Hi I am looking in getting the Sigma f/1.4 30mm or the Sigma f/1.8 24mm for my Rebel XT. I already own the Tamron 17-50 f/2.8. I have been studying the DOF using the online DOF calculator and it appears that the DOF is the same for a given focal length regardless of the lens. The DOF of the 30mm at f/1.4 is probably very small for me to use most of the time. Given that the DOF of the Sigma 30/24mm are the same as the Tamron at those focal lengths and 2.8 aperture, can I expect to have much faster shutter speeds with the larger aperture lens?
On a side note, anyone have much experience with either of the Sigma 24 f1. /30mm f1.4 lenses? I like the 24mm on my Rebel XT, but the f1.4 is very tempting too. I plan on using the lens to take pictures of kids indoors as they romp about without using a flash.
Edited by vijay venkat on Dec 17, 2007 at 06:02 PM GMT
On the technical side, the difference between f/1.4 and f/2.8 is two stops. If you're shooting at a shutter speed of 1/50 at f/2.8, you will gain 2 stops of shutter speed to 1/200 at f/1.4.
The difference between f/1.8 and f/2.8 is 1 and 1/3 stops. If you're shooting at 1/50 at f/2.8, you would be shooting at 1/125 at f/1./8
Depth of field is all your preference. Some shooters like a bit more depth of field (f/4-f/8 or even more for some applications) while some shooters prefer very small DoF (f.1.2 to f/2.8). Note, these are all on the short end (85mm or shorter).
The 350D isn't the fast in the Auto Focus department, so you may end up not shooting wide-open so that you have a bit more room to play with in terms of misfocus.
So, I won't be getting a faster shutter speed at f/2.8 with these wide aperture lenses? Am I correct in understanding that if I don't take pictures in the f/1.4 - f/2 range, then these lenses are pretty much useless.
No. If your shutter is 1/100 at f2.8 on a 24mm, it will be 1/100 at 200mm. The focal length does not enter into the equation here. OK, for all you pedantic types, yes there are small differences, but not enough to affect camera settings.
I wouldn't say these lenses are useless. They give the option of shooting at f/1.4-f/2 that the f/2.8 cannot do, and at f/2.8 they are stopped down, so their optical performance is improved.
