Your camera’s soul purpose is to create images. …Okay, if you’re using a video camcorder you’re probably using it as your audio recorder as well, but that’s another topic. We’re just talking about how to use a camera here…
Let’s start with the basics of how an image is captured. It’s all about the exposure. The first picture is under exposed. The second picture is over exposed. But the third picture is just right. (Pictures courtesy of Goldilocks).
Three things control the exposure:
1) The aperture, AKA the opening in the iris.
2) The shutter speed.
3) The sensitivity of the imaging device.
They all add up like this:
The larger the aperture the more light comes into the camera.
The longer the shutter is open, the longer light is allowed to fall onto the imaging device.
The less sensitive the imaging device is to light the more light it needs to make a good image.
Film compared to Video
Apertures in film and video cameras work the same way.
With motion picture cameras, the shutter is a metal half-circle that spins just in front of the film. When the shutter is out of the way, the film is being exposed. When the shutter is blocking light from falling on the film, the film is being advanced to the next frame. Most of the time, motion picture cameras have a shutter speed of 1/48 of a second.
With video cameras… Okay, are you ready for this? I’m going to blow your mind. Despite the fact that video cameras have a feature you can set called “shutter speed”, THERE IS NO SHUTTER IN A VIDEO CAMERA! …Have you recovered from that yet?
Alright, let me explain…
Film cameras have shutters because the film needs to be advanced to the next frame. If there was no shutter, the film would be exposed while it’s moving to the next frame, causing nasty vertical streaks.
Video cameras use CCDs. When light hits the CCD it begins to build an electric charge. The longer light is allowed to hit the CCD the larger the charge becomes. Also, the more intense the light the faster the charge builds. When the CCD is discharged, the image is captured and the CCD returns to zero charge. The process repeats.
Normally, an NTSC video camera has a “shutter speed” of 1/60 of a second (PAL is 1/50). That means the CCD is discharged after 1/60 of a second. If the “shutter speed” is 1/1000, that means the charge on the CCD is only allowed to build for 1/1000 of a second before it’s discharged. Obviously, the shorter the time a charge is allowed to build the smaller that charge will be, resulting in a darker image.
This has the same affect as shutter speeds have with film, but it does it without a physical shutter.
Whew… glad that’s over.
Sensitivity of the imaging device:
Okay… That’s a bad choice of words to describe how film cameras work, because they don’t use an imaging device, they use… well, er, film.
However, the sensitivity can change depending on the particular film stock used. The sensitivity of film to light is expressed in the ASA number and referred to as the “speed”. The “faster” a film stock is (or the higher the ASA number) the less light it needs to capture a good image. The trade off is that fast film is more grainy. Most movies use 250 ASA film for daytime scenes and 500 ASA for nighttime and indoor scenes.
The sensitivity of a CCD depends on it’s engineering. Some video cameras use three CCDs and as a result need more light (but have better colour reproduction).
Video cameras have a feature called “Gain”. This boosts the video signal to make it look brighter, but does so at the expense of making the image more grainy. It’s very much the same effect as using a faster film stock. In most cases it’s better to add more light to a scene rather than increasing the gain.