"The Question": IS and f4 or non-IS and f/2.8?
How many times has The Question been asked? Hundreds on this site alone.
This question reaches a crescendo when the 70-200mm is considered because all four combinations of IS and Lens speed are available on a 70-200mm.
Here’s the deal: IS and faster lenses are not answers to the same question. Asking do I need IS or a faster lens is like asking do I need a Ford F150 or Angioplasty?
The Question is really two questions: Do I need IS? Do I need a faster lens?
Faster Lens are the answer for the following needs:
I need better focusing accuracy and/or focusing speed.
I need a brighter viewfinder.
I need greater subject/background separation.
I need better low-light performance on stationary subjects.
I need fast shutter speeds in low light conditions.
Image Stabilization is the answer for the following needs:
I need to shoot shutter speeds slower than the "1 / focal length" the rule of thumb.
I need to be able to blur motion or panning shots without a tripod.
I need better low-light performance on stationary subjects.
I need to shoot higher f-stops.
I need to be able to go sans tripod in more situations.
These lists might not be complete but notice that the only overlap in the two lists is better low-light performance with stationary subjects.
Folks considering going with a faster lens usually don’t think about the focusing and viewfinder improvements but these can be a big deal to many photographers.
The most oft missed advantages of adding IS to a lens is shooting higher f-stops without tripods or sacrificing ISO performance.
Just my unsolicited thoughts on the IS vs. f/2.8 discussion and it didn't even cost you the 2¢.
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