Speaking of rectangular, flat, retro SLRs with off-centre viewfinders, this is what the original Pen F looks like next to a Fujifilm X-T1. While it’s not Rollei 35 tiny, it is nearly as pocketable. Half frame 35mm film is 15% larger than an APS-C sensor, mathing its standard 38mm lens to the APS-C equivalent of ~33mm (virtually the same FOV as 50mm on FF).
The 38mm f/1,8 Pen Zuiko returns a DOF just barely deeper than the 35mm 1,4 XF Fujinon lens, or roughly on par with how an APS-C 33mm 1,5 lens would capture images. Finally, it's got that 1950’s Canon rangefinder lens look, which is to say: proudly SLR.
And at 137 grams, it won’t weigh you down.
While I respect many of Fujifilm’s XF lenses, I dislike the inconsistent hardware interface. So, I use adapt lenses from various systems. Despite size differences in capture area, Pen F lenses work pretty well with APS-C digital cameras such as the X-T1 and X-Pro 1. And they make Leica lenses look big.
Most are reasonably sharp and contrasty and while not Leica-tough, are built to last.