[quote name='mahinthan5' timestamp='1288767926' post='3930']
1) EF-S 18-55 IS + Sigma 70-300.....
2) EF-S 18-55 IS + EF-S 55-250 IS
3) EF-s 18-135 IS + Sigma 70-300...
1st option is cheap but I'm loosing some focal legnt between
2nd Option Good but I'm limited to 250 (I like to catch Birds)
3rd Option Expensive may be i can go
IQ is my consideration..
[/quote]
IQ is a good consideration. But keeep the following in mind:
a) the 18-135 is actually very good at f/8 througout its focal range. So landscapes and anything not moving to fast in decent light can be shot at very high quality and printed up to the largest poster sizes because of the IS. At wide open aperture at least the center image area is very decent, so portraits and all other subjects which dont extent to the edges/borders are also possible without iq sacrifice.
<img src='http://forum.photozone.de/public/style_emoticons/<#EMO_DIR#>/cool.gif' class='bbc_emoticon' alt='
' /> how big are you going to print? If you dont print larger than 11x14" (A4/20x30cm) the sharpness of all the lenses in the mix is perfectly adaquate, even wide open.(they all distort heavily,at wide angle). For the sake of putting IQ into perspective: I had a double page spread in Time Magzine taken with an old 28-105mm lens, the quality of which was alot worse than any of the lenses you mentioned here. Dont let yourself get fooled by 100% crops. They are only relevant for comparing lenses, checking Af precision and as a guideline for huge poster prints. If you print smaller than 20x30 cm (or even 30x45cm) they are irrelevant.
c) How often do you print posters from wide open shots with the border/edges containing important elemnts in focus? If you rarly or never do this, by all means go for the more flexible lens (which could either be the one with a greater focal range or the one with a wider aperture). Getting the shot is more important than having the sharpest shot with a bad composition!!!!!Changing lenses is one of the most frequent causes for missed shots!
d) My recommendation: If you dont intend to print bigger than 20x30cm from wide open shots on a regular basis get the 18-135 plus a 28mm f/1.8 or 35mm f/2 or 50mm f/1.8 or 85/1.8 lens for low light and shallow dof applications. If you want a longer tele, you can always get a 70-300 IS later (get the tele instead of the fixed focal length if you shoot many long distance objekts, such as wild animals or birds.
The greater overlap of the 18-135 and the 70-300 is a huge advantage in practice, because you dont need change lenses back and forth if you cant gauge the reuqired focal length excactly. With a 17-70+70-300 or 18-55+55-250 combination, you will very often find that you have the wrong lens attached if your subject requires you to shoot in the 60-80mm or 50-70mm area respectivly).
If the great majority of your shots is under low light AND you need to zoom in these conditions, OR you require a very shallow dof at shorter focal length, get the Sigma 17-50 OS plus a tele zoom.
Finally, if you want a tele , go for a canon original. The narrow depth of field of tele lenses requires the AF to be very precise and Sigmas have not a good reputation in this regard (my own expereince). The Af is also faster, generally speaking, with Canon originals, an improtant factor for birds.
Canon 18-55 plus 70-300IS may also be a viable combination if cost is an issue. This way you may have to zoom with yur feet in the 55-70mm range, but you can catch your birds at 300mm. With birds you can never have enough focal range. You can then substitute the 18-55 for a better lens later.
