Pentax SMC DA 18-250mm f/3.5-6.3

Gary Wolstenholme takes a look at this all purpose superzoom for use on Pentax digital SLR cameras that covers a range equivalent to 27 – 375mm on a 35mm camera.

Pentax's 18-250mm superzoom costs around £500 and is ideal for the photographer who wishes to travel light, but still requires a large focal range. It is the only lens of this type in their line-up adding an extra 50mm to the usual 18-200mm normally found on marque superzooms.

Third party manufacturers have long dominated the superzoom market, and both Sigma and Tamron make lenses with equivalent specification. Tamron's 18-250mm costs around £380 and looks uncannily similar to the Pentax optic in terms of size. Sigma's 18-250mm OS HSM costs around £400 but adds optical stabilisation you can see working through the viewfinder and a silent focusing motor.

Pentax SMC DA 18-250mm f/3.5-6.3: Handling and features
This lens appears to be reasonably well built, with the barrel being constructed from high quality plastics. The zoom mechanism is nice and smooth and the lens extends in two sections, more than doubling in size at maximum extension. A petal shaped hood is supplied with the lens which clips into the bayonet fitting around the 62mm filter thread. I had no issue with swift zoom creep during use as the zoom system has just enough resistance to make it stay put.

Focus is performed internally and unfortunately isn't all that quick, sometimes hunting for a second or two at the long end of the zoom range. This lens also lacks the full time manual focus feature found on so many other Pentax lenses. As the maximum aperture is only f/6.3 at the long end, the viewfinder image can appear quite dull, especially when not shooting in the brightest conditions, which may make manual focusing difficult anyway. As the focusing is internal the filter thread does not rotate, making this lens perfect for use with polarising filters.

Pentax SMC DA 18-250mm f/3.5-6.3: Performance
Constructing a lens with such a huge zoom range involves all sorts of compromises. Unfortunately as far as resolution is concerned these compromises lead to reduced performance. At 18mm and the maximum aperture the sharpness can only really be described as disappointing, being fair in the centre and slightly less than fair towards the edges. Stopping down does improve matters, but the aperture needs to be closed down a fair way before the resolution starts to exceed good levels here. Peak quality is achieved at f/16 where sharpness is very good in the centre and good towards the edges.

Matters improve somewhat when the lens is zoomed to 70mm. Here the resolution is approaching good levels at maximum aperture and peak quality across the frame is achieved at f/11, where the sharpness in the centre approaches excellent levels and the quality is good towards the edges.

Finally at 250mm the performance at f/6.3 returns to slightly disappointing levels. As this lens will spend most of its time at maximum aperture in auto mode, this is where the performance is most critical. Here the sharpness in the centre is fair and will probably satisfy those wishing to make 6x4inch prints of their holiday snaps just fine. Stopping the lens down improves matters and peak quality across the frame is achieved at f/11 where sharpness in the centre is very good.

Resolution at 18mm	Resolution at 70mm
Resolution at 250mm	How to read our graphs The blue column represents readings from the centre of the picture frame at the various apertures and the green is from the edges. Averaging them out gives the red weighted column. The scale on the left side is an indication of actual image resolution. The taller the column, the better the lens performance. Simple. For this review, the lens was tested on a Pentax K-7 using Imatest.

Chromatic aberrations are well controlled for a lens of this type, with colour fringing staying well below acceptable levels throughout the zoom range. Fringing across the frame reaches levels that may pose a problem in very large prints at 250mm and f/6.3.

Chromatic Aberrations at 18mm	Chromatic Aberrations at 70mm
Chromatic Aberrations at 250mm	How to read out charts Chromatic aberration 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-7 using Imatest.

Falloff of illumination towards the edges of the frame is kept to reasonable levels. At 18mm and f/3.5 the corners are 1.43 stops darker than the image centre and visually uniform illumination is achieved at f/5.6. Vignetting is reduced slightly at 250mm with the corners only being 1.16 stops darker than the image centre. Again, f/8 results in visually uniform illumination here.

Distortion is surprisingly well controlled for a lens of this type with Imatest recording a level of 3.36% barrelling at 18mm. Although this amount of distortion will be noticeable in images with straight lines, it is much less than I expected from this optic. Again at 250mm there is a surprisingly low level of distortion. Here Imatest recorded only 0.248% pincushioning, which can be quite difficult to see. The distortion pattern throughout the range is uniform, so should be easy to correct in software afterwards if needed.

Click on the thumbnails for a high resolution image.
18mm on a Pentax DSLR provides a useful wideangle equivalent to 27mm.		This image, taken from the same position as the one to the left shows the extent of the zoom range.

Flare can be an issue when shooting into the light, resulting in a distinct loss of contrast and flare patterns that may dominate your image. A petal shaped lens hood comes supplied with the lens normally, but unfortunately it was missing from the sample provided for review. Still, I'd recommend leaving it in place all the time as light sources outside of the frame can also result in loss of contrast and beautiful, yet dominant flare patterns.

DxOMark provides objective, independent, RAW-based image quality performance data for lenses and digital cameras to help you select the best equipment to meet your photographic needs. Visit the DxOMark website for tests performed on the Pentax SMC DA 18-250mm f/3.5-6.3.

Pentax SMC DA 18-250mm f/3.5-6.3: Verdict
As I mentioned earlier, lenses with huge zoom ranges like this often result in compromise of some sort. It seems unfortunate in this case that the aspect of performance to lose out is sharpness at maximum aperture, which is how I believe a lens like this will be used most of the time.

Still, the quality is good enough to satisfy those after postcard size prints for their holidays with the minimum of fuss, so I suppose you could argue that Pentax did their market research when designing this lens. Whether a lens like this will be worth £500 to you will be entirely subjective. I know I'd rather spend my money elsewhere.

Pentax SMC DA 18-250mm f/3.5-6.3: Pros
Low distortion for a superzoom
Low CA levels
Impressive zoom range

Pentax SMC DA 18-250mm f/3.5-6.3: Cons
Disappointing resolution at maximum aperture
Slow AF
Susceptible to flare

FEATURES
HANDLING
PERFORMANCE
VALUE FOR MONEY
OVERALL

Pentax SMC DA 18-250mm f/3.5-6.3: Lens specification

Price	£500.00
Contact	www.pentax.co.uk
Filter size	62mm
Format	APS-C
Construction	16 elements in 13 groups
Angle-of-view	76 - 6.5 degrees
35mm equivalent focal length (on APS-C body)	27 - 375mm
Internal focusing	Yes
Image stabilisation	No (Pentax Stabilisation system is in body)
Minimum focus	45cm
Maximum aperture	f/3.5-6.3
Minimum aperture	f//22-45
Weight	455g
Size	85.5mm x 75mm
In the box	Front cap Rear cap Lenshood

The Pentax SMC DA 18-250mm f/3.5-6.3 costs around £500.

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