HD Pentax-DA 20-40mm f/2.8-4 Limited DC WR Lens Review

Handling and Features
Performance
Verdict
Specification



This 2x zoom lens provides an angle of view equivalent to a 30.5-61.5mm lens on a 35mm format camera, when mounted on a Pentax Digital SLR, which is neither very wide, nor very telephoto. It sports a relatively fast variable maximum aperture of f/2.8-4 and costs around £850, which isn't cheap for a lens covering this range. The price may be explained in part by this being the first weather-resistant zoom lens in their 'Limited' range. In this review we'll take a look at how it performs.

HD Pentax-DA 20-40mm F2.8-4 ED Limited DC WR Handling and Features

The build quality of this lens is very solid indeed with a metal lens mount and a mixture of high quality plastics and metal used for the construction of the lens barrel. It appears to have a touch of retro styling going on, with a zoom ring reminiscent of how the focus rings were on the old screw-mount Takumar lenses from the era of the Spotmatic SLR. The lens is lightweight, weighing only 291g with the lens hood attached and it balances well with the Pentax K-5 IIs body used for testing.

Focusing isn't performed internally, so the lens barrel extends during focusing and during zooming. Even so, the filter ring does not rotate, which makes it perfect for use with polarising and graduated filters. A shallow metal hood is supplied with the lens that screws into the 55mm filter thread.


The manual focusing ring rotates during auto-focus, so care may need to be taken to it catching your fingers as it moves back and forth. Manual focusing action is smooth although there is very little damping on the focus ring, which can make applying fine adjustments tricky. The Pentax 'Quick Shift' focusing system has been implemented on this lens, which means manual adjustments can be applied once the lens has focused automatically. Auto focus is powered by a motor in the lens, but it isn't as quiet as you might expect from an ultrasonic lens. The minimum focus distance of 28cm is typical for a lens of this type and it is close enough to pose no issues when shooting in claustrophobic environments.

HD Pentax-DA 20-40mm F2.8-4 ED Limited DC WR Performance

At 20mm and maximum aperture, sharpness is outstanding in the centre of the frame, but the lens doesn't perform as well towards the edges of the frame, only producing fairly good sharpness. Stopping down to between f/5.6 and f/8 produces excellent sharpness across the frame at this focal length.

Zooming to 30mm results in a slight increase in sharpness across the frame at maximum aperture. Here clarity is outstanding in the centre, but falls just short of good sharpness towards the edges of the frame. Stopping down to between f/5.6 and f/11 results in excellent sharpness across the frame at this focal length.

Finally, at 40mm, sharpness is very good in the centre of the frame at f/4, but still only fairly good towards the edges of the frame. Again, stopping down improves performance greatly, with peak sharpness across the frame being realised at f/5.6.

MTF@20mm

MTF@30mm

MTF@40mm

How to read our charts 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-5 IIs using Imatest.

Chromatic aberrations are extremely well controlled, barely exceeding a quarter of a pixel width towards the edges of the frame throughout the zoom range. This level of fringing will be difficult to spot, even in harsh crops from the edges of the frame along high contrast edges.

CA@20mm

CA@30mm

CA@40mm

How to read our 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-5 IIs using Imatest.

Falloff of illumination is fairly typical for a standard zoom. At 20mm the corners of the frame are 1.25 stops darker than the image centre and at 40mm the corners are 1.21 stops darker. Visually uniform illumination is achieved with the lens stopped down by two stops from maximum aperture or beyond throughout the zoom range.

Distortion is also fairly typical for a lens of this type. Imatest detected 2.92% barrel distortion at 20mm and 0.627% barrel distortion at 40mm. Fortunately the distortion pattern is uniform across the frame, which should make it relatively easy to apply corrections in image editing software afterwards.

During testing, this lens proved itself very resistant to flare and contrast levels are good, even when shooting into the light. The shallow metal hood doesn't provide much shade for the front element, but is useful for preventing bashing the front of the lens, at least.

Pentax HD Pentax-DA 20-40mm f/2.8-4 ED Limited DC WR Sample Photos

Value For Money

Being priced at around £850, this lens is quite expensive for a standard zoom with a variable aperture, especially when you consider that for around the same price, you can pick up a Pentax 16-50mm f/2.8 DA* ED zoom lens. Not only does the 16-50mm lens have a more useful zoom range, but it has a constant f/2.8 maximum aperture and internal focusing.

Another alternative, covering a similar zoom range is Sigma's 18-35mm f/1.8 DC HSM, which sports a fast f/1.8 aperture throughout the zoom range and costs around £650. With alternatives like this available, it makes it increasingly difficult to justify the expense of this Pentax 20-40mm lens.

HD Pentax-DA 20-40mm F2.8-4 ED Limited DC WR Verdict

As far as variable aperture standard zoom lenses go, this is one of the better ones as far as optical performance is concerned. Unfortunately, due to the price, it is easy to expect more from a lens like this. The zoom range is reminiscent of making do with what was available when APS-C DSLRs first came out and the maximum aperture range is nothing special.

In summary, this lens may represent poor value for money for many. Others who like the excellent build quality and retro design may appreciate the small niche this lens fills.

HD Pentax-DA 20-40mm F2.8-4 ED Limited DC WR Pros

Very good sharpness in the centre at maximum aperture
Excellent sharpness when stopped down
Very good build
Weather resistant construction
Low CA
Retro styling

HD Pentax-DA 20-40mm F2.8-4 ED Limited DC WR Cons

Expensive for a variable aperture standard zoom
Odd zoom range is not really very wide, nor very telephoto
Neither focus or zoom are performed internally

FEATURES
HANDLING
PERFORMANCE
VALUE FOR MONEY
VERDICT

Pentax HD Pentax-DA 20-40mm f/2.8-4 ED Limited DC WR Specifications

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