Originally Posted by
neuroanatomist
Yes, I've used my EF 85mm f/1.8 on my 7D - it's a great lens, and I'm very happy with the results! With any prime, you're giving up flexibility - meaning you probably want to know what you're going to use the lens for before you mount it on your camera. I love the 85mm f/1.8 for close-up portraits of my daughter; it also works well for head/torso portraits of an adult. 135mm on FF is the 'classic' head/shoulders portrait length - enough compression to be flattering, but not too much compression; the 85mm lens is the 1.6x equivalent of 135mm. I have been thinking that 135mm on a crop body might be too long for indoor work; in another thread, Jon Ruyle stated that he really likes the 135mm f/2L for tight portraits indoors.
I do think the 85mm f/1.8 is an excellent value, even more than the 50mm f/1.4 (the 85mm has ring USM, and it's sharper at f/1.8 than the 50mm is at f/1.4). Compared to f/2.8, the narrower DoF with the primes makes a noticeable difference in OOF blur, to me. There's not really much difference between the two primes in terms of DoF for the same framing, though (e.g. 85mm @ f/1.8 and 10 ft distance has the same DoF as 135mm @ f/2 and 15 feet distance, but the 135mm will be a little 'flatter' in terms of telephoto compression).
If you're considering indoor portraits as a use for the 85mm or 135mm lenses, one thing you might consider is setting your 100-400mm to 135mm, just to get an idea of framing at that focal length in places where you'd be shooting. If you like the framing, the 135mm may be the way to go - it's a spectacular lens, would also be great for twilight backyard shots of your dogs, for example. But if 135mm seems long, consider the 85mm f/1.8. Or, just use your 100mm f/2.8L Macro and see if you'd want to go a little longer or a little shorter, or even just stick with the 100mm f/2.8 if you're happy with the OOF blur from that aperture.