Wednesday, June 18, 2008

SONY EX1: CMOS and the Rolling Shutter

You know what, I was going to save this for later in the discussion but let's just get this one out of the way.

As people delve deeper and deeper into these cameras, this one issue has become the keystone consideration and possible deal-breaker in their decision-making. So let's address it now. It's just one of the many things that will come into play decision-wise. But it's a biggy.

The EX-1, unlike its competitors here, utilizes a CMOS chip to capture its images. Now this is both good and bad.

The fact that the CMOS chip is more heat and energy efficient, allows it to be larger - 1/2 inch instead of 1/3. This means great resolution and low-light sensitivity, and the potential for a shallower depth of field. Not 35mm film depth of field, but something a little nicer than 1/3 allows.

Also, the CMOS chip avoids vertical banding when we hit bright light. If you have a camera with a CCD chip, point it at a bright light at night and you'll get vertical bright line from the top of the screen to the bottom. Banding. Gone with CMOS.


All is not perfect in CMOS-land.

The way CMOS chips are designed, they use a rolling shutter, and this can lead to several problems when filming, especially when motion is involved.

While global shutters used with CCD chips take a single snapshot of what is in front of them with each frame - everything all at once - rolling shutters sequentially record the image in rows from top to bottom, one after the other, so that not all parts of the image are recorded at exactly the same time. Therefore, the subject being recorded may actually have moved a bit from when part of one frame is recorded to when another part of the same frame is recorded. This can distort the image.

When the EX1 came out, it was quickly hailed as the "HVX killer." That may or may not be, but many people's issues with the rolling shutter and the artifacts it creates are, at a minimum, keeping the HVX on some pretty strong life support.

Among the problems rolling shutter can create are:

Skew: The image bends diagonally in one direction or another as the camera or subject moves from one side to another, exposing different parts of the image at different times.

Jello-vision: This is most common in handheld shots. The rolling shutter causes the image to wobble unnnaturally and bizarrely. This typically only shows up in somewhat extreme circumstances, but we'll get into that later...

Partial Exposure: This one also sucks. If a camera flash goes off, it may only be present for some of the rows of pixels in a given frame. The top 1/3 of the picture may be brightly lit by a flash, while the bottom 2/3 of the picture is dark and unlit. The difference between the two parts of the frame can be very dramatic and even for a frame, is visible and jarring to the human eye. Similar problems can arise with police sirens, some flourescent lighting, and so on. That makes rolling shutters an issue for things like weddings, red carpet and event filming, and narrative filming situations that involve these issues.

Anyway, take a breather.

We'll get into some real-life examples and clips of the above phenomena in the next posts.

And don't go and eliminate the EX1 from consideration just because I opened with this issue. We're gonna cut rolling shutters apart a little bit, but we'll give this camera a chance to redeem itself.

Additional Links and Readings:

You can also find a good ARTICLE on the subject by Barry Green, an expert in digital video and also a strong proponent of the Panasonic HVX.

Here also is a POST by another video pro on blogspot that goes over CCD vs. CMOS. Tell him JC sent ya.



And finally, an ARTICLE by Kyle Doris.

1 comment:

Anonymous said...

great read man, thank you