4 Dec 2008 The ISO Lo 0.3 – 1 in D700
The ISO Lo 0.3 to 1 setting in D700 is said to be equivalent to ISO 160 to 100. But equivalent means not the same.
The usual way for the camera to adjust the ISO value is by controlling the gain of amplifier of signal comming out from the sensor before analog to digital conversion. If our sensor have unlimited dynamic range, there is no problem. But as our sensor have a limited headroom. A photo site is limited by a fixed quantity of photon it could receive before it is full. If more photo is received after a photo site is full, it lost track of how many more photon is received and we got highligt clipping.
Because of that, when using ISO lower than the base ISO value, the base ISO value of a sensor is the ISO level at which the signal comming out from the sensor to do need an amplification (the amplifier gain is 1 in this case), we run the risk of higlight clipping.
So, is the ISO 200 the base ISO value of the D700? I have tried to shoot the a scence with ISO 200 with manual exposure control so that part of the image have highlight clipping. For the same scence with the same expsosure value at ISO Lo 1, I observed that the image have exactly the same highlight clipping area. Therefore, it could be concluded that the base ISO value of the D700 is 200.
What is the consequence? That would means that when in situation that needs longer exposure time, it would be better use the ND filter to acheive the effect than using this ISO Lo 1 setting. If using this setting is not avoidable, we should be paying more attention to the highlight clipping problem.
