cameras) so the control of the aperture is automatic only.
The minimum focusing distance is only 0.28 m which allows to produce interesting close-up shots at the wide end.
The buyer receives front and rear caps, stylish case and pretty big petal hood PH-RBM 67 mm in the box. The hood can be mounted reversed for storage or transportation purposes.
The lens is equipped with Supersonic Drive Motor (SDM) so the autofocus is virtually silent and very fast with Pentax K-5 IIs camera used for testing. The Quick-Shift Focus System is supported — the photographer can make manual focus adjustments by just rotating the manual focus ring after the camera has done with the autofocusing. There is no AF/MF switch on the housing of the lens.
As for the accuracy of the autofocus — I had some problems with it. The SDM motor often couldn’t provide the accurate focusing due to its inability to make accurate and at the same time tiny movements on the focusing scale. I almost permanently had to use the Quick-Shift Focus System by turning the manual focusing ring so that the camera could make the shots. In fact the point-and-shoot method is simply impossible with my copy of the lens.
The lens works excellent in bright direct sunlight even without dedicated PH-RBM 67mm hood. The overall contrast never drops. Very rarely you could observe the artefact of moderate size and intensity when the sun is located in the corner of the frame.
Thanks to focal length of 17 mm it is possible to make a beautiful shots with impressive perspective and wide angle of view. Visually the lens is sharp all over the frame on most landscape combinations of focal lengths and apertures. There is a little softness at 70 mm at the center and especially at the edges of the frame but the sharpness increases with further closing of the aperture.
The lens shows visible vignetting at 17mm F/4 but such combination of focal length and aperture almost never used for landscape shooting. With further closing of the aperture the effect diminishes and from F/8 becomes imperceptible.
The lens shows large barrel distortion near the focal length of 17 mm (this is one of the big disadvantages of the lens) but as in the case of lateral chromatic aberrations the distortion could be fully corrected in one click using Adobe Photoshop Lightroom. However the strong distortion at such focal lengths is the typical disadvantage even of the prime lenses (despite the fact that optical schemes of prime lenses are simplier than of zooms which theoretically makes it possible to adjust the distortion better).
Moderate lateral chromatic aberrations appear at 17 mm when the background contains lots of small contrast subjects (for example, tree branches, foliage etc). Such effect fortunately only can be seen at the borders of the frame, but in some cases it may impair the visual perception of the image. However this is the usual disadvantage of optics with similar focal lengths. Just one click in Adobe Photoshop Lightroom or similar software and the problem is gone. The photo of the forest below provided in two variants — without and with corrections of lateral chromatic aberrations made using Adobe Photoshop Lightroom 5.6. The aberrations will decrease with zooming and disappear at 70mm.
Theoretically the lens is good for street shooting thanks to its pretty wide focal range, fast autofocus and with the help of in-body image stabilizator. In reality however the street shooting is difficult due to SDM motor problems.
The lens is not wide enough for the interior shooting — you won’t make any interesting photos in confined spaces. Besides the lens is not fast enough so you have to use the tripod in order to obtain high quality interior pictures.
The quality of bokeh is so-so. The background blur is neutral at 70mm F/4 and sometimes even nervous. The out-of-focus highlights are evenly illuminated and round at 70mm F/4 at the center of the frame but are deteriorated at the borders and take the form of lemons. In general the lens does not exhibit an impressive bokeh.
The smc Pentax-DA 17-70mm F/4 AL IF SDM is a compromise model which does not make much of an impression. Certainly the manufacturer tried to develop a convenient lens with a fairly wide range of focal lengths and a constant maximum relative aperture F/4 and substantially very good sharpness. But in reality the lens is not sharp enough near the 70mm, has SDM motor problem which affects the accuracy of autofocus and the bokeh is neutral and leaves something more to desire.
If the Pentax 18-55 kit lens became a limitation for you and you would like to purchase a pro lens, it makes sense to buy the smc Pentax-DA* 16-50mm F/2.8 ED AL [IF] SDM and not to make compromises. Believe me it’s wise to purchase a high quality pro lens and enjoy all the benefits of such lens (outstanding image quality, excellent build quality, dustproof and water resistant body, ultrasonic motor) than to buy a budget lens which you will be soon trying to sell on a secondary market.
Author of the review and the photos: Evgeny Artemov, email@example.com