>> Given how similar the 85/1.8 SMCT and 105/2.8 SMCT are in most >> respects,
>Well, the SMCP 85/1.8 and the SMCP 105/2.8 are ~very~ different in >their bokeh characteristics. I meant that their angle of view is pretty similar. I wouldn't normally put both an 85 and a 105 in my bag. Normally, I'd opt for the faster lens which is why I've carried something like an 85/1.8 for years. Like most photojournalists, I wouldn't know bokeh if it bit me in the ass, at least not explicitly. I might have an impression or feeling that the renditions of the two lenses were different without being able to wrap the concept of "bokeh" around my observations. It is also possible that the SMCP and SMC Takumar lenses do differ, even if their optical formulas are supposedly the same. >They also have a very different optical design. The 85/1.8 employs >6 elements in 6 groups, while the 105/2.8 uses a somewhat simpler 5 >in 4 configuration. This may account for the difference. I suspect the 85 was optimized for better wide open performance than its predecessor. In a large-aperture telephoto, this often hurts maximum sharpness at optimum apertures. The simpler 105 design is likely to be worse wide open but sharpen up more at optimum apertures. One of these days I'll buy an 85/1.9 S-T, which looks almost the same as the 85/1.8 SMC-T but is a telephoto design inside and likely performs more like the 105 across focal distance and aperture ranges. Straight telephotos tend to be a little weaker at close range than Gauss designs, so I can see why Pentax would have changed from a telephoto 85 design to a Gauss one for such a common portrait focal length. Nikon redesigned its legendary 105/2.5 in much the same way and for exactly the same reason. They also took an 85/1.8 that was supposedly much like the K85/1.8 and replaced it with and 85/2.0 that is much like the M85/2.0 >> a finding that one was substantially sharper than the other would >> lead me to carry that one and not the other under most conditions. >My experience is that the 85/1.8 and the 105/2.8 are both quite >sharp lenses. Mine too. This is why I was surprised that at f6.7 the group of pix I shot with the 85 appeared substantially softer than the group I shot with the 105. Upon further testing it appears to be primarily operator error that was responsible for the difference. I did find the 105 to be sharper on careful testing at that aperture, but not by much. I'm hoping to develop the film test I shot tonight so as to eliminate the possibility that the *istD had something to do with the observed relative sharpness. DJE
