Super Steady Shot with Full Frame Sensors
February 1st, 2008 - 12:00:00 AM:
In Internet forums you often hear the claim “Sensor based image stabilisation (Super Steady Shot) does not work with full-frame sensors”. The two main arguments are:
- The mirror box is too small. The sensor has no room to move.
- The image circle of existing lenses is too small. The sensor would move outside the image circle.
The first argument is complete nonsense. SSS is not added to an existing camera. The camera is a new development. There's no law that forbids to make the mirror box a bit wider and higher.
The second argument requires closer analysis. All depends on two factors:
- How far does the sensor have to move?
- How large is the image circle really?
To answer the first question, you can devise a simple test: Use a camera without image stabilisation (or turn it off) and make a long handheld exposure. Then check how far the image has moved across the sensor during exposure.
I've used the Sony A700 with SSS turned off, the 100/2.8 Macro, focused at intermediate distance, and made a handheld 1 second exposure. Note that 1 second at 100 mm is far beyond the effectiveness of 2 to 4 stops that most makers claim. With a 2 stop gain you could shoot at 1/25 second, with a 4 stop gain you could shoot at 1/6 second. For 1 second you'd need between 6 and 7 stops of gain, which currently no maker can provide.
|Test with 1 second exposure, 100 mm|
But still, when checking the resulting image, it can be seen that the image only moved ±30 pixels during the shot. This translates to an absolute movement of about ±0.17 mm. In other words, the sensor would have to move ±0.17 mm to counter the image movement.
Now what about full frame? You can do all the math to translate the image movement to angular movement, then relate that to the angle of view of lens and APS-C sensor, compare with the angle of view of the same lens and a full frame sensor, translate back to the image movement on the larger sensor, and - surprise, surprise - you'd end up with the very same value. This is easily understandable. It's the lens that creates the image, and the same angular movement of the same lens causes the same shift of the image on the sensor, no matter how large the sensor is.
Now let's be generous. Let's say we're using a 600 mm lens. It has a 6 times narrower angle of view, which causes a 6 times larger image shift for the same angular movement. With our 1 second exposure the sensor would have to move ±1.02 mm to compensate. Remember that we're operating way beyond the range of current systems. But we're extra generous and assume that the photographer had two espresso with his lunch, so he'll shake a bit more, let's say ±1.5 mm. Now take a step back. Does anybody really think it's impossible to move a 36×24 mm sensor ±1.5 mm for image stabilisation? And does anybody really think you can't make the mirror box 1.5 mm larger in each dimension to give the sensor the room to move?
Now let's look at the image circle of existing lenses. Most people who claim that it's not large enough appear to never have seen it. With all lenses the image circle is larger than 43.3 mm in diameter. The 36×24 mm frame is not that tightly squeezed into the image circle. Furthermore, the edge of the image circle is not a hard cut-off. It's a rather soft transition. Especially with telephoto lenses, where you need the largest sensor movements because of the narrow angle of view, there is quite a wide image circle with little vignetting. Vignetting is generally a larger problem with wide angle lenses, but with these you also need only small corrective sensor movements.
Also consider lenses with a smaller image circle designed for APS-C cameras. With these SSS works just as well as with full frame lenses.
The sensor has to make only small movements to counter shake, and it's
independent from the sensor size. Furthermore, the image circle of existing
lenses is large enough for sensor-based image stabilisation, even for full-frame