ISO setting effect on noice in raw format?

Started Sep 1, 2009 | Discussions thread
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GordonBGood Veteran Member • Posts: 6,301
Re: ISO setting effect on noice in raw format?

Graystar wrote:

OS74 wrote:

I wonder if anyone can enlighten me on how the ISO setting affects the sensor reading.

I don’t know if anyone can, but I can tell you there certainly won't be a shortage of people who think they can!

Recently i had to photograph in low light underwater hand held, and deemed that 1/40 and 2.5 (max aperture) was the limit acceptable for motion blur and camera shake. I saved the pictures in RAW format. (Ricoh Caplio GX100)

Are you saying that you were using manual mode and set the speed and aperture yourself? (as opposed to letting the camera control one or the other...)

I tried to use both the camera's setting ISO 80 and ISO 200. However in post processing, I increased exposure a stop or so. Now I still liked the result on ISO80 better.

If you were in manual, and both images were taken with the same shutter and aperture, then both images are exactly the same because both took in the exact same amount of light (theoretically, that is. A cloud could have passed over you while switching the ISO to 200...that could have a significant effect on the image.)

My question is if the ISO affects the gain when the pixels are read, before digitization or after when JPEG is generated? And how does ISO-change apply actually?

ISO gain is applied to the analog sensor levels before being “digitized,” then the RAW is created, and if set to do so, a raster image is created from the RAW. That raster image is then compressed using the JPEG compression scheme and then stored on the flash card.

If you actually did as you described, where the camera was in manual mode, set to a specific shutter and aperture, and all you changed was the ISO, then here’s what you did... In your ISO 80 image the RAW file simply contains the sensor data. In your ISO 200 image the RAW file contain an amplified version of your ISO 80 image. The reason that is so is because you didn’t change the shutter or aperture. Both images took in the same exact amount of light. Remember that changing the ISO doesn’t affect the light levels...only what’s done with the light after it’s captured.

So now you’re editing. You should be able to increase the Exposure Value of the ISO 80 image by just over 1 stop (1.2? I forget exactly what it is. Someone will know and pipe in) and get an image that is practically indistinguishable from the ISO 200 image. Why? Because by increasing the EV by +1.x you essentially did exactly what the camera did when shooting at ISO amplified the signal. The only difference is that much testing has shown that the camera usually does a slightly better job (has to do with quantization errors...not worth discussing now.) The key to understanding all this is to realize that you can only do so much with the light that is taken in during a capture. If two captures take in the same amount of light then you'll get similar results from either high ISO in camera, or EV adjustments on the computer.

That's (log base whatever of (200 divided by 80)) all divided by (log base whatever of 2) = 1.32 where you can use log (base 10) or ln (base e) as log base whatever

Other than the quantization errors as Graystar mentions which you won't even see for this small a boost, some cameras have more deep shadow noise at low ISO's relative to the full scale limit than they do at higher ISO's and thus applying an EV boost in raw convertsion from a lower ISO may produce more noise in the resulting image that if the image was shot at the higher ISO in the first place. This is due to noise introduced by the analog processing chain including the amplifier and the Analog to Digital Converter (ADC). I have never analysed the Ricoh Caplio GX100 so don't know if this is true or not.

Also, many cameras apply Noise Reduction (NR) even to raw for higher ISO's so you may prefer the look of a raw conversion boosted lower ISO with presumably less NR if you don't like the smudged detail and coarser blotchy noise look resulting from the NR and would prefer the more honest look of the actual fine grained noise. Again, I don't know if your Ricoh applies NR to raw or not.

Regards, GordonBGood

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