Light sensitive chip is transferring image of falling light on set of electric signals. These electric signals make digital photo possible.
Sensitivity of sensor is indirect relation between quantity of falling light and level of outgoing signal. The chip more sensitive, the less light we need for sufficient level of outgoing signal to make a high-quality photo.
Adjustment of chip sensitivity is normally expressed in grades ISO (50, 100, 200, 400...). Double ISO express half quantity of falling light per time unit for obtaining sufficient outgoing signal. For example: for ISO we need 32x less light than for ISO 50.
The Model is elementary, problem is, that real chip sensitivity remains the same. Light – tension curve is not changing. Adjustment of sensitivity defines only the part of characteristic light – tension for getting the outgoing signal and how it is executed by camera firmware. Level, gamma corrections, noise reduction, balance of white color...
Generally there is a linear dependence between light impulse and outgoing signal. Bigger the light impulse is, bigger the outgoing tension is. The outgoing signal is not increasing, if there is too much of light. Chip loses its sensitivity. The blow appears – white area without drawing. Contrary to little amount of light, when the outgoing signal doesn’t change. Be careful. Signal has the same level but we can see only dark colored noise, noise with sign of picture. Maximal sensitivity (minimal ISO) is consuming almost all characteristic. If the value ISO is increasing, there is only suitable low part of chip characteristic used. For example: ISO 1600 is using only 1/32 characteristic of dark photo area.

Application of four elementary influences of “high sensitivity – high ISO” to the final picture:

1) depreciation of light and color depth – pictures are flat, color range is lower
2) picture is sensitive to balance white color – colors are not so similar and often there is a dichroic fog
3) picture is less sharp
4) there is a noticeable grade. Single grade is bigger, more colored and intrude