I'm interested to buy a Pentax K-30. I'm wondering : can I buy not stabilized optics because of body' stabilized sensor ? Is the overall image quality comparable to an optic with stabilizer (like Nikon and Canon) ? If no, what happen if i mount a stabilized optic ? Thanks
Yes, you can buy non-stabilized optics for a Pentax. In fact, very few stabilized lenses are available, because all Pentax digital bodies since the 2006 model K100D (excluding K110D) have provided the Shake Reduction sensor stabilization, so there's no need to have stabilization in lenses.
Yes, image quality is comparable to optic stabilization since it is optical stabilization, just executed at different point in optic path compared to in-lens stabilization.
When mounting a stabilized lens, the sensor stabilization and lens stabilization should not be enabled at the same time, as that would lead to overcompensation. Use either one, or disable both stabilization systems.
I'm interested to buy a Pentax K-30. I'm wondering: can I buy non-stabilized optics because of body' stabilized sensor?
Yes. In fact, the vast majority of lenses available for the Pentax K-mount are non-stabilized because they were either:
There are a few exceptions to this rule.
Is the overall image quality comparable to an optic with stabilizer (like Nikon and Canon)?
That depends on several variables. Ultimately you have to compare lens based and camera based optical image stabilization systems by comparing one specific camera body + lens combination to another specific camera body + lens combination. In general, though, camera based systems tend to work better at shorter focal lengths while lens based systems tend to work better at longer focal lengths.
As the focal length gets longer, the camera based system is required to move increasingly faster and further to get the same effect as it can with a shorter focal length lens. Eventually one runs out of room for the sensor to move with regard to the edge of the image circle cast by the lens as well as room for the mechanisms needed to move the sensor faster and faster at greater and greater distances. Lens based systems do not suffer from the image circle size problem because the correction is made inside the lens in front of the final group(s) of lens elements that project the image circle. Only the size of the image cast by the stabilizing element (or group of elements) must be enlarged to insure a full enough sized image is cast on the succeeding elements in the lens.
If no, what (will) happen if I mount a stabilized optic?
If you are using a lens that provides optical image stabilization inside the lens, you can choose whether to turn off the lens based system, the camera based system, or both. There have been other camera makers, notably Olympus, that have begun experimenting with combining both type of stabilization at the same time. But, as of December 2017, Pentax has not introduced such a camera.
¹ Remember, the 1/focal length rule-of-thumb says you can handhold a camera using good technique at a shutter time no slower than 1/focal length (expressed in 35mm equivalent angle of view). You can shoot with a 50mm lens on a 1.6X 'crop body' at 1/80 second and expect good results, but to get the same results with a 300mm lens on the same camera would require shooting at 1/500 second (300 x 1.6x = 480).