Originally posted by Ginnipe I have been busy traveling around here in Greece so I apologize that this response is so delayed. I'm really in a pickle right now with the 24-70 2.8 vs that 28-105 lens. Everything you say about it is true, its optically great, lighter, and has a great range. My issue with it however is that variable aperture. I'm not worried about sacrificing some ISO to get a shot, that's fine, what im worried about is the fact that at 105mm and f 5.6, the depth of field isn't shallow enough to isolate subjects well. When I schlep around the 24-70, I know that if I quickly have to go into portrait mode and throw that background away, I can. I just go right down to 2.8 and get within a few feet of the person I'm shooting and bam, perfect portrait.
With the 28-105, that will be much much more difficult. Id want to zoom in to at least 50mm if not 80mm to get the facial features rendered nicely, and by then I'd be at around 5.6 (please let me know if that is incorrect) which would mean that I would have to get very close to my subject to isolate it via DOF.
It really is a conundrum that I've been toiling with the whole time I've been here in Athens. I want that extra lightness and range the 28-105 brings, but I don't know if I can give up that constant 2.8. I could obviously buy a prime or two with the money I'd save by selling the lens but then I have to worry about switching lenses mid walk and that brings up a whole other concern.
Let me know what your thoughts are on this issue! and if you have any portraits taken with the 28-105 I'd love to see them and how it well it can throw out the background!
I also have the D FA 24-70mm f/2.8, pretty well for the reasons you identify. Specifically, one of the reasons for buying the K-1 was for studio portraiture, and the D FA 24-70mm is what I expect to use there. Although even there I expect to use it stopped down, perhaps to f/5.6 or f/8, simply because my experience is that this will be necessary to get the whole head and enough of the body in focus. Outdoors, if I had a formal portraiture session, I would use the D FA 24-70mm, or possibly the D FA 70-200mm f/2.8.
At the following link, I apparently used the D FA 28-105mm at f/7.1, and the background is still pretty obvious. I used it because I had it with me! I was at an airshow, where I use the D FA 150-450mm f/4.5-f/5.6 for aircraft in flight. That just wasn't the right lens in this case. I didn't have the other lenses with me, because of the weight. They were in the car. (Had I have been doing landscapes or interiors using the D FA 15-30mm f/2.8, I would still have had the 28-105mm with me, but not the others).
Photos showing the quality of the 28-105mm lens
If you can get close, (which I couldn't in that case), it would be much easier to throw the background out of focus. You can see from those photos that even though I was using the lens at 105mm, the subject was not large. (Perhaps I could have used the 150-450mm after all!) I think the D FA 70-200mm would have been my best lens for the purpose, but it was in the car!