Jump to content


Photo

What is CMOS?


  • Please log in to reply
7 replies to this topic

#1 Federico Pedroza

Federico Pedroza
  • Basic Members
  • PipPip
  • 12 posts
  • Director
  • Mexico City

Posted 08 August 2008 - 03:42 PM

I'm thinking about buying a new JVC camera that has CMOS technology, wich is replacing the previous version that had 3CCD. The salesperson tried to explain how instead of the 3 Chips through the prism set up we all know; now the CMOS technology is all about a larger chip that recieves each color separately. He says it's better, but he didn't look like he knew what he was talking about; I really didn't understand if this technology is better than 3CCD. I tend to think that a larger chip size is better due to depth of field issues.
Has anyone compared both? Which is better?

Be well

Federico Pedroza
  • 0

#2 John Sprung

John Sprung
  • Sustaining Members
  • 4635 posts
  • Other

Posted 08 August 2008 - 04:44 PM

First, whether a camera uses one chip or three is independent of whether the chip(s) are CCD or CMOS. Arri's D-21 and Red are large single chip CMOS cameras. Panavision's Genesis and Sony's F-35 are large single chip CCD's. At this time, the small chip (2/3" and under) three chip cameras that I'm aware of are all CCD, though theoretically they could be designed with CMOS as well.

Three chip cameras use a glass prism block to separate the light into three primary colors. This restricts lens design, because the back element can't be where the prism block has to be, and it restricts your stops to no faster than f/1.4. Single chip cameras use an overlay pattern of colored dye filters to permanently assign each photosite to only one of three primary colors. This requires some digital processing to convert the data into co-located pixel form, and results in lower resolution than you'd get with three chips with the same number of photosites on each.

Both CCD and CMOS convert photons into electrons in pretty much the same way. They differ in what they do next with the electrons.

CCD's shift the image very quickly to one edge of the chip, and read out each column of pixels one by one. Some of them have a whole frame's worth of covered storage area, so the image can in effect be "pulled down" much like an exposed film frame, only very much faster, and read out while the next frame is being exposed. This lets them have effective shutter angles of just barely under 360 degrees. (Cheaper ones without this storage can have vertical streaking issues on extremely bright objects, like the sun or practical lights in frame.)

CMOS chips read out each photosite where it is, no charge shifting. To achieve continuous data flow, they use a "rolling shutter", which has some artifact issues, especially with things like strobe lights. These shuttering issues are probably the biggest practical difference between them for the cinematographer.

CMOS chips can have a lot of digital processing built right onto the same chip. Because of how the charge shifting works, CCD's need to have that done on separate chips.



-- J.S.
  • 0

#3 Saul Rodgar

Saul Rodgar
  • Basic Members
  • PipPipPipPip
  • 1682 posts
  • Cinematographer

Posted 08 August 2008 - 10:10 PM

The Sony XDCAM EX1 and EX3 both have 3 1/2" CMOS sensors.

At some point CMOS sensors were not very gracious to images shot in low light. Apparently, the Sony EX cameras' sensors have been designed to counter this. The images produced with these cameras look OK, but to the naked eye in normal viewing conditions, it is practically impossible to tell well-lit images produced with CMOS chips apart from equal images produced with CCD's -except in a side by side comparison and by trained eyes.

Edited by Saul Rodgar, 08 August 2008 - 10:10 PM.

  • 0

#4 Federico Pedroza

Federico Pedroza
  • Basic Members
  • PipPip
  • 12 posts
  • Director
  • Mexico City

Posted 09 August 2008 - 12:07 AM

Would you then say that a cmos chip does bad in low light conditions?, more noise perhaphs?
I heard that the RED camera has a cmos chip, If I got the name correctlly Isn't this camera really high end?
How abouth the chip size? doesn't cmos tend to be larger than a regular ccd chip in similar categories?
  • 0

#5 Saul Rodgar

Saul Rodgar
  • Basic Members
  • PipPipPipPip
  • 1682 posts
  • Cinematographer

Posted 09 August 2008 - 01:40 AM

Would you then say that a cmos chip does bad in low light conditions?, more noise perhaphs?
I heard that the RED camera has a cmos chip, If I got the name correctlly Isn't this camera really high end?
How abouth the chip size? doesn't cmos tend to be larger than a regular ccd chip in similar categories?


CMOS used to be less efficient in low light, yes. The RED is high end, how much? Depending who you ask. The debate still rages on. About the size of the RED sensor, it is super 35 mm so it would be around 24.89 mm × 18.66 mm.
  • 0

#6 David Auner aac

David Auner aac
  • Sustaining Members
  • 1117 posts
  • Cinematographer
  • Los Angeles

Posted 09 August 2008 - 02:09 AM

At this time, the small chip (2/3" and under) three chip cameras that I'm aware of are all CCD, though theoretically they could be designed with CMOS as well.


Sony's HVR-V1U was the first small 3-chip CMOS camcorder AFAIK. It features 1/4" chips.

Cheers, Dave
  • 0

#7 John Sprung

John Sprung
  • Sustaining Members
  • 4635 posts
  • Other

Posted 11 August 2008 - 02:01 PM

The Sony XDCAM EX1 and EX3 both have 3 1/2" CMOS sensors.

Thanks, Saul and Dave. I've corrected my database.



-- J.S.
  • 0

#8 Sam Wells

Sam Wells
  • Basic Members
  • PipPipPipPip
  • 1751 posts
  • Cinematographer

Posted 11 August 2008 - 03:14 PM

Nikon D3 dslr (and now D700 w/ same chip) is CMOS and better in low light than just about anything else at the moment......

-Sam
  • 0


Broadcast Solutions Inc

Media Blackout - Custom Cables and AKS

Rig Wheels Passport

Tai Audio

Wooden Camera

Abel Cine

CineLab

Opal

Visual Products

Aerial Filmworks

Willys Widgets

Glidecam

Technodolly

FJS International, LLC

Gamma Ray Digital Inc

The Slider

CineTape

Ritter Battery

Paralinx LLC

Metropolis Post

rebotnix Technologies

Media Blackout - Custom Cables and AKS

Opal

Ritter Battery

FJS International, LLC

Willys Widgets

Wooden Camera

The Slider

Broadcast Solutions Inc

Technodolly

Abel Cine

CineLab

rebotnix Technologies

Aerial Filmworks

Visual Products

Glidecam

Paralinx LLC

Rig Wheels Passport

Tai Audio

Metropolis Post

Gamma Ray Digital Inc

CineTape