Originally posted by crossing Now I got... then 17-50 f2.8 its not for me. I dont shot only in low light , I need it also for landscapes, outside pictures.. f2.8 I think its more for low light..
Well, f/2.8 is useful for at least two things: low light, and shallower depth of field (nice sometimes in portraits and many other types of photos).
But let's be clear about something: A zoom lens with a fixed aperture throughout its range isn't ipso facto superior to one with a variable aperture, especially if you consider other qualities of the lens like zoom range and price. There are some fixed aperture zoom lenses out there that are only so-so as far as image quality goes, while there are quite a few variable aperture zoom lenses (including the Sigma 17-70 f/2.8-45 DC) that have rather good image quality. I wish my Sigma 17-70 f/2.8-4.5 were a fixed f/2.8, like the Tamron 28-75 or 17-50 lenses. But notice that the Sigma has a wider zoom range than either of the Tamron lenses, and it costs less, too. I wish my Pentax 18-250 were a fixed f/1.8 throughout its zoom range, but nobody to my knowledge makes such a lens and if they did it would cost $20K.
Not everybody needs the advantages of a fixed-aperture lenses at all, and even those of us who need those advantages sometimes, don't need 'em all the time. Indeed, I dare say that MOST photographers don't care about having fixed aperture lenses, most of the time. If they did, then kit lenses would have fixed apertures - but they don't.
What's the advantage of a fixed aperture - like you get on the Tamron 17-50 f/2.8, or the Tamron 28-75 f/2.8, or the Pentax 16-45 f/4, and many others? The main advantage is that, if you're shooting in low light, at the limits of the camera's capabilities, if you can manage an acceptable exposure at one focal length, you can zoom in or out to a different focal length without having to change your camera's settings (or without having them changed for you automatically). For example, if you're shooting in a dark church, and, at a focal length of 20mm, f/2.8 + 1/30th sec + ISO 1600 gives you a just barely acceptable exposure - if the lens has a fixed aperture, you can zoom in tighter on the subject safely, without having to worry that you're now going to be taking a grossly underexposed shot.
But most users do NOT shoot a lot in such difficult circumstances, and for a good reason. It's hard to get a good photo in bad light! Most users, most of the time, either shoot in fairly decent light - quite frequently outdoors during the daytime - or are able to use flash or some other form of supplementary light.
Now, the advantage I described above is pretty much the only advantage of a fixed aperture zoom lens. You can zoom without changing your exposure settings. That's it. Note that, when you zoom, even keeping the aperture constant, your depth of field is going to change, because, well, because depth of field is directly and intimately related to distance from the focal point of the photo.
Anyway, the bottom line is, if you aren't facing fairly extreme exposure + zooming challenges on a regular basis, then you may not give a darn whether your lens has a fixed aperture or not. The best thing we can all do to improve our photos is get better light on the subjects. And if you can shoot in better light, then having the aperture narrow to f/4.5 when you zoom in tighter may not matter much to you. I go over to the Dallas Arboretum to take some photos quite often. Sometimes I have a bride or a portrait subject with me. In that case, I probably don't grab a zoom lens at all: I grab a couple primes. But if I'm going just to enjoy the scenery and take some shots for fun, I probably grab a zoom, and the Sigma 17-70 f/2.8-4.5 is one that I'm likely to grab, because it's a good lens, and versatile. Most of the time when I'm at the Arboretum, it's daytime and the light is very good.
I'm not KNOCKING fixed-aperture zoom lenses. Just trying to counter the possible misunderstanding that they are somehow inherently better for all photographers. 'Tain't true.
Will