text: 35mm Summaron 2.8 vs C-biogon 2.8 - Rangefinderforum.com
  I'd go with the zeiss unless you want the Leica brand or "character" filled look. Don't see why I'd pay more for a 50 year old lens when I can get a brand new one with better technical performance for less.
  Two very different lenses, IMHO. The Summaron will give you a much lower contrast, vintage look; the Biogon has a much more modern rendition (and will deliver crisper colors). It all depends on what lens signature you'd like more, but if I were shooting on the M8, I may just choose the latter.
  I have to agree with Horosu, the Biogon is perfect. I don't know how to describe it, but it just seems every picture taken with it, whether BW or color, just comes out looking great, no flaws. I think if you're looking for a vintage look, you can spend a lot less than the cost of the Summaron but if you're looking for just great images, the Biogon wins.
  Summaron V. Zeiss
  I bought my Summaron in 1966 new, I've babied it and it still functions as new. It was removed from the body it was on less than half a dozen times and always sported a UV filter(s). I bought my Zeiss a couple of years ago. If you are doing available light out doors, and street photography the Summaron is superior to the Zeiss. It is also wonderful for portrait work. It takes years off the ladies. It is far more ergonomic than the Zeiss. It tends to flare more, but the Zeiss can make for halation flare too. The Summaron is noticeably wider than the Zeiss, and yet has little distortion.
  The 35 C-Biogon is my favorite lens, period. I like it even a bit more than my old 35 Summilux ASPH. Its only weakness is that it vignettes a bit when wide open. On film I think this looks fantastic but on digital it could be a nuisance. Other than the vignetting, it has essentially no technical flaws, and its rendering is gorgeous. At a given aperture it seems to have a bit more OOF than one might expect for a 2.8, perhaps because of its great contrast at 2.8 and the way it renders the transition from critical focus to OOF. I have not seen any 35 with bokeh that I like better. Untitled by Semilog, on Flickr
  Summaron for digital, biogon for film.
  If one can't take a decent photo with the Summaron switching to the Biogon probably isn't going to help I fear ...
  Please note that the Biogon has a different filterthread than the Leitz lenses, 39mm or 46mm doesn't fit, also the less common 40,5mm used by older Zeiss and FSU lenses doesn't fit. I believe the Biogon has 43 x 0.75mm filter thread and the Summaron has plain 39mm. Not important perhaps but for some a consideration..
  You may want to consider also the new Summarit 35/2.5 - I have seen some results from this lens on Flickr - and Steve Huff has some interesting articles about it too (see here, here and here)
  it's a minor difference but if you have a perfect sample of each the f2 biogon is a bit sharper; both in that it hits a higher maximum cycles (nearly 90cycles MTF40 at f4) and that it's MTFs stay above the c-biogon's for most of the field.
  whats Not to Love about either...Tough Choice
  Summaron Again... classic Old Leica Glass at its BEST
  C Biogon Modern Magic...
  C Biogon best bang for the Buck...
  Its only weakness is that it vignettes a bit when wide open.
  The OP is looking to use the lens on an M8 (but he will soon graduate up to film I hope ) on which the crop will remove that vignette. It does, of course, change the look of many lenses that have softer corners you may or may not see that as an advantage.
  If you are doing available light out doors, and street photography the Summaron is superior to the Zeiss.
  it's a minor difference but if you have a perfect sample of each the f2 biogon is a bit sharper; both in that it hits a higher maximum cycles (nearly 90cycles MTF40 at f4)
  You mean 90% (=0.9) modulation transfer at 40 cyc, not 90 cycles. And even that is not correct. Both the 2.0 [Zeiss MTF at Rockwell's site] and the 2.8 [Zeiss MTF at Rockwell's site] peak at about 80% MTF at 10 cyc, not 40 cyc. 40 cyc is the bottom pair of lines in each set, not the top.