In another answer @ital writes that:

For DSLRs, the difference is that optical stabilization is visible in the viewfinder while sensor-shift is not. For telephoto lenses, this makes it easier to aim but that does not mean stabilization is any less effective.

Why is this the case? Isn't it at least visible in video mode?

  • "... but that does not mean stabilization is any less effective." Well, unless the degree of stabilization provided by a lens based system would require the sensor based system to move beyond the edge of the lens' image circle or the edge of the light box or would require servos faster than what will fit in the camera's form factor... Longer focal length lenses with lens based IS can provide degrees of stabilization that sensor based systems can not match. – Michael C Dec 22 '17 at 17:25

A DSLR uses a mirror to reflect light coming in from the lens into the viewfinder. So when framing, the sensor is not involved. In fact framing can be done with a DSLR power-off since it is all optical.

Anything that happens to the sensor, such as movement for stabilization, therefore has no impact on what you see in the viewfinder.

Video is off-topic here but you should at least know that on a DSLR, the video is not previewed by the viewfinder, instead it is shown on the rear LCD which does get its data from the sensor. In theory that means that you could see the effect of stabilization but most camera turn off sensor-shift when video is being recorded because the sensor-shift system is noisy and would be recorded in the audio (it is similar to the buzzing sound of an insect while motors are working to move the sensor along up to 5 axis to compensate for movement).

  • Now I get it. I was thinking that only mirrorless cameras have in-body stabilization, but it turns out some mirror cameras have it as well. – JonathanReez Dec 22 '17 at 15:20

Very simply, for a DSLR, you don't see the image from the sensor through the viewfinder, you're seeing directly through the lens.

If you use the rear screen for live view or video, the mirror is up & you cannot use the viewfinder. Sensor shift would then be a viable option on the screen.

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