What happens as we turn the ISO down?

[Mike Burke]

In other words, when shooting and recording, all the ASA rating means (other than for monitoring purposes) is that you are biasing your exposure either towards more highlight information or more shadow information. The sensor is what it is in terms of sensitivity and you are just exposing for a certain range of the real world subject.

I am assuming that any specs related to rating this sensors' sensitivity are
based on viewing in the "Raw mode".
That being said, when viewing or monitoring in "Red Space",
there is close to a stop difference in sensitivity, when compared to Raw.
REC 709 is somewhere in between.

This would seem to allow for potential errors in exposure, depending on what method of file transfer/post/finish is chosen.
Can anyone shed any light on this?

 

[A. Järvinen]

So, just to summarize, RED's dynamic range is the same all the time since there is no way to change the variable gain amplifier. Here's how dynamic range goes on a DSLR (I'll use the 1D3 for example). This for a fixed exposure (same f-number and shutter speed, but different ISO):

• ISO 100 has the most dynamic range and the least photon shot noise throughout the exposure range. Read noise is the strongest, but it's at the lowest exposure zone, so contributes the least to the total image as long as shadows are not pushed. 11.8 stops of dynamic range using 1:1 SNR per pixel. 7.5 stops of dynamic range using a standard of 4:1 S/N per pixel.
• ISO 200 - one stop of the highest exposure zone is clipped, but read noise is *much* better than ISO 100. Almost enough to make dynamic range the same (more shadow room, less highlight headroom). 11.6 DR @ 1:1 S/N. 6.8 DR @ 4:1 S/N.
• ISO 400 - Read noise is again improved very much, but not as much as the improvement from 100 to 200. 11.3 DR @ 1:1 S/N. 6.0 DR at 4:1 S/N.
• ISO 800 - Same trend. Read noise improved somewhat, but not as much as 200 to 400. 10.8 DR @ 1:1 S/N. 5.2 DR @ 4:1 S/N.
• ISO 1600 - Same trend. 10.1 DR @ 1:1 S/N. 4.2 DR @ 4:1 S/N.
• ISO 3200 - Trend has topped out. No more improvement in read noise. 9.1 DR @ 1:1 S/N. 3.2 DR @ 4:1 S/N.

Thanks for your explanation!
So in this case analog gain allows to 'see more into the shadows', about the same amount that was clipped from highlights and so the dynamic range is almost same? Did I understand correctly? Is read noise from converting the charge in photosites to digital signal?

A. Järvinen

 

[Daniel Browning]

So in this case analog gain allows to 'see more into the shadows',

Yes.

So in this case analog gain allows to 'see more into the shadows', about the same amount that was clipped from highlights and so the dynamic range is almost same?

Whether it's "almost" or not depends on the circumstances. For the 1D3, if you accept a relatively noisy 2:1 S/N, the dynamic range from ISO 100 to ISO 200 is reduces dynamic range by less than 1/3 stop. If you require a cleaner 4:1 S/N for your shadows, though, then going from ISO 400 to ISO 800 drops dynamic range by almost an entire stop.

What you can tolerate for acceptable S/N depends on the display resolution and display size (standard definition DVD can get away with much higher noise levels than 4K projection), as well as taste and discretion.

Is read noise from converting the charge in photosites to digital signal?

Yes. There are two basic sources of noise in an image:

• Natural Noise
• Un-natural Noise

The only natural noise is called Photon shot noise. It's part of the basic and fundamental nature of light itself. The amount of photon shot noise present in the image is determined by the exposure. Higher exposure (i.e. brighter parts of the image) have less photon shot noise. Dimmer exposure (i.e. midtones and shadows) have more photon shot noise.

All other types of noise come from the necessary imperfection of our photographic tools: read noise, pattern noise, thermal noise, hot/dead pixels, amp noise, PRNU, etc.

At base amplification, for most cameras, photon shot noise is the only noise present in the midtones and highlights. Shadows are a mix of photon shot noise as well as read noise.

As gain is increased, read noise starts to affect more of the exposure zones, pushing "photon shot noise only" into higher and higher regions. Improvements in read noise allow the image to be underexposed without contaminating high exposure zones with more read noise: photon shot noise will still get worse, but in S35-sized sensors there is room for plenty more photon shot noise before the image is ruined.

 

[J. Eric Camp]

Good thread

 

[Harry Clark]

Wow.
I had not looked at this thread since my original reply.
But lots of good info, guys!
Cheers,
Harry

 

[DS Williams]

Daniel Browning you are a gold mine of information!