Please explain the Digital Sensor chip

[Skinner]

This question might better belong in one of the other forums. If so please move it where it belongs and I'll follow it for my answers.

OK. Here is my question.....

How can you change the ISO from one shot to another shot using any Digital Camera (and specifically the Rebel) and have a change in the "grain" of each shot.

From what I have been reading and listening to in the Epson Print Academy course when you increase the ISO you in effect add "grain" to the picture.

In the "Old School" days I would use one film with a fine grain and a slower speed for the shots I had plenty of light for. If you changed to a faster film you got a coarser grain and thus a grainer print. With Digital you are exposing the images on the exact same surface (The sensor chip) so how does the "Grain" change??

[Olegis]

The film grain is equivalent to "noise" in digital, more or less. The higher the ISO setting - the higher noise levels and the more "grain" you get.

In fact, you don't change the light sensitivity of the sensor, but instead you change the amplification of the signal coming from every photosite - higher ISO setting equals more amplification. Stronger amplification causes more noise and worse signal-to-noise ratio - that's the reason for all those coloured spots you see in the digital pictures.

[Mark_Cohran]

In digital photography, you don't have grain, but rather you have noise. This noise (usually seen in the shadows or dark areas of the photograph) is a result of electronic amplification. In other words, it doesn't come from light hitting the pixel, but from internally generated signals inherent in all electronic equipment. In a well lit seen, this noise is low compared to the all the light information reaching the pixel and low amplification (a low ISO setting) is necessary to produce a good photo.

In lower light situations, this "signal to noise" ratio is lower and longer exposures or increased amplification (higher ISO), results in the noise being more visible. The key concept is that at higher ISO values(which is simply increasing the gain or amplification of the sensor), the noise will be more amplified as well, and at very high ISO settings, this amplification of noise is a much more than the amplification of the light which results in a much lower signal to noise ratio and much more visible noise (not grain ) in the resulting photo.

You should also be aware than increasing the sensor temperature also increases this noise.

Mark

[daveh]

Even with film you could use tricks like push/pull processing to change the effective film speed during development.

[PacAce]

Remember when you used to push a certain speed film to a higher ISO number just by exposing at a lower EV? Then you'd process it so that the negs came out as if it was exposed normally? You got the shot at the expense of more grain, right?

Well, basically, it's almost the same principle with the sensor and the different ISO ratings you use with it. By setting the ISO to a higher speed , you are essentially "pushing" the sensor. Because you are actually getting less exposure at a higher ISO than you would at the lowest ISO setting, the image has to be "push" processed so tha the image will come out as if it were normally exposed. What that does is increase the noise level that's inherent in the sensors which is synonymous to the grains of film.

Of course, it's a little more complicated than the analogy I used but hopefully it'll give you a little better idea of what's going on inside a digital camera...sort of.