Underexposure vs Higher ISO

EchoingWhisper · Oct 28, 2011

Why would someone use a higher ISO just to reduce the noise? I don't think there is a big difference and increasing ISO just reduces dynamic range which I don't like.

Snakeguy101 · Oct 28, 2011

How is properly exposing an image reducing the dynamic range?

analog.universe · Oct 28, 2011

If it's underexposed, you're also throwing away dynamic range. In any case you're going to need to boost the final exposure, the question is whether to do it digitally in post, or in analog via ISO. The analog data doesn't quantize when you scale it like the digital data does. So while you may be amplifying the same noise, to possibly similar levels even, you still limit the flexibility and accuracy of your data by scaling it in the digital domain instead of in camera.

EchoingWhisper · Oct 28, 2011

I mean, I said it wrong. According to some people, they would overexpose their picts even if that means they need higher ISO and later pull the levels down. Why? What's the point? Exposing it properly doesn't throw away dynamic range does it?

analog.universe · Oct 28, 2011

You're speaking of expose to the right, which in oversimplified terms is the practice of always bringing the highlights just short of clipping, and then adjusting exposure in post. The reason this is done is to get the best accuracy out of the analog to digital converter. The conversion is done from a continuous voltage curve, into a 14-bit binary representation. If you underexpose, for the purpose of avoiding ISO noise, you may only be using 9 or 10 of those bits, instead of all 14. ISO is an amplification of the continuous voltage in the analog domain, before the digital capture is performed. Increasing the ISO will increase the noise floor, but so will boosting exposure in post, the difference is that if you boost ISO, you still get to use the full range of the A to D converter, and so any adjustments done in post have more data to work with.

So, does exposing properly throw away dynamic range? It may not change the absolute range.. but if by properly you mean "so it looks right", then in certain cases, it does throw away digital dynamic range, which in the end is more restrictive than throwing away analog dynamic range by boosting ISO. You may only get 12 of the 14 bits utilized in a "proper" exposure, meaning if you want to do work to the shadows in post, you'll see more quantization error than if you had boosted the ISO, even though you may still see the same absolute noise floor.

I'm not sure if I'm explaining myself well.....

analog.universe · Oct 28, 2011

If you can expose to the right without boosting ISO, then in post you can effectively lower the noise floor below the base level of the sensor...

Noxire · Oct 28, 2011

Simply put; darker tones contain less data.

You can test this at home.
Set camera to 100 iso take a picture of a white wall where you deliberately underexpose it just before the shadows clips then boost it in post just before it clips to white.
Then take a second picture with iso 800 on the same wall where you overexpose just before whites are blown out.

Now compare noise.

clanthar · Oct 28, 2011

analog.universe said:
You're speaking of expose to the right, which in oversimplified terms is the practice of always bringing the highlights just short of clipping, and then adjusting exposure in post. The reason this is done is to get the best accuracy out of the analog to digital converter. The conversion is done from a continuous voltage curve, into a 14-bit binary representation. If you underexpose, for the purpose of avoiding ISO noise, you may only be using 9 or 10 of those bits, instead of all 14. ISO is an amplification of the continuous voltage in the analog domain, before the digital capture is performed. Increasing the ISO will increase the noise floor, but so will boosting exposure in post, the difference is that if you boost ISO, you still get to use the full range of the A to D converter, and so any adjustments done in post have more data to work with.

So, does exposing properly throw away dynamic range? It may not change the absolute range.. but if by properly you mean "so it looks right", then in certain cases, it does throw away digital dynamic range, which in the end is more restrictive than throwing away analog dynamic range by boosting ISO. You may only get 12 of the 14 bits utilized in a "proper" exposure, meaning if you want to do work to the shadows in post, you'll see more quantization error than if you had boosted the ISO, even though you may still see the same absolute noise floor.

I'm not sure if I'm explaining myself well.....

I think well explained and dead on right. :thumbup:

MLeeK · Oct 28, 2011

Dear Lord, that was mind boggling to read!

Simplify: When you reduce exposure you begin to eliminate or hide the noise generated. When you boost any of the exposure you are creating more noise than was originally visible. If you underexpose to avoid using the higher ISO with the intention of either increasing exposure or brightening in post you will have more noise than if you had exposed properly or to the right (over) and either left alone or reduced exposure. http://www.luminous-landscape.com/tutorials/expose-right.shtml
Lightroom Tutorial – Expose to the Right

EchoingWhisper · Oct 28, 2011

analog.universe said:
You're speaking of expose to the right, which in oversimplified terms is the practice of always bringing the highlights just short of clipping, and then adjusting exposure in post. The reason this is done is to get the best accuracy out of the analog to digital converter. The conversion is done from a continuous voltage curve, into a 14-bit binary representation. If you underexpose, for the purpose of avoiding ISO noise, you may only be using 9 or 10 of those bits, instead of all 14. ISO is an amplification of the continuous voltage in the analog domain, before the digital capture is performed. Increasing the ISO will increase the noise floor, but so will boosting exposure in post, the difference is that if you boost ISO, you still get to use the full range of the A to D converter, and so any adjustments done in post have more data to work with.

So, does exposing properly throw away dynamic range? It may not change the absolute range.. but if by properly you mean "so it looks right", then in certain cases, it does throw away digital dynamic range, which in the end is more restrictive than throwing away analog dynamic range by boosting ISO. You may only get 12 of the 14 bits utilized in a "proper" exposure, meaning if you want to do work to the shadows in post, you'll see more quantization error than if you had boosted the ISO, even though you may still see the same absolute noise floor.

I'm not sure if I'm explaining myself well.....

I see. I understand now. Would you expose to the right even if you needed to boost your ISO?

MTVision · Oct 28, 2011

EchoingWhisper said:
I see. I understand now. Would you expose to the right even if you needed to boost your ISO?

From what I've read - yes!! Check out the link MLeeks gave - (luminous landscape). Great article on ETTR

MLeeK · Oct 29, 2011

EchoingWhisper said:
analog.universe said:

You're speaking of expose to the right, which in oversimplified terms is the practice of always bringing the highlights just short of clipping, and then adjusting exposure in post. The reason this is done is to get the best accuracy out of the analog to digital converter. The conversion is done from a continuous voltage curve, into a 14-bit binary representation. If you underexpose, for the purpose of avoiding ISO noise, you may only be using 9 or 10 of those bits, instead of all 14. ISO is an amplification of the continuous voltage in the analog domain, before the digital capture is performed. Increasing the ISO will increase the noise floor, but so will boosting exposure in post, the difference is that if you boost ISO, you still get to use the full range of the A to D converter, and so any adjustments done in post have more data to work with.

So, does exposing properly throw away dynamic range? It may not change the absolute range.. but if by properly you mean "so it looks right", then in certain cases, it does throw away digital dynamic range, which in the end is more restrictive than throwing away analog dynamic range by boosting ISO. You may only get 12 of the 14 bits utilized in a "proper" exposure, meaning if you want to do work to the shadows in post, you'll see more quantization error than if you had boosted the ISO, even though you may still see the same absolute noise floor.

I'm not sure if I'm explaining myself well.....

Click to expand...

I see. I understand now. Would you expose to the right even if you needed to boost your ISO?

Most especially when you are using higher ISO's. Go even higher to ETTR-otherwise you are looking at more noise issues in your shadow areas than you care to deal with.

EchoingWhisper · Oct 29, 2011

MLeeK said:
Dear Lord, that was mind boggling to read!

Simplify: When you reduce exposure you begin to eliminate or hide the noise generated. When you boost any of the exposure you are creating more noise than was originally visible. If you underexpose to avoid using the higher ISO with the intention of either increasing exposure or brightening in post you will have more noise than if you had exposed properly or to the right (over) and either left alone or reduced exposure. http://www.luminous-landscape.com/tutorials/expose-right.shtml
Lightroom Tutorial – Expose to the Right

Lol. Thyroid disease?