Matthew Newton recently discussed the 135mm f3.5 on this forum. I own a 135mm f2.8 and, fired up by his enthusiasm, I decided to buy a 135mm f3.5 to compare them. The 135mm f2.8 appears to be one of a family that includes the 100mm and 180mm. They are all f2.8 and appear to be versions of the same design (5 elements in 5 groups). The 135mm f3.5 has a similar design to the 200mm f4 (5 elements in 4 groups) but the OD is scaled down by more than 135/200, possibly to make the lens filter size 49mm, hence the slower speed of f3.5. Well, it's nothing too scientific, I just photographed a still-life and compared a small part of the scene. I chose to crop to a hurricane lamp at the edge. The crop corresponds to an area of the negative of about 1.5mm x 1.5mm. The aperture was f5.6 and the shutter speed was 1/250th. I don't think there's too much to choose between the sharpness of the images and it's similar to my 50mm f1.8 lens (i.e. sharp!). What is really noticeable is that the 135mm f2.8 image displays more chromatic aberration (CA), in particular blue colour fringing. At the level at which the sharpness of the images could be compared, it's the colour fringing that distinguishes them. Some years ago I carried out a similar comparison of the 200mm f4 and the 180mm f2.8. I found the same difference: the 180mm f2.8 exhibited quite noticeable blue colour fringing but the 200mm f4 was quite well corrected. It seems that the acromatic doublet in the front lens group of the 135mm f3.5 and the 200mm f4 does a good job of correcting CA. The 100/135/180mm lens design seems to be more aimed at maximising aperture. I have no experience with the 100mm f2.8 so it would be interesting to hear from anybody who has.