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Dave Smith
01-05-07, 17:37
I now always shoot in RAW for astrophotos or situations where post processing is likely to be severe. I was under the impression that when shooting in RAW you were saving the output of the sensor without the camera doing any processing. I therefore assumed that the ISO setting on the camera would have no effect.

However, if I do change the iso setting, the camera does make changes to the aperture or exposure time depending on which mode I am in.

So what is happening in RAW mode?

If I use manual mode and take two raw images with diferent iso settings will there be any difference?

I would be very grateful for clarification on this.

Dave

yelvertoft
01-05-07, 18:52
The ISO setting of the camera does impact the RAW data.

The ISO setting is effectively how much amplification (gain) is applied to the sensor to produce a given output. If you wind up the ISO you are increasing the amplification of the sensor; it requires less light to fall on the sensor to produce a correctly exposed image. This setting is a fundamental sensor hardware function, not a function of the processing engine which processes the data after it has been recorded by the sensor. Thus it is independent of whether you use raw or jpeg, tiff, etc.

You ask "If I use manual mode and take two raw images with diferent iso settings will there be any difference?"
The answer is yes. If you use a higher ISO, the sensor will have a higer gain and thus will need less light on it. This will be achieved by either reducing the shutter speed, or closing down the aperture. Using the higher amplification on the sensor will also amplify the electrical noise within the pixels, causing the grainy effect you are no doubt familiar with if you push the gain high enough.

Hope this helps.

Canis Vulpes
01-05-07, 19:18
The main thing about RAW is the transfer of 'RAW' Bayer data from the sensor to the card. This data is then interpreted by your RAW processing software where different 'in camera' settings can be applied as if from the camera itself.

Dave Smith
01-05-07, 20:21
Many thanks to you both. I had always assumed that the amplification came after the sensor.

Another lesson learned:)

Dave