I have the 135 4.0 and the 165 2.8. For me, the difference between 4.0 and 2.8 is significant, especially when the light is low or I want less depth of field. I use the 165 2.8 with the shortest extension tube from the 67 auto extension tube set for tighter head shots, 3/4 portraits work fine without the extension tube. I don't lose a stop of light with the short extension tube, it's more like 1/3 stop. I make almost exclusively black and white portraits and, when I use the in camera meter and the short extension tube, no adjustment is needed for exposure. I occasionally make adjustments for exposure when I don't use the in camera meter. You will have to check the exposure issues out carefully if you plan to use color slide film and an extension tube. I bought the 165 2.8 for portraits because the 135 portraits I tried made faces looked more distorted than I wanted when I got up close for tight head shots. Some people like the distortion or don't notice it, others sometimes really don't appreciate the way their face looks with the 135mm. I have used the 165 2.8 hand held and tripod mounted with no problems. There have been many occasions when I wanted to use this combination in low light and I was able to make the shot work with Delta 3200 and the 2.8 lens but wouldn't have been able to with a 4.0 lens like the 135mm or 200mm. This usually happens when I am trying to hand hold the camera or my portrait subject is not so good at remaining motionless. I haven't tried the 150mm lens. The 135mm lens can work for group or environmental portraits without out introducing distortion issues, or you can back off a little when you take the photo and crop the negative when you print it.