To get your question answered, send it via email to ross@thephotovideoguy.ca  You'll be glad you did! I've actually known CJ since he was an infant, because I know his folks.  He's been into photography since his teens and as a young adult is starting to make his way as a professional photographer.  He's been published and is a very talented fellow.  He recently asked me this question, after adding a beauty dish to his lighting kit.  "What's the best lens for portraits?  I'll be doing head shots and half body shots.  I've narrowed it down to 2 lenses, the Canon 85/1.2 L and the Canon 100/2.8 L Macro.  What do you think?"

Well CJ, it's a great question.  My preferred portrait lens is actually the Canon 70-200/2.8. It's awesome but I hold +Scott Kelby accountable since it was his articulate treatise on the subject that led me this way.  That said, I own and like both the lenses you asked about and I will hold my answer to one of your selections, (which is what I actually did do).

CJ is using a Canon 7D.  We tend to toss this off as "oh a crop sensor camera" as if that meant some kind of disease.  The ONLY think that really matters with having a crop sensor is the effect it has on relative focal length.  A crop sensor sees a smaller image circle, so if a lens that produces a full frame image circle is used (as in these two lenses), the sensor only sees part of the total image giving you the effect of shooting a longer focal length.

This is true for crop sensor built lenses too with the difference that they WON'T work on full frame cameras.  If you think you'll ever move from crop to full, or have both, only buy glass that will produce the image circle required by a full frame sensor.

The Canon sensor has a crop factor of approximately 1.6x.  So simply this means to get what the effective focal length is, multiply the physical focal length by the crop factor.  In the example of the 100mm lens, this means it gets the look of a 160mm lens.

This can be awesome and horrible.  For sports and long distance it's wonderful.  For super wide it's a nightmare.  But CJ asks about portraits.

Back in the olden days, there was an ongoing bun fight over what lens was better for portraits, the 85mm which allowed you to get really tight, had a super large maximum aperture and had lovely focus falloff, or the 105mm which allowed you a bit more standoff distance and gave slightly more perspective compression.  I know CJ asked about the 100mm, but when I was coming up, the portrait lens I yearned for was the Nikkor 105/2.5 AI.  It was SO good.  Well that bun fight still goes on.

The reality is that either of the 85mm or 100mm will do lovely portraits if you do your part.  The 85mm that CJ asks about is the f/1.2 variant.  Think sees in the dark.  Also think very razor thin depth of field wide open.  On a headshot with focus on the eye, the tip of the nose is definitely soft.  It's an incredible look if you use it properly.  The lens also has wonderful bokeh (no rants on mispronunciation or vendor BS dumps about Bokeh - I Promise).  Out of focus areas are really rendered beautifully.  The downside is that the AF performance is slow.  Like you can watch the lens turn slow.  And this is the II iteration which is faster than the first series.  It's also surprisingly heavy.  On a 7D, it acts like a 136mm/1.2 lens which is really wonderful for faces and still works for half-lengths if you stop down a bit, say f/5.6.

The 100mm f/2.8 is a different animal entirely.  This is by design, first a macro lens.  It delivers up to 1x life size on the sensor without additional kit.  It's tack sharp and focuses very quickly, given the sophistication that goes into macro lens design.  It's fast enough optically but doesn't produce that razor thin depth of field as we find in the 85mm.  It does produce beautiful bokeh, because that is a design criteria for top end macro lenses.  This is not well known but may help explain why so many photographers love the bokeh in macro lenses.

There is a lot of noise about the number and style of the blades.  Odd numbers of blades produce star effects with twice the number of points as blades, even numbers of blades produce star effects with the same number of points as blades.  This has NOTHING to do with the choice of lens for portraits.  More blades tend to produce rounder apertures as do curved blades and many people think that this produces more pleasing out of focus highlights.  I'm one of those people.

Having shot both lenses a lot, I favour the 100mm most often.  I love the look of the 85mm but since I shoot most often with a full frame now, in my opinion, the 85mm focal length pushes me too close to the subject for a headshot, particularly if the subject is not a model who may be more comfortable with big glass in her (or his) face.  The 100mm give just a bit more standoff and I have not found that to be a problem in the areas where I shoot these types of portraits.  I also love getting really close (eyes are awesome) and the macro is wonderful for that sort of thing.  If I'm doing low natural light work, that's really where that f/1.2 comes into play on the 85mm.  Here are a couple of shots of my wonderful model Sondra shot today and attempting to get a similar perspective with the different lenses.  For those all gear interested, lighting is a Bowens 500 Pro tripped by radio via Pocket Wizard at lowest power shooting through the Bowens Beauty Dish with the added diffusion sock.  Camera is a Canon 1Dx in manual mode at 1/100 and f/9.0 ISO 50, no exposure compensation.

Yes, I should have brushed her hair.  Bad me.

Again, although it wasn't in the criteria that CJ asked about, I encourage you to take some time to think about a 70-200/2.8  Both the Canon and Nikon variants are really exceptional and they are extremely versatile lenses.  Both are extremely sharp with excellent distortion control so great for head shots, plus the zoom gives you very quick compositional changes.  The downside of this route is always going to be the physical size and intimidation factor.  Please also note that I would never go with a lens optically slower than f/2.8 in such a zoom if portraiture was part of my expected outcome list.