Just curious. Why would someone pick a 50mm f/1.8 with IS over a 50mm f/1.4 (no IS)? Or vice versa.
Many, many threads have weighed the answer to that question. I'll take a crack at it here, but understand that the physics majors and the working pros on this thread probably see this argument quite differently.
Why Slower with IS is better
With some exceptions, a narrower max aperture lens like a F/1.8 or F/2 should be smaller and lighter than a F/1.4 lens. That absolutely will be the case with Canon's 50 F/whatever IS -- expect it to be about as big as the recently 35mm F/2 IS, as the non-L IS refreshes seem to be paired housing/size-wise.
For comparison's sake, the new Sigma F/1.4's dimensions are very close to the Canon 24-70 F/4 zoom
when closed/at 24mm:http://www.the-digital-picture.com/Reviews/Lens-Product-Images.aspx?Lens=823&LensComp2=0&LensComp=824
(mouseover the 'Select View' below the pictures and you can make the hoods disappear for a truer read on size.)
Probably more important for the shooters on this forum, presuming (for the sake of example) it's a 4-stop IS system, provided the subject is not moving
, you have a lens that is 3-ish stops better for handholding in low light. That advantage can be used a few different ways:
- For a given shot that the non-IS lens would make (assuming it's not right at F/1.4), you can get the same shot while still moving your ISO back down to a more reasonable level, and minimize noise.
- Or, you can stop down the lens to gain sharper image and/or more working DOF. Keep in mind that an F/1.4 non-IS lens in a dimly lit room may have to be slammed fully open to net a low-light shot without cranking the ISO too high, and that can be a fairly limiting DOF to work with, plus no lens is anywhere near being sharp corner to corner at F/1.4.
I shoot a fair amount in low-light, handheld, and without a flash. So IS is a big help for me.
Why Faster without IS is better
You can generate a smaller DOF, which is awesome in some compositions.
If you have quickly moving subjects (and you aren't gunning for a blurry one), the upside of IS effectively lost. IS helps with longer shutters and unsteady hands, but it can't freeze (for example) a five year old with happy feet. So if your subject is moving, take the aperture over the IS.
Some folks really rave about the color and draw of the really big F/1.4, F/1.2 and F/1.0 lenses. Quite simply, you are letting more of the light in and it shows. Folks often claim such large aperture work has a 'magical' feel or tone to it. On my F/1.4, when I shoot it wide open, I note richer colors + healthy vignetting on my FF rig that simply looks more memorable at first glance. You can't/shouldn't shoot wide open all the time, but it's a killer look for some shots.
So there are two schools on this. There are a good (I'd guess) half the people in this forum that would take the speed of the F/1.4 without IS at this focal length. For what I shoot, I wouldn't -- I'd probably take the IS.