Does IBIS reduce image resolution? How does it compare to lens based IS?

Question

I heard a lot about in-body image stabilization recently and I wonder how this technology works. I don't mean the mechanical part (I understand how the sensor moves along different axes to compensate), but the technique in principle:

When I take my 5D III and a lens like the 70-200 2.8 II IS USM with activated stabilization, the IS will constantly move one glas element within the lens in such a way that it directs the cone of light exactly onto the sensor, so that you can always use the full sensor area.

When the cone of light goes through an unstabilized lens and you need to move the sensor - how can it capture the same amount of light at all? The light enters the lens down a fixed axis right through the center of the lens. When the sensor currently moves down, for example, to compensate for a shake, I can't see how it then is still able to capture the full resolution? When a part of the sensor is below default/calm position, then there must be missing the same part on the opposite site?

There's only one way I could image IBIS to work without resolution reduction: if the sensor area was significantly bigger than the area exposed to light, so that there's still sensor available when it currently moves out of its initial position. But wouldn't this (partly) be the same as digital stabilization because the final image gets calculated from the full sensor area minus the not exposed area?

I really would like to understand this and it would be great if someone could "shed some light on this".

Answer 1

There's only one way I could image IBIS to work without resolution reduction: if the sensor area was significantly bigger than the area exposed to light, so that there's still sensor available when it currently moves out of its initial position.

It's actually exactly the opposite. The image circle — the result of that cone of light hitting the imaging plane — is larger than the sensor. It has to be, first because that's the only way to cut a full rectangle out of a circle, but also because the edges of the circle aren't clear cut ­— it's kind of an ugly fade-out with a mess of artifacts. So, the circle covers more than just the minimum.

It's true that for IBIS to be effective, this minimum needs to be a bit larger. To give a concrete example: a full frame sensor is 36×24mm, which means a diagonal of about 43.3mm. That means the very minimum circle without moving the sensor needs to be at least 43.3mm in diameter. The Pentax K-1 has sensor-shift image stabilization, allowing movements of up to 1.5mm in any direction — so, the sensor can be within a space of 36+1.5+1.5 by 24+1.5+1.5, or 39×27mm. That means the minimum image circle diameter to avoid problems is 47.4mm — a little bigger, but not dramatically so.

But then, the resolution of the sensor cut from the circle is still the same. It's just shifted by a bit.

It's actually pretty easy to find some examples which demonstrate the image circle concept, because sometimes people use lenses designed for smaller sensors on cameras with larger sensors, which results in less-than-entire-frame coverage. Here's an example from this site... don't pay too much attention to image quality, as this is clearly a test shot taken through a glass window (with a window screen, even). But it illustrates the concept:

You can see the round circle projected by the lens. It's cut off at the top because the sensor is wider than it is tall. This sensor measures (about) 36×24mm, but the lens is designed for a smaller 24×16mm sensor, so we get this effect.

If we take the original and draw a red box outlining the size of the smaller "correct" sensor, we see:

So, if the lens were taken on the "correct" camera, the whole image would have been that area inside the box:

You've probably heard of "crop factor". This is literally that.

Now, if IBIS needs to move the sensor quite a lot (here, the same relative amount as that 1.5mm travel limit on Pentax full frame), you might see this, with the lighter red line representing the original position and the new one the shift. You can see that although the corner is getting close, it's still within the circle:

resulting in this image:

Actually, if you look at the very extreme bottom right corner, there's a little bit of shading that shouldn't be there — this contrived example goes a bit too far. In the extreme case of a lens which is designed to push the edges of the minimum (to save cost, weight, size, etc.), when the IBIS system needs to do the most extreme shift, it's actually possible to see increased artifacts like this in the affected corners of the image. But, that's a rare edge case in real life.

As Michael Clark notes, it's generally true that image quality falls off near the edge of the lens, and if you're going for maximum resolution (in the sense of captured detail), shifting off center can impact that. But in terms of pixels captured, the count is identical.

In addition to the centering issue, this can also affect composition: if you are trying to be very careful about including or excluding something from one edge of the frame, but aren't holding still, you could be something like 5% off of where you thought you were. But, of course, if you're not holding still, you might get that just from movement.

In fact, Pentax (at least) actually uses this to offer a novel feature: you can use a setting to intentionally shift the sensor, allowing different composition (the same as a small amount of shift from a bellows camera or a tilt-shift lens). This can be particularly useful with architectural photography. (See this in action in this video.)

Also: it's worth thinking about what's going on over the course of the exposure. The goal is to reduce blur, right? If the camera is perfectly still (and assuming perfect focus, of course), every light source in the image goes to one place, resulting in perfectly sharp drawing of that source in your image. But let's examine a fairly long half-second shutter speed during which the camera moves. Then you get something like this:

… where the movement of the camera during the exposure has made it draw a lines instead of points. To compensate for this, image stabilization, whether in-lens or sensor-shift, doesn't just jump to a new location. It (as best it can) follows that possibly-erratic movement as the shutter is open. For video, you can do software-based correction for this by comparing differences frame to frame. For a single photographic exposure, there's no such thing, so it can't work like in your quote at the top of this answer. Instead, you need a sophisticated mechanical solution.

Answer 2

Both IBIS and lens based IS (LBIS) have their own set of advantages and disadvantages. Which is "better" depends on a large number of variables. The hierarchy of those variables will be different from one photographer to the next and even from one specific photo to the next. As with many things in photography, you pay your money and you make your choice of which you think will work better for you and the types of photos you wish to make.

mattdm's answer explains very well that the image circle must be larger than the sensor's diagonal for IBIS to work. The corollary of this is that when the sensor is shifted, the center of the lens' optical axis is projecting to a spot other than onto the center of the sensor. Thus, one edge (or corner) of the sensor is closer to one edge of the image circle than when the sensor is centered. The main disadvantage of this is, of course, that the edges of image circles are usually not as good optically as the center of image circles. There's usually more distortion, more light falloff, lower resolution, etc. on the edge of the image circle than in the center.

Does IBIS reduce image resolution?

For most lenses, if the lens can not outresolve the sensor on the edges of the image circle, on one side of the frame it will. On the other side of the frame it will increase resolution slightly, with that edge/corner of the sensor closer to the lens' center axis. It can be noticeable in photographs if the resolution at the very edge of a lens' image circle is enough lower than the resolution at the point in the image circle that would be projected on the edge/corner of the sensor when the sensor is centered on the lens' optical axis.

If, on the other hand, the lens can still outresolve the sensor, even at the edge of the projected light circle, then there will be no noticeable difference. The obvious takeaway is that on a very high megapixels sensor with very small photosites, the effects will be more noticeable than when using a camera with a much larger photosites (pixel wells) that can't show the limits of the lens' capabilities.

The main advantage of IBIS that marketers used to tout is that it can be used with all lenses.¹ But older legacy lenses may have smaller image circles than newer lenses designed for a camera with IBIS. This is not an issue if the older lenses are for 135 format 35mm film and the camera has an APS-C sensor, but it can be an issue if the camera is a full frame 135 format (36x24mm) digital camera with IBIS. Presently the main advantage of IBIS emphasized is the ability to correct for camera movements on axes for which lens based IS can not compensate. The main disadvantage of IBIS is that longer focal lengths that need IS the most get the least amount of benefit from IBIS in that the same amount of sensor movement can counteract less camera movement with a longer telephoto lens than with a wide angle lens.

With lens based IS, the movements of the IS element/group introduce mild misalignment of the lens. This is viewed as acceptable if the blur introduced by the less than 'perfect' optical alignment of the lens² induced by an IS movement is less than the blur that would otherwise be introduced by the motion of the lens/camera. The largest advantage of LBIS is that for narrow angles of view (long focal lengths) much more correction can be done than can be done by shifting a sensor that is limited by both the size of the image circle and the speed and distance at which in-camera servos can move the sensor while remaining relatively compact and efficient with regards to battery/energy consumption/cost.

_{¹ Supposedly, this results in cheaper lenses for systems that use IBIS when compared to systems that use IS lenses. Try a survey of comparable lenses between, say Canon's EF line of IS lenses or Nikon's F mount line of VR lenses and Sony's non-IS A-mount and early non-IS E-mount lenses with IBIS. They may try to sell you on the argument that putting stabilization (and even a focus motor) in the camera body makes the non-stabilized lenses (sometimes with no focus motor needed) cheaper, but a comparative look at the prices of Sony's non-stabilized lenses versus Canon's/Nikon's stabilized lenses indicate there is little, if any, difference in price between lenses that are otherwise comparable. Even third party lenses that include stabilization for Canon/Nikon mounts are priced very close if not identically to their non-stabilized Sony mount counterparts.
² There's no such thing as a 'perfectly' aligned compound lens, even among non-IS prime lenses. There are always manufacturing tolerances to be considered.}

Answer 3

The sensor moves with, not against the image projected by the lens, and as long as it does not force itself so far off the center that it leaves the imaging circle of the lens (or enters its fringes where beyond-tolerable image faults are suffered), there should be no problem. Actually, some cameras take advantage of the movable sensor to increase resolution by taking multiple exposures with the sensor shifted by a half pixel each time, then interleaving the pixels (so called pixel shift mode).