Luminous Landscape, one of the web's premier sources of camera geekery, has posted an "open letter" to camera manufacturers about lenses and sensor designs. I'm pissed and this helps to explain a few issues that I've had with cameras in low-light situations that I've simply explained as sensor noise.
Basically, large aperture lenses are great because they let in more light, but also let in more light from oblique angles. The light coming at the sensor from these more extreme angles is one of the reasons that large apertures gives you that shallow depth of field for buttery portraits.
This worked perfectly for film since the light-sensing aspect of film were tiny crystals in a substrate on the surface of the film, thus they were sensitive to light coming in from many angles. Digital cameras, on the other hand, are a sensor which points straight ahead. If the light is not coming from directly above, the sensor doesn't seem the image. This is the reason why the 4/3's format from Olympus and Co. was called the first fully digital camera system, because it took into account all of the unique aspects of digital photography.
All of the camera companies are trying to come up with ways to make their sensors better, including greater sensitivity, small lenses above each sensor site, etc. If this article from LL is to be believed, it's not enough and the camera companies have switched to cheating us.
Luminous Landscape, in association with DxOMark, have found that light loss at high apertures can get downright extreme. And to combat this, when using paired lenses, the camera will boost ISO WITHOUT YOU KNOWING. WTF?!
This is unacceptable. I buy pro-level tools to have manual control, not to give it up without my knowledge. The end results are all good, certainly, but I buy expensive stuff to get good photos in extreme light and movement conditions; the very places where these issues actually matter.
An Open Letter To The Major Camera Manufacturers