Lens HD Pentax DA 70mm F2.4 AL Limited

 

This compact telephoto lens provides an angle of view equivalent to the classic 105mm focal length on a 35mm format when mounted on a Pentax Digital SLR. It costs around £565, sports a fast maximum aperture of f/2.4 and is part of the Pentax Limited range, which means it's one of their premium lenses.
 

HD PENTAX-DA 70mm f/2.4 Limited Handling and Features

As you might expect from lenses in the Pentax Limited range, build quality is excellent. Even so although this lens isn't heavy, weighing only 147g with the extendable hood attached, due to the use of aluminium in its construction. The lens is very compact for a fast aperture telephoto lens, being only 26mm long. The lens balances well with the Pentax K-5 IIs body used for testing, and it will feel just as good when used with compact Pentax K-mount camera bodies.

Focusing isn't performed internally, with the lens barrel extending by around one centimetre at the closest marked focusing distance. Thankfully, the filter ring does not rotate, which makes it perfect for use with polarising and graduated filters. The 49mm filter thread size is standard for Pentax, which should make it easy to find accessories and filters to fit.
 

The manual focusing ring rotates during auto-focus, so some care may need to be taken to avoid it catching your fingers as it moves back and forth. Manual focusing action is smooth and well damped, which makes it a pleasure to apply manual adjustments. A useful hyperfocal scale is marked on the lens barrel, which has marking between f/4 and f/22. The Pentax 'Quick Shift' focusing system has been implemented on this lens, which means manual adjustments can be applied once autofocus is complete.

Auto focus is screw-driven, although it still manages to lock onto subjects very quickly. The older screw-driven system does generate more noise than newer silent focusing lenses do. The minimum focus distance of 70cm is typical for a lens of this type and focal length.
 

HD PENTAX-DA 70mm f/2.4 Limited Performance

This lens puts in an excellent performance as far as sharpness is concerned. At maximum aperture, sharpness in the centre of the frame is already very good, although the clarity recorded towards the edges of the frame is only fairly good at this aperture. Stopping down improves performance across the frame, with peak sharpness across the frame being achieved between f/5.6 and f/8. Here sharpness in the centre is outstanding, whilst clarity towards the edges of the frame is excellent here. 
 

MTF @ 70mm

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, with levels of fringing only just reaching half a pixel width between f/2.4 and f/2.8 towards the edges of the frame. Stopping down to f/4 or beyond reduces fringing to very low levels, which should be very hard to spot, even if you're looking closely for it.
 

CA @ 70mm

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 well controlled for a lens with a maximum aperture of f/2.4. At f/2.4 the corners of the frame are 1.08 stops darker than the image centre and visually uniform illumination is achieved with the lens stopped down to f/5.6 or beyond.

Distortion is also very well controlled. Imatest could only detect 0.35% pincushion distortion, which isn't much at all and will be difficult to spot.

This lens is very resistant to flare and contrast levels remain good when shooting into the light thanks to the HD coating applied to optical surfaces. A circular hood is supplied, which screws into the 49mm filter thread. The hood has an extendable front section, which can simply be pulled out when more shade is required.