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Lens review

Pentax smc DA 17-70 mm f/4.0 AL [IF] SDM

10 June 2009
Arkadiusz Olech

9. Ghosting, flares and transmission

On the Pentax 17-70 mm barrel there is an abbreviation “SMC” which means that a multi- layer coating process was used on this lens. The SMC coating was first applied back in 1971 and from that time it became a model to follow for other manufacturers.

We had an opportunity to check the multi-layer coating really thoroughly because this lens’s transmission was tested by using a spectrophotometer, an instrument that allows you to measure the wavelength from 380 6to 900 nanometers with the highest precision. The appropriate chart is presented below.


Pentax smc DA 17-70 mm f/4.0 AL [IF] SDM - Ghosting, flares and transmission



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We like the shape of this chart very much. In the 480 to 640 nanometers range we get a flat function graph with the values on the 92-94% level which means small illumination loss and good colour reproduction. It is an excellent result because 12 lenses groups mean as much as 24 glass-to-air surfaces, each of them causing a transmission decrease. If the transmission is on 93% level for the whole instrument it means that at one surface the lens loses only 0.3% of illumination – a very good result indeed.

If you want to pick holes in the lens’s performance here you can have small reservations about how it fares in blue and violet spectrum. For 400 – 420 nanometers the transmission decreases to the level of 60-70%, which might cause some concern.

Let’s progress to the against the light work. The coatings are responsible here but not only them. The mechanical construction of the lens has something to say too and the cooperation of its layers with the whole camera sensor system. Here we’ve got an unpleasant surprise because the Pentax’s performance is weak, what can be seen on the shots below. In the category of vignetting the Pentax was the best in its class, in the case of flares we have one of the weakest results.

Pentax smc DA 17-70 mm f/4.0 AL [IF] SDM - Ghosting, flares and transmission

Pentax smc DA 17-70 mm f/4.0 AL [IF] SDM - Ghosting, flares and transmission

Pentax smc DA 17-70 mm f/4.0 AL [IF] SDM - Ghosting, flares and transmission

Pentax smc DA 17-70 mm f/4.0 AL [IF] SDM - Ghosting, flares and transmission