[quote name='PuxaVida' timestamp='1299060435' post='6425']
Is it possible that underexposed shot + exposure slide in ACR vs. high ISO setting usage can have same consequences in resulting image?
[/quote]
No, albeit the effect is quite similar.
When you expose correctly, the following happens:
1) Each sensor-cell produces an analog output-voltage.
2) This voltage is amplified by an analog amplifier (which is controlled
by the ISO setting you selected).
3) The amplified voltage is digitised by an A/D circuit.
When you underexpose, the following happens:
1 )Same as above.
2) The voltage is amplified less, due to the lower ISO setting. It will therefore
not reach the same maximum level that is reached above.
3) The amplified voltage is digitised by an A/D circuit.
4) The digitised value is enlarged
It might seem, that there is no difference between doing an analog amplification and
a digital amplification ... but there is ... it is hidden in step 3.
With a properly amplified value as output of step 2, the A/D step can make use the
full bitdepth ...while the output value of the underexposed image will never reach
the possible maximum, and therefore the A/D step will always produce "low"
values (due to the underexposure). But even if you enlarge these values later, the
point is, that you started with a reduced number of values ... and the number of
distinct different values does not increase.
Example ... a properly amplified signal produces digital values between
0 and 1023 after digitization (10bit) ... an underexposed signal (by 2 stops)
will only produce values between 0 and 255 ... and even if you multiply
by 4 (and therefore increase the range) to 0 to 1023 ... you will not use
all possible values. ... 0*4 = 0 ... 1*4=4 ... the 1 and the 2 and the 3 will not
be used. ... You will see this effect as posterization especially in the dark areas.
Is it possible that underexposed shot + exposure slide in ACR vs. high ISO setting usage can have same consequences in resulting image?
[/quote]
No, albeit the effect is quite similar.
When you expose correctly, the following happens:
1) Each sensor-cell produces an analog output-voltage.
2) This voltage is amplified by an analog amplifier (which is controlled
by the ISO setting you selected).
3) The amplified voltage is digitised by an A/D circuit.
When you underexpose, the following happens:
1 )Same as above.
2) The voltage is amplified less, due to the lower ISO setting. It will therefore
not reach the same maximum level that is reached above.
3) The amplified voltage is digitised by an A/D circuit.
4) The digitised value is enlarged
It might seem, that there is no difference between doing an analog amplification and
a digital amplification ... but there is ... it is hidden in step 3.
With a properly amplified value as output of step 2, the A/D step can make use the
full bitdepth ...while the output value of the underexposed image will never reach
the possible maximum, and therefore the A/D step will always produce "low"
values (due to the underexposure). But even if you enlarge these values later, the
point is, that you started with a reduced number of values ... and the number of
distinct different values does not increase.
Example ... a properly amplified signal produces digital values between
0 and 1023 after digitization (10bit) ... an underexposed signal (by 2 stops)
will only produce values between 0 and 255 ... and even if you multiply
by 4 (and therefore increase the range) to 0 to 1023 ... you will not use
all possible values. ... 0*4 = 0 ... 1*4=4 ... the 1 and the 2 and the 3 will not
be used. ... You will see this effect as posterization especially in the dark areas.