Pentax Lens Designs for Digital

desertscape · 10-13-2009, 11:20 AM

As an optics nut, I'm always intersted in seeing what Pentax is up to regarding their new designs, so I went to their German website to see the cross sections of their digital lenses. I was not expecting these to look all that different from their film designed lenses. Boy, was I wrong. The most noticeable difference was the large number of elements used in all types of lenses ( zooms, primes, macro etc). If done correctly, an increased number of elements translates to better aberration correction. The frequent use of aspheric lens surfaces and use of low dispersion glass (ED) seems to be common now in their digital designs. There are big optical chages at Pentax since the 1990's.

jogiba · 10-13-2009, 04:06 PM

The FA* 80-200/2.8 ED [IF] came out in 1994 with 16 elements in 13 groups and the FA* 250-600/5.6 ED [IF] came out in 1991 with 18 elements in 16 groups.

By comparison the SMC PENTAX-DA* 1:4 60-250mm ED (IF) SDM has 15 elements in 13 groups.

desertscape · 10-13-2009, 06:07 PM

Okay Jogiba I get your point. I'm too used to looking at their 6x7 lens cross sections, where they are much more basic in design. Some of the digital designs I saw do not fit into any category I'm familiar with. However the 14mm f/2.8 EDIF DA had some similarity to the current 55mm f/4 used on the 6x7 and to the 90-180 zoom (67).

flyer · 10-13-2009, 06:38 PM

One big difference in wide angle design is the fact that imaging sensor don't like to have light coming at it from a sharp angle, so to avoid darkened and muted colors on the edge, the light must hit the sensor at close to 90 degrees. On the other hand, films don't care much if the light is coming at 45 deg. or 75 deg. So, for a digital imaging sensor, the wide angle lens needs to have the light coming at it "straightened", and this adds to the complexity of designing a wide angle lens.