I would heartily recommend the EF 50/1.4 for your purposes. You seem to be aware, on your crop sensor camera it will act as a short telephoto prime (not a "standard" lens as it would on a full frame 5D camera).
The EF 50/1.8 II is also quite capable of making a very nice image, the background blur or bokeh isn't quite as nice, but you really have to look closely to tell the difference. It also doesn't seem to have as good light baffling or coatings, so has a bit lower contrast and less color saturation than the more expensive EF 50/1.4. But, again, you almost have to compare images from each of them side by side to see the differences. Optically, the 50/1.8 II is a real bargain. It's very low cost (Canon's cheapest lens), but it also is lightly built and doesn't have nearly as good auto focus performance as the USM of the EF 50/1.4. The 50/1.8's AF is be noisier, slower and less accurate. It particularly tends to slow down and hunt in low light situations.
Now, a 50/1.4 is a "short telephoto" on your camera. It's especially ideal for portraiture. A Canon EF 28/1.8 makes for a very nice "standard" lens on your crop camera. And an EF 85/1.8 makes for a top notch moderate telephoto/portrait lens that requires more working space, good for more candid shots, but it might be a bit long indoors, in tighter rooms. There is also the EF 100/2. All these are in your price range.
Personally I use the 28/1.8, 50/1.4 and 85/1.8. They are all within your budget. And it's nice that they all share 58mm filters.
This was shot with the EF 50/1.4 at f2.0 (I'm 3 or 4 feet from subject and the background is about 18" or 2 feet behind him)....
This was shot with the 85/1.8 at f2.0 (I was about 15 feet from subject, who was lit only with the modelling lamps in a large light box on the left, there's no background to consider, but notice how his ears, hands and the wine bottle are a bit out of focus. That's pretty shallow DOF, even from a distance.)...
I also am currently using the Tamron SP 60mm f2.0 Macro/Portrait lens. It takes the place of three lenses in my camera bag, when I don't want to carry a lot (I leave the 50/1.4, 85/1.8 and my 100/2.8 macro at home). This lens is nice, but slower focusing than the Canon USM lenses. It's fine for most portraits and macro shots, just not usable for sports. The f2.0 aperture is large enough to render strong background blur in many cases. Oh, and this is a "crop only" lens, too (and uses 55mm filters).
There are some superb zooms you might consider... the Canon EF-S 17-55/2.8 IS is a good example. A top notch zoom for your crop sensor camera. For portraiture with crop cameras, one of my favorite zooms is an EF 24-70/2.8L. However, to get f2.8 you will need to spend more (the 17-55 is more than double what you want to spend), you will get a smaller range of focal lengths, the lens itself will be larger and heavier, and it will "only" be f2.8. It would be more of a replacement your 18-135mm.
I have not used the Tamron or Sigma lenses you mention. I have and use or have had and have used some other Tamrons and most have been very fine lenses. The Tamron 17-50/2.8 "non-VC" is often touted as an excellent lens optically, at a budget price (the stabilized version is said to not be as sharp). The main complaint I hear about it is slower and more erratic auto focus. It's not got the "better" USD (similar to Canon's USM) that Tamron has started putting in their premium lenses.
I would be concerned about the focal length range of the Sigma 18-35/1.8 for portraiture. As some others have noted, with wider lenses you do get some perspective distortion (big noses, tiny ears) that can either be humorous or a problem for portraiture. Other types of photography, it might not matter much... But with portraits in particular you have to be very careful using a wide lens very close to your subject.
This is an example of perspective distortion (and this was a 24-70mm lens at 66mm, but on a full frame camera... so would be equivalent to using a 41mm lens on a 70D)...
Another form of wide angle distortion (I don't have a good example image to share)... It's anamorphic distortion, where a person's arm or leg near the edge of the image appears stretched and larger than the one that's farther away. It's the "Hellboy" or "elephant leg" look. Not very nice! It can be avoided by being careful to keep the subject well away from the edge of the frame, if using a wide lens for portraits.
So I would recommend you go with a nice prime lens instead of a zoom. A reasonably fast prime gives you up to two full stops larger aperture than any zoom, with good overall performance that complements your "walk-around" zoom (instead of replacing it), while remaining fairly affordable.
And if portraits of your newborn are your main concern, for use on your 70D I'd recommend something in the 50mm to 100mm focal length range. The EF 40mm f2.8 is a neat and ultra compact lens... nice price, too. But it will have the potential for perspective distortion like the above if used too close, and it's only f2.8. Heck, you have to be a little careful getting close with even a 50mm lens (on crop camera). I was sort of pushing the limit with the portrait at the top.
If you get the 50/1.4, definitely get the matching lens hood for it.... and use it!seems to protect it nicely. Many users think the 50/1.4's AF gets damaged when the lens gets a hard bump on the front barrel (that protrudes slightly when you focus closer)... or if the focus ring gets knocked about when the lens is stored. The hood helps in both situations. While some folks have had problems, my 50/1.4 was bought used more than 10 years ago and is still going strong. It doesn't get much special care, but is always used and stored with the hood (as are all my lenses).
A filter is up to you. I see you have a Hoya Pro 1 and that or their HD/HD2 are high qualiyt. Personally I don't see the point and don't leave a filter on any of my lenses all the time. I've been shooting for going on 40 years, have used hundreds of lenses and never damaged one in a way that might have been "saved" by a thin piece of glass up front. I do have "protection" filters, but I only install them when actually needed (out shooting in a sandstorm, for example). There are just way too many times I'm shooting in challenging lighting situations, where even the best filter can cause some issues with my images. So I leave them in my camera bag, until actually needed.
If you get the 50/1.4, you will find it's not at its sharpest wide open at f1.4. My copy of the lens is certainly usable at that aperture, but most of the time I set it to f2 minimum by default, which really sharpens it up nicely and still renders all the background blur I can possibly use in most situations. If needed, I'll open it up. This is fairly common... most lenses benefit from stopping down a little. Some feel the older Sigma 50/1.4 is sharper wide open. I didn't see enough difference to worry about, comparing the two side by side. The Sigma was more expensive at that time, and is a lot larger and heavier. So I stuck with my Canon lens. I have not compared with the new Sigma 50/1.4 "Art" now being offered... but that's a much more expensive lens.
Hope this helps with your decision.
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Alan Myers
San Jose, Calif., USA
"Walk softly and carry a big lens."
GEAR: 5DII, 7D(x2), 50D(x3), some other cameras, various lenses & accessories
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