Best Lens for Family portraits?

No experience with either but my first impression:

* both have 7 blades for the aperture diaphragm. Usually makes for smoother bokeh than cheaper lenses made with 5 or so.
* focal length range is a bit too wide for my taste in portraits. I'd probably be shooting mostly at 50mm and 70mm which is at one end of their focal length range. Zooms typically don't perform well at either end fo their range.
* I like the constant max aperture of the 18-50mm while the 17-70mm gets a bit slow on one end.. perhaps a bit too slow for indoor natural lighting.
* The 50mm f1.8 is a bargain at under $100. Makes for a great portrait lens... there's no reason not to get it..
the 18-50 2.8 does have a constant 2.8, which is nice, but optically (from reviews and tests i've read) the 17-70 outperforms it.

I'm going to have to disagree with the 50mm 1.8 suggestion. it's definitely some great glass for the price, but the focal length is too limiting for families on a 1.6x crop camera. If you are shooting individuals or groups of 3-4, then it is great. for large families it is just plum too long, as you can't always step back very far.

sigma makes a 30mm f1.4 that supposedly is very very good, you may want to consider that. if you don't have a wide zoom at all, you should get that. the flexibility is quite handy. i'd recommend the 17-70 out of the ones you asked about, but I would recommend the new tamron 17-50mm 2.8 over both of them. it is an excellent zoom, and the constant 2.8 is a great asset.
I'd go with a kit zoom lens (18-55mm) and a 50mm f1.8 and also a nice off-camera flash for approximately the same price as one of those zooms. This will give a great blend of flexibility and performance.

As thebeginning quite rightly says, with a wedding, you are constrained on space and a 50mm can be a bit too tight for group shots with an APS-C sized sensor camera.

It depends on the camera. This is repeating some of what others have said, but for the small sensor digital cameras, 35mm for group shots and 50mm or 85mm for single portraits. For the full-frame digitals and film cameras, 50mm for the group shots and 85mm for the portraits, though the 50mm could be used for some portraits if you don't want to get very close.

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