Hey guys, I got to wondering the other day. Recent cameras such as the 7D, 5D and 1DmIV are doing great things in the iso technology. I was wondering why such a camera doesn't have the feature of taking light away. It would be such an easy alternative to fumbling around with ND filters. A simple push of the ISO button and a scroll away from exposure where you want it in a bright environment. Is there a reason we haven't seen this yet?
Why can't there be "negative iso"?
- Thread starter seven.b
- Start date
Jester2138
Well-known member
Huh, never really thought about that. I suppose ISO 100 is generally as low as normal sensors can go while staying "reliable?" I guess people are usually more concerned with the upper numbers. I'm very interested in the actual reason.
)) ehe funny but realy must be a technical reason which i have no idea..
AndrewGentle
Well-known member
It would be because once there is too much light on the sensor, the output goes to its maximum. ISO is amplification of the signal from the image sensor. Negative ISO would be attenuation but the problem is that once the different light levels hit the ceiling voltage, dividing them down to something smaller isn't going to separate them again into a usable image.
PerroneFord
Deals in Lead
Probably to keep fools from dialing it down to -1000 and shooting the Sun (or it's electronic equivalent) for an hour.
Dang, that's exactly what I was hoping to do.
Anyway, thanks guys, this explanation makes sense though. Oh well, off to buy some ND's
Taylor Rudd
Veteran
I'm no expert, but I just don't think it would work after a certain point. That's my professional advice for the day.
Moz
Active member
umm ISO is an exponential value, so you can't have a negative number. That's mathematically and physically impossible.
100 is 50 to 25 to 12.5 to 6.25 to 3.125 etc etc, you can divide in half infinitely.
each divide is 1 stop of light, come on this is basic photography stuff and actually is a pretty stupid question.
100 is 50 to 25 to 12.5 to 6.25 to 3.125 etc etc, you can divide in half infinitely.
each divide is 1 stop of light, come on this is basic photography stuff and actually is a pretty stupid question.
Last edited:
Thanks for sharing your intellect with us.
I had never thought of this before, but after reading through some of the responses in here it makes sense (Why it's not possible).
Because in a former life I mixed sound, I immediately thought of microphones. You can turn a mic down all you want, but if somebody is screaming into the mic it's going to sound awful. Period.
AndrewGentle
Well-known member
Just out of interest, you can read all about ISO on Wikipedia
Green Hornet
Veteran
The biggest reason is that the sensor needs a certain voltage to stay operational.
For example, if the battery is 8 volts, then it can only provide 8 volts to the sensor max.
(Unless you have a voltage amp).
So at 8 volts your sensitivity is maxed.
When you lower the voltage, the sensitivity goes down.
Say at 5 volts, you are at iso 200.
At 4.5 volts you are at iso 100
At 4.4 volts, the sensor is really noisy.
At 4 volts the sensor shuts off.
From my understanding, the base ISO is determined when you photograph an 18% grey card,
and photograph it outside at noon on a clear day.
You set your lens at F16, and take a shot at srgb color space.
When the Y component of the YCbCr output reaches a value of 118 (out of 255)
The amount of exposure required will tell you the ISO.
For example, if it takes an exposure of 1/250 sec then your iso is 200.
Following this example, if your voltage to sensor was 5 volts, then lowering the voltage will not produce more output, but less,
so sensitivity is down. The best performance will be at a point near the low end of operation. You have more range going higher, as the sensor will cease to function quicker by lowering the voltage.
To increase sensitivity, you apply more voltage to the sensor, there will be a point where saturation occours.
About a half stop below saturation, is where your max iso will be.
For example, if the battery is 8 volts, then it can only provide 8 volts to the sensor max.
(Unless you have a voltage amp).
So at 8 volts your sensitivity is maxed.
When you lower the voltage, the sensitivity goes down.
Say at 5 volts, you are at iso 200.
At 4.5 volts you are at iso 100
At 4.4 volts, the sensor is really noisy.
At 4 volts the sensor shuts off.
From my understanding, the base ISO is determined when you photograph an 18% grey card,
and photograph it outside at noon on a clear day.
You set your lens at F16, and take a shot at srgb color space.
When the Y component of the YCbCr output reaches a value of 118 (out of 255)
The amount of exposure required will tell you the ISO.
For example, if it takes an exposure of 1/250 sec then your iso is 200.
Following this example, if your voltage to sensor was 5 volts, then lowering the voltage will not produce more output, but less,
so sensitivity is down. The best performance will be at a point near the low end of operation. You have more range going higher, as the sensor will cease to function quicker by lowering the voltage.
To increase sensitivity, you apply more voltage to the sensor, there will be a point where saturation occours.
About a half stop below saturation, is where your max iso will be.
Last edited: