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I have the XF 50-140mm and the XF 100-400mm zooms for Fujifilm X-T2.

I heard for crop sensors to use the 1/(2× focal lenth) rule rather the 1/(1× focal length) for full frame. So, without OIS on a crop sensor, I would use the 1/2× (focal length); e.g. if I use 200mm, my shutter speed is 1/400.

If I use OIS then how does that rule change? E.g 1/x, or 1/.5?

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First, see Where does the ¹/shutter speed = focal length rule for hand shake come from?

It's important to recognize that this rule is definitely more of a guideline than a solid rule, and is very situation-dependent. It was also created sometime in the 1970s, and people generally were making smaller prints rather than examining with a loupe — the equivalent pixel peeping.

You also wonder if sensor size is a factor. Since we tend to enlarge more from these smaller sensors (that is, an 8×10 from APS-C is enlarged more than one from full-frame), it's generally true that the rule of thumb should be scaled up as well — see Does the shutter speed and focal length rule of thumb apply to cropped sensor cameras?

That said, Fujifilm claims 4-4.5 stops of stabilization. Each stop is equivalent to doubling the time the shutter is open, so if 1/400th adequately freezes camera shake without, one stop would allow you 1/200th, two stops 1/100th, three stops 1/50th, four stops 1/25tg, and another half stop a little bit more.

So, in theory, you could go down to 1/20th of a second or even a bit slower. In practice, that's probably somewhat ambitious in reality, though, and at those speeds the subject movement is probably a big factor too.

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