Can I buy one of the EF 100mm Macro lens instead of EF 85mm f/1.8 USM?

danferrin wrote:

Unless I've forgotten my math, the depth of field of a 100mm f/2.8 lens is essentially the same as a 50mm f/1.4 lens.

No, not correct I'm afraid.

Assuming you adjust your shooting distance to maintain the same subject framing (otherwise it wouldn't be a comparable shot) then the depth of field depends only on the f-number - so the 50/1.4 would give you much shallower depth of field.

You are probably thinking of the amount of distant background blur, which depends on the diameter of the entrance pupil - and that is indeed the same for those two lenses.

However it's a mistake to think that shallow depth of field is desirable for the majority of portraits. It's fine when you want to use it as a special effect, but what is actually most often wanted is subject separation from the background, not eye separation from the face!! We want both eyes in focus, not just one, and in fact we probably want the hair in focus too. Yet we want the background blurry so the subject stands out. The lens choice which will help with this is the *longer* lens, because it allows you to use a smaller aperture for enough DoF, while keeping the desirable background blur.

Of course there are other factors in lens choice, and one for example would be that portraits taken with longer lenses tend to be more 'detached'. I'm responding purely to the point about the numbers - not saying which is the 'better' portrait lens.