Canon 100mm f/2.8 Macro or 85mm f/1.8 for Portrait Photography
I myself posted numerous threads to forums asking for such opinion. But this one actually seemed a bit out of discussion. Of course what is given below is based solely on my own opinions.
Many of those asking above question are really concerned about the difference in maximum aperture of the lenses. I wonder why! Most of them indicate that they will use the lens on a full frame camera and are going to shoot portraits. I doubt they actually have done much of portraiture! If I shoot studio portraits most probably I do not need a super wide aperture to let the sensor lit well (I have studio strobes!) so I just deliberately adjust the aperture to control the depth of field. The point is here. With a full frame camera and around 80 to 100 mm of focal length if I set the aperture to say 1.8 the depth of field is ridiculously thin (especially for tight compositions). Imagine you cannot even keep a whole one eye or so of the subject in focus! Let me tell you, for most portrait cases which requires full face focus or couple shots the useful aperture on a full frame camera (in the above range of focal lengths) is most probably above 2.8. So please do not be worry about the DOF difference of the two lenses that much cause it is NOT a practical concern.
Another concern is Bokeh. Of course it is only practical when the portrait is going to be taken outdoors or in front of a patterned backdrop/background (not the solid color seamless backdrops). To be honest unless you compare the images taken with both lenses side by side there would be no real concern regarding Bokeh as both lenses especially on a full frame body are cable to render complete out of focus and smooth blurred background. But in total if you are a real Bokeh lover I may say the rendered Bokeh of 100mm lens could be defined nicer considering the effect of its curved aperture blades, it is relatively longer focal length (same subject area to total photo area ratio) and also its dedicated macro construction. But again it is really a personal taste.
Other factors such as price, construction, focusing speed and focal length difference are the same or completely impractical if the ultimate intention for them is portraiture.
So please consider that the most important part of producing a good photo is the photographer him/her self. My advice is to take whichever of the lenses (of course 100mm macro if you are going to shoot some bugs portraits) and start shooting portraits. The more you shoot (wisely) the better you get. The gears are there to make an already good photo better so try to become the good one.
Again I emphasis that both lenses are decent (and from a real giant in the imaging industry) and both are capable of producing award winning photos in the hands of the well trained photographers.
The views and opinions expressed in this article are those of the author and do not necessarily reflect the views and opinions held by dpreview.com or any affiliated companies.