in quickly reading all the responses, there are a couple of things to note here.
first of all the 1/focal length rule was developed for 35mm film, printing to a basic 8 x 10 print.
If you consider the "crop factor" of the ASP-C sensor, the impact is that you are making an enlargement 50% bigger, therefore, you need to multiply by the sensor crop factor.
This means for the same size print, you need for a pentax DSLR to consider without shake reduction, shutter speed = 1 / (Focal Length x Crop Factor)
If you intend to make enlargements greater than 8 x 10, or if you crop your image, you would need to increase the shutter speed accordingly by the enlargement or crop factor ratio relitive to your full image on 8 x 10 print.
Also remember that this is somewhat variable, individual users and their techniques can impact this.
With shake reduction ON, and again depending on technique, you can usually get 2-3 stops improvement, therefore instead of a shutter speed of 1/(FL x 1.5) or 1/200 for your 135mm lens, you should be able to shoot at 1/200, and might get away with 1/50 or 1/25
again depending on your camera holding technique
Note however shake reduction, while good has 3 things it cannot account for,
- first is panning, if your subject is moving, shake reduction cannot easily determine the difference between panning motion and shake, in these instances it is usually recommended to turn it off
- second, shake reduction cannot account for subject motion, if your subject is not perfectly still, even if you have shake reduction off it may not give you a sharp image due to subject motion
- third, and many people for get this point, shake reduction only applies for motion in the focal plane, i.e. up and down, left and right, and on pentax cameras rotationally, but it cannot account for chang in subject to image distance and the impact on focusing errors induced by motion of the photographer. This becomes extremely important the closer you get to the subject, or the wider open the lens is (shallow DOF) and the length of the lens (since long lenses have shallow DOF)
I havve found shake reduction is good, as far as it goes but it can;t correct everything.
For the OP
IN reading the follow on comment, about shooting 1/30 with a 50mm, this is equal to 1/90 with a 135mm and 1/10 with the 50mm is equal to 1/30 with the 135mm.
Also 1/10 with a 50mm lens is 3 stops and about the limit of SR, 1/10 is also almost too slow unless you are shooting ab building to insure subject motion itself does not give you a problem
for Marc
I keep telling you you should get the 135 F2.5