The FujiFilm X-H1 has in-body image stabilization that requires the focal length to be set on the camera when using manual-focus lenses with adapters. This is a bit cumbersome with zoom lenses, and I often forget to change the setting. I've read that some people recommend setting the camera to use a focal length somewhere in the middle of the zoom range. However, when doing this, I've noticed that IBIS sometimes introduces motion blur. What setting can I use to get some benefit from IBIS without constantly changing the focal length setting and without introducing additional blur?
Do not just set a value in the middle of the zoom range and hope IBIS will work appropriately. Larger sensor movements are required to compensate for longer focal lengths, which will introduce blur at shorter focal lengths.
Some options to consider:
Turn image stabilization "off". FujiFilm image stabilization is never truly "off", so this option isn't very useful, except to determine whether a problem is being caused by image stabilization. You will receive no benefit from IBIS at any focal length, but you will also avoid letting IBIS introduce blur.
Set the camera to the lowest focal length your zoom supports. For instance, if you are using a 24-105mm zoom, set the camera to 24mm. This will (not effectively) reduce motion blur at longer focal lengths without introducing blur at shorter focal lengths.
Set the camera in stages based on the focal lengths you plan to use. For instance, suppose you have a 24-105mm zoom. You can divide the focal range into 24-50, 50-70, 70-105. Set the camera to the lowest value in the range. For instance, use 50mm for the 50-70 range. If you forget to change the setting when increasing focal lengths (70-105), IBIS will simply be less effective. But if you forget to change settings when dropping to lower focal lengths (24-50), IBIS will introduce blur.
Set the camera to the specific focal length you will be using. This is basically just a finer-grained version of the previous approach. Instead of only a handful of stages, you have 80. As before, if you forget to change the setting when dropping to lower focal lengths, IBIS will introduce blur.
In principle, this approach will give you the best results from IBIS, but may not be practical with manual zoom lenses, where the exact focal length is often unknown. Consider rounding down by about 10mm.
Try starting out with the second strategy (lowest focal length lens supports). Then move to the third strategy (focal lengths in stages). As you become accustomed to changing the camera settings before you zoom, you can increase the number of stages to whatever you can handle. However, the X-H1 only allows two custom focal lengths beyond 35mm, so the limit will likely be only around 3-4 stages.
Also, consider turning on continuous image stabilization. The stability of the view in the EVF will remind you to change the focal length setting as needed.