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JJBurke
June 11th, 2006, 07:38 PM
Hi!

I have read in the past (and can't find it now) of a procedure to “push” the exposure of the 5D like we did with film in the past. The purpose is even cleaner low-light images.

Does anybody have the right formula? If I recall, set the camera to a lower ISO, such as 800, set Exposure Compensation to -2, then, in post, push the exposure.

Did I get this right? Any other thoughts?

Thanks!

Lee Jay Fingersh
June 11th, 2006, 08:30 PM
Does anybody have the right formula? If I recall, set the camera to a lower ISO, such as 800, set Exposure Compensation to -2, then, in post, push the exposure.


That's the right procedure but don't ever do what you said. What you proposed is exposure index 3200 (800 pushed by 2 stops). If you need 3200 either shoot at 3200 or push 1600 by one stop (underexpose by one stop and push by one stop in RAW conversion).

1600 is the camera's maximum native ISO setting (highest amplifier gain). 3200 is 1600 pushed by one stop in the camera. I've successfully pushed ISO 1600 by 3 stops on the 5D converted with RSP. That's exposure index 12,800. I personally find that pushing 1600 by 2 stops is better than pushing 3200 by 1 stop, perhaps because the external RAW converters use some better math in the conversion. It also could be my imagination.

Lee Jay

Asher Kelman
June 11th, 2006, 08:48 PM
That's the right procedure but don't ever do what you said. What you proposed is exposure index 3200 (800 pushed by 2 stops). If you need 3200 either shoot at 3200 or push 1600 by one stop (underexpose by one stop and push by one stop in RAW conversion).

1600 is the camera's maximum native ISO setting (highest amplifier gain). 3200 is 1600 pushed by one stop in the camera. I've successfully pushed ISO 1600 by 3 stops on the 5D converted with RSP. That's exposure index 12,800. I personally find that pushing 1600 by 2 stops is better than pushing 3200 by 1 stop, perhaps because the external RAW converters use some better math in the conversion. It also could be my imagination.

Lee Jay

Lee, This is something that is entirely testable. Want to try?

Asher

Lee Jay Fingersh
June 12th, 2006, 02:33 PM
Lee, This is something that is entirely testable. Want to try?

Asher

I have tried it. The difference isn't much, if it's there at all. I think I see a little less noise and a little smoother tonal gradations when I push from 1600 compared to pushing from 3200.

Lee Jay

JJBurke
June 12th, 2006, 09:55 PM
Awesome! This is why I posted!

Thanks!

JJ

Michael Tapes
June 13th, 2006, 05:04 AM
I would also think that it depends on the lighting conditions. I have always believed in the shoot at he require ISO, as pushing it will deliver lessor results. Based on this post I will do some experiments and report back.

Bart_van_der_Wolf
June 14th, 2006, 03:11 AM
I would also think that it depends on the lighting conditions. I have always believed in the shoot at he require ISO, as pushing it will deliver lessor results.

Yes, I agree. First decide on what you want to capture and decide on the trade-off between aperture and shutterspeed. Then worry about the ISO needed to get a technically correct exposure level (i.e. "expose to the right").

However, from a technical point of view, from most reliable tests I've seen the evidence seems to indicate that up to and including ISO 1600 the 'sensitivity' boost is performed with an 'analog gain' amplification of the original signal levels prior to quantization (the analog gain is an input parameter for the ADC). At ISO 3200 the signal is basically the quantized ISO 1600 result, multiplied by 2, before linear to adjusted gamma conversion.

The reason that 'analog gain' produces lower noise, is because at the lower ISO settings each DN (digital number) is built from multiple photons per DN (even the shadow ones) so it is less prone to (quantization) rounding errors. There comes a point in low exposure levels where very few photons are converted to electrons, maybe 1 per digital number (especially in the shadows). Then there is no averaging benefit possible, and any method of boosting the signal levels will produce the same result as postprocessing.

The above is somewhat simplified to explain the principle, and I am aware that a signal level of 1 electron will in most cases be swamped by Photon shot noise and read noise, to mention a few noise sources.

Another point of attention is that by boosting the ISO, the dynamic range is reduced . But then that's just a reminder to use as low an ISO as feasible to allow a good capture.
There is little use in getting a technically good shot that suffers from motion/camera shake artifacts. Getting the shot is the prime concern.

Bart