F5.6 IS vs F4.0

[parkkibum]

to be more specific
i currently have 55-250 F4-5.6 IS
I'm thinkin about switching over to 70-200 F4 non IS
how will 200mm F5.6IS compare with 200mm F4

will 1.5 difference on F be good enough that I won't need IS ?

I guess similar question can also apply to 70-200F2.8 with 70-200F4IS

I'm not sure if IS is even comparable with aperture, but way I see IS is that with slower shutter speed it alows me to think that i'm shooting in faster speed because it stabilizes it.
IS->stabilizes at slower shutter speed->faster shutter speed?
lower aperture->allows me to take at faster shutter speed.

[OsmosisStudios]

I would definitely recommend IS at that length, even with f/4. f/2.8 is a different story.

Remember, f/4 and f/5.6 is only one stop, but with stationary targets the IS is 2-3 stops, which means you're still down at least a stop. For action, its irrelevant, as IS does nothing for moving subjects, but still.

[inkista]

Originally Posted by parkkibum

how will 200mm F5.6IS compare with 200mm F4

This review gives some side-by-side comparisons of the lenses, including image quality. The L is optically better, more expensive, works on a full-frame and is a lot bigger and heavier than the 55-250.

will 1.5 difference on F be good enough that I won't need IS ?

Aperture doesn't go by a linear scale. It goes by a square-root-of-two scale, so the smaller the numbers the get, the bigger the differences are between them. The difference between f/4 and f/5.6 is a full stop, which is a doubling of the light. It's the same difference, exposure-wise, as doubling the iso, or halving the shutter speed.

I'm not sure if IS is even comparable with aperture, but way I see IS is that with slower shutter speed it alows me to think that i'm shooting in faster speed because it stabilizes it.

That's pretty much how it's sold, and why IS is typically described in stops, as in "two-stop IS" or "three-stop IS". The stops, in this instance, though, are typically in terms of the shutter speeds at which you can hold things. Without IS, 1/focal_length is the rule of thumb for achieving a handheld shot without camera shake blur. That is, with a 200mm lens, you need a shutter speed of 1/200s or faster. With two-stop IS, that limit lowers to 1/50s.

But you still HAVE to use that slower shutter speed to get a good exposure. And if your subject is moving, IS will do absolutely nothing to stop subject motion blur. If your subject is still, however, you're ok. Think of IS as being similar to using a monopod or tripod.

With a larger max. aperture, however, you actually can get a faster shutter speed, which can be very useful for "freezing" action.