John Riley puts one of Pentax' mid-range zooms to the test, the Pentax-DA 17-70mm f/4 ED IF SDM.
Posted: 05/01/2016 - 14:31
Handling and Features
Performance
Verdict
Specification
There are several mid range zoom lenses in the Pentax range, including this 17-70mm with a constant aperture design. Is this one of the best choices for Pentax users, or has it been overtaken by more recent lenses? Let's find out.
SMC Pentax-DA 17-70mm f/4 ED IF SDM Handling and Features
The SMC Pentax-DA 17-70mm f/4 ED IF SDM lens is for the Pentax range of APS-C format DSLRs. The 35mm format equivalent is 26-107mm, a useful range to cover, taking in a reasonably wide angle through the standard focal length and on to a short telephoto. It is a compact design, weighing just 485g without hood, and making extensive use of high-quality plastics in its construction. The finish is to a high standard.
A lens hood is supplied as standard, but was not included with the test lens provided. The Pentax K-5 was used for the review and the lens balances very well with the camera body. Controls on the lens are minimal as there is no aperture ring, just a zooming ring and a manual focus ring towards the front. The zoom action is firm and smooth. The manual focusing is a very short throw of less than 90 degrees and is typical in its lack of feel for an AF lens. It is not too loose however, and can also be employed at any time when the AF point has been found, or as a full manual focus when the camera body is switched to MF. This QuickShift feature is very useful for final manual tweaks to the focusing point after AF has locked, especially in macro and other close up photography.
Lens construction is 17 elements in 12 groups, including two aspheric elements and ED (extra low dispersion) glass. Minimum focus is 0.28m (0.92 feet) and the lens achieves a maximum magnification of around 1:3, one third life size. That is not as close as a macro lens, but nonetheless a very useful magnification.
Focusing is via the Pentax SDM (Super-Dynamic Motor) motor, integral to the lens. This system is silent and finds focus very reliably, but is not the fastest to lock on. As the mount version on this lens is KAF3 there is no AF coupler to engage the in-body AF system that all Pentax camera bodies have. Later bodies (K10D with firmware 1.3, K20D and later) will support the SDM focusing via the Power Zoom contacts in the mount. If the lens is used on older bodies, it will be manual focus only. As time goes by and older bodies move out of circulation, this will be unlikely to be an issue.
The f/4 maximum aperture is constant, so using flash, for example, will be easier as the aperture does not change when zooming. The lens is SMC coated and adds SP (Super Protect) coating to the front element. This repels water, grease and dirt, making the lens easier to keep clean. The filter size is 67mm.
There are no issues with using the lens, it gives a crisp clear viewfinder image and is easy to focus manually as it matches the focusing screen very well. The zoom is firm enough not to creep when carried, so no lock is needed. The focal length range is excellent for general purpose shooting. 17mm is wide enough for much wide angle shooting and the 70mm end caters well for portraits and some closer sports photography.
SMC Pentax-DA 17-70mm f/4 ED IF SDM Performance
At 17mm central sharpness is very good at open aperture and peaks at f/5.6, just one stop down. After that sharpness slowly reduces, but is still very good at f/16 and good at f/22. The edges never really approach the central sharpness, being only fair throughout the range but peaking at a good level at f/11. CA is controlled tightly at the centre, being barely one quarter of a pixel wide at all apertures. An excellent result. The edges though show quite a bit of fringing, with high CA values exceeding 2 pixels width throughout. This edge CA can clearly be seen in images, but can of course be tackled in software. Distortion is an obvious -3.24% barrel, which again can be corrected in software.
At 35mm Central sharpness again starts off as very good, peaking one stop down at f/5.6 and remaining very good through to f/16. It remains good at f/22. Edge sharpness is poor at f/4 and f/5.6, rising to good levels by f/11 and f/16 before trailing off again to fair at f/22. CA however is much better than at 17mm, again excellent at the centre with exceptionally good figures. The edges are also much better, less than one pixel width. Pincushion distortion measures +1.18%, correctable but reasonably low in any case.
At 70mm, although the figures are slightly lower, the central sharpness is still very good from f/4 to f/16, peaking at f/5.6. It is still good at f/22. The edges fare less well, being poor at f/4 and f/5.6, fair at f/8 and good at f/11 and f/16. Sharpness falls to fair levels at f/16.
Pentax DA 17-70mm f/4 AL [IF] SDM MTF Charts
How to read our MTF charts
The blue column represents readings from the centre of the picture frame at the various apertures and the green is from the edges.
The scale on the left side is an indication of actual image resolution and sharpness as LW/PH and is described in detail above. The taller the column, the better the lens performance.
For this review, the lens was tested on a Pentax K-5 using Imatest.
CA remains very well controlled, especially at the centre of the field. Pincushion distortion measures +1.15%, another good result.
Pentax DA 17-70mm f/4 AL [IF] SDM Chromatic Aberration Charts
How to read our CA charts
Chromatic aberration (CA) is the lens' inability to focus on the sensor or film all colours of visible light at the same point. Severe chromatic aberration gives a noticeable fringing or a halo effect around sharp edges within the picture. It can be cured in software.
Apochromatic lenses have special lens elements (aspheric, extra-low dispersion etc) to minimize the problem, hence they usually cost more.
For this review, the lens was tested on a Pentax K-5 using Imatest.
Bokeh is a bit fussy with this lens, not quite as smooth as lenses with more diaphragm blades. However, in general images were punchy and had a very good overall sharpness. The edges did let the lens down at longer focal lengths and wider apertures, although this could be taken advantage of with some subjects, perhaps close portraits.
Flare can be induced, despite the SMC coatings, and this is not helped by the lack of lens hood. Fortunately the hood is provided in the package with new lenses. Shielding the lens shows that given a hood or some other shield the resistance to flare becomes good. Into the light shots then retain contrast, saturation and sharpness very well.
Pentax DA 17-70mm f/4 AL [IF] SDM Sample Photos
Value For Money
There are now a few options in the Pentax range if we want to upgrade to a more interesting zoom than the standard kit lens. The SMC Pentax-DA 18-135mm f/3.5-5.6 ED AL IF DC WR lens (£379) extends the range at the cost of a slightly less wide view but also adds the faster DC motor and weather resistance. The HD Pentax-DA 16-85mm f/3.5-f/5.6 ED DC WR (£499) widens the view but only adds a small amount to the telephoto end, and also offers DC motor and WR construction. This compares to the £449 being asked for the reviewed lens.
Sigma also offers an alternative, the 17-70mm f/2.8-4 DC Macro HSM (£329) which gives that slightly wider maximum aperture at the widest end.
SMC Pentax-DA 17-70mm f/4 ED IF SDM Verdict
The lens is a very good performer in many ways, with just a couple of weak areas as mentioned, designed with the central performance given priority. The questions to ask revolve around whether or not lenses from the same range that have wider ranges, sometimes better sharpness, faster focusing and weather resistance might not be a better option.
In isolation, the 17-70mm is capable of some fine images, looked at against more recent Pentax lenses it just may have now been overtaken.
SMC Pentax-DA 17-70mm f/4 ED IF SDM Pros
- Very good central sharpness
- Excellent central CA control
- Focuses very close
- Silent AF
- Constant aperture design
SMC Pentax-DA 17-70mm f/4 ED IF SDM Cons
- Lower edge sharpness
- CA at edges at 17mm
- AF not the fastest
Pentax DA 17-70mm f/4 AL [IF] SDM Specifications
| Manufacturer | Pentax | |
| General | ||
| Lens Mounts | Pentax K SMC-DA |
|
| Lens | ||
| Focal Length | 17mm - 70mm | |
| Angle of View | 23 - 79 | |
| Max Aperture | f/4 | |
| Min Aperture | f/22 | |
| Filter Size | 67mm | |
| Stabilised | No | |
| 35mm equivalent | 26mm - 107mm | |
| Internal focusing | No Data | |
| Maximum magnification | No Data | |
| Focusing | ||
| Min Focus | 28cm | |
| Construction | ||
| Blades | 7 | |
| Elements | 17 | |
| Groups | 12 | |
| Box Contents | ||
| Box Contents | No Data | |
| Dimensions | ||
| Weight | 485g | |
| Height | 93.5mm | |
Members gallery photos using: smc PENTAX-DA 17-70mm F4 AL [IF] SDM
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