CMOS rolling shutters, film shutters, flashes, etc

John Sprung · January 17, 2008

I've been wondering why the rolling shutter thing is such a big deal, and how it compares with the film shutters that we're all accustomed to. So, I took the lens off a film camera, looked in the hole, and slowly turned the inching knob a few times.....

Suppose we have a film camera running 24 fps with a 180 degree shutter, and a CMOS camera shooting 24p with a 1/48 second exposure time. Any point in the aperture of the film camera sees image light for 180 degrees, and darkness for 180 degrees. Any one pixel on the CMOS chip is sensitive to light and accumulating charge for the equivalent of the same 180 degrees, and then it's read out and inactive for the rest of the cycle. Those are the similarities.

The big difference is that the edge of the shutter blade on the film camera spends maybe 45 to 60 degrees passing over the aperture, while the CMOS camera is designed for a continuous uniform flow of data from the chip, so the readout "edge" takes the whole 360 degrees to sweep over the whole image and start over. Therefore, worst case, the top and bottom of the film frame are 60 degrees apart in time, while the top and bottom of the CMOS image are nearly 360 degrees apart, six times as long.

Another difference is that the CMOS "edges" are absolutely horizontal and pixel-boundary sharp. The film shutter being between the lens and the film casts a soft-edged shadow, and is only horizontal at the middle of the frame, sweeping across the rest of the frame at an angle. (Or it's vertical at the middle in cameras with the shutter under rather than alongside the gate).

So, what happens if we have a very brief bright flash of light, one that goes on and off very quickly rather than fading up and down?

Do a bunch of flashes at random, and there's some chance that you'll catch the film shutter edge during its rapid pass over the aperture. But mostly they'll either hit when the shutter is open, and go on the film, or hit when the shutter is closed, and go into the viewfinder. (That's something to watch out for when you operate -- If you see the flash in the finder, you *didn't* get it on the film, and vice versa.)

On the CMOS camera the "start" and "stop/readout" edges are always in the frame somewhere. So, no matter when the flash happens, you're going to catch the "shutter" edges. To fix that, you'd have to sync both the beginning and end of the flash to the camera, with the flash starting when the readout edge is at the top of the frame, and ending when the turn-on edge is at the bottom of the frame. In this example, the flash would have to have a duration of 90 degrees or 1/96 second.

Another approach would be to read the CMOS out faster, and pause between frames, which would be more film-like. But to get the same time difference as a film camera, you'd have to read the chip six times faster, which would be equivalent to being able to shoot 144 frames per second in continuous rolling mode. That's not cheap or easy, which is why it doesn't happen in real world cameras.

So, did I figure that out right?

-- J.S.

Mitch Gross · January 17, 2008

This is essentially correct.

The Phantom HD has an adjustable shutter that can go from 1 microsecond (that's 2/500,000th of a second) up to a virtual 360 degrees (just five microseconds short to allow for a buffer's reclocking). The scan time of the shutter is a function of the exposure time.

John Sprung · January 17, 2008

Thanks, Mitch.

I guess that for all but big budgets, the thing to do with CMOS is shoot without strobes, and fake them in post. Just make some one or two frame events in the list and color time them blazing hot to totally blown out. You save on the strobe rentals, and you get to pick exactly where you want them to hit, too.

-- J.S.

Mitch Gross · January 19, 2008

Not if you have a well-designed camera you don't.

There is a period where the sensor is all-exposing and no lines are being scanned off. During this period it is safe to use a strobe. The Phantom HD has a Strobe output that is designed to interact with strobe lighting systems such as Unilux and Lightening Strike which will insure that every frame is fully, properly exposed. There is also another output which sends frame sync information, telling other units not only when each frame begins but the dration of exposure. This is for multicamera syncronization (great for 3-d or blowing up a model at 1000fps) as well as motion control and other synchronized devices.

The only time there is a danger of a partial exposure is when there is a short-duration illumination that cannot be synced to the camera, such as a lightening flash or camera strobe that is not tethered to the Phantom. In the case of lightening, it is only an issue when shooting high speed and pulling still frames, as we have successfully shot lightening at more than 140fps on the Phantom 65 and while the shutter sync issue was apparent when looking at individual frames, it blended perfectly when played back at 24fps.

Again, it is a REALLY limited issue we're discussing here.

John Sprung · January 22, 2008

There is a period where the sensor is all-exposing and no lines are being scanned off. During this period it is safe to use a strobe.

OK, as I understand it, to have the whole sensor active at once, you'd have to either:

a.) Shoot with the equivalent of a 360 degree shutter, or

b.) Read the chip faster and have a pause between frames in the data flow from the chip. That could be smoothed out with a frame buffer.

Doing some math, to clock out, say, 1080 lines 24 times a second, we'd have about 38.6 microseconds per line. Figuring from how a strobe works as a timing light for a car engine, it seems that flash durations in the low single digits of microseconds are very possible. So, our syncing problem is to land an under 5 microsecond flash in a 38 microsecond window that opens once every 41.67 milliseconds. That sounds like a doable thing. It might be good to have a delay adjustment you can use to dial it into the middle of the window.

Thanks --

-- J.S.