Image Stabilization and Bokeh

 

How does IS/VR/SSS/OIS affect bokeh?

I have lately run across several conversations about image stabilization and its effect on bokeh.  Several people with long stabilized lenses have been reporting strange artifacts in foreground and background blurred objects.  Thin lines appear as double lines, like Nisen Bokeh, but they are only present when using the image stabilization, and NOT when using a tripod and IS/VR turned off.  I first heard of it at this link:


http://luminous-landscape.com/forum/index.php?showtopic=19642


Since then I have been trying to understand why this would happen, and again, for me visualization is the key to understanding this.  Anyone who has done any panorama work knows about parallax error.  If your camera does not rotate exactly around the entry pupil of the lens (not the nodal point, as is often incorrectly asserted), your images won’t line up from one shot to the next.  This is essentially the same thing that causes these defects in stabilized lenses (and also in sensor shift technology).















































The above diagram demonstrates parallax in an image stabilized lens.  The central spot is kept sharply in focus, as it is “set” in the lens as the location which must be kept stable.  But the other spots will move depending on which direction you rotate the lens.  If however, the lens can be rotated exactly at its entry pupil, I believe that this effect should be virtually eliminated.  Why do you get a double line instead of just a straight smear?  When you are shaking the lens, you move it one direction and then another.  When you turn from one direction to the other, you have to go through a point of deceleration, then a full stop, followed by acceleration.  Therefore, more time is spent at and near the place where you switch from one direction to the other, therefore more exposure happens there.  So the question remains, is this effect mitigated when you have a lens with superb Bokeh, or does it affect mirror lenses and lenses with excellent bokeh equally?

































Despite the fact that the diagrams only show how this happens in lenses with in-lens image stabilization, exactly the same principle works for those with in body stabilization. 






How much movement do you need to reproduce the effect?

So, after thinking about this a bit more, I realized that I simply do not have the mathematical skill to determine this.  It depends on so many different factors, I just don’t know how to control for them.  Sensor size, diameter of outer lens element, f-stop, degrees of motion, and also some stuff about the internal workings of the lens itself.  One also has to take into account lens distortions, and any aberrations that the shifting lens itself might make.  I cannot do this kind of math.  I did, however, post this question to the Luminous Landscape Forum, where some people have looked at it.  According to one poster who tried to figure this out (http://luminous-landscape.com/forum/index.php?showtopic=19642&view=findpost&p=197102) if you take a 600mm lens, put a subject at 65 feet away and have a background at 80 feet, you should expect about 1 pixel of motion due to parallax shift if you have a 0.5 degree rotation around an axis that is NOT at the entrance pupil (4 inches in front).  Things that will increase the effect include:


  1. 1) increasing the angle of rotation

  2. 2) increasing distance between the axis of rotation and the entrance pupil

  3. 3) increasing subject to background distance.


Conversely, decreasing these will lesson the effect.


The reason I got interested in this in the first place was someone’s comments about their Canon 100-400 f/4 L and how bad the bokeh was when the IS was turned on, and not getting the effect when IS was off.  Others noted it on the 600mm L lens as well.  I think it is relevant that the comments from these were all on VERY LONG lenses.  I doubt there is much of an effect on a 24 mm lens.  But I also think that, while a single pixel may not make much of a difference, several pixels might, and in the math calculated by Olaf on the Luminous Landscape forums, there was only a 15 foot distance between subject at 65 feet.  One would not expect all that much bokeh anyway on that situation, (although a 600mm lens has a PRETTY narrow depth of field) and I think the more interesting question would be when you have a background at 80 feet and a subject at 10 or 20 feet.  At that point, the difference in Parallax may be significantly more, and one may end up with a shift in position on the sensor of significantly larger than 1 pixel.




Is this effect Real?

I have still been trying to figure out if this is a real effect or not, and I think it may be.  Here is my reasoning...


Imagine taking the same picture with a 600mm lens, once with IS on and once with IS off.  Suppose you handhold at 1/80th of a second.  One would expect the image that does NOT have IS to be blurry, right?  The entire image should be blurry, both the background and what you are focussing on.  Nobody would doubt, I think, that the blur would be more than a few pixels-- easily visible in your blurry picture.  When using IS, the focal part of the image stabilizes completely.  What about the rest of the image?


Suppose we had a magical f/5000 600mm lens (and some magic that allows you to avoid diffraction effects).... it would have so much depth of field that everything in your image would be sharply in focus.  Now we take pictures of 3 light bulbs.... one green at 1000 meters away, one yellow at 100 meters, and one red at 10 meters.  We take a picture with focus on the yellow light (100 meters) and can measure on the sensor plane exactly how far the image moved on the sensor.