Easy question about noise

[Carlos Richter]

I asked a question at the bottom this thread but maybe no one is seeing it there: http://www.reduser.net/forum/showthread.php?t=23110&page=5

If you shoot at 320 ISO and then open the clip up in RedAlert (or any software) at 320 ISO, is there any situation where you should see noise?

 

[Noah Kadner]

Sure- if you underexposure you can easily see noise.

Noah

 

[Carlos Richter]

This is the part that is eluding me though. If I underexpose on film or digital stills I get a darker image but no noise. Are you saying that with Red there will be noise in an underexposed image even if I don't compensate in post or try to boost the exposure in RedAlert?

 

[Jannard]

You can correct to some extent in post for underexposure... but you can't "find real light" in post. If you already have underexposed footage you need to save, make sure to process carefully in RED Alert! and then use some NR if needed. In the future, try not to underexpose using all the features included on a RED ONE... like false color, shadow zebras, histograms, etc.

As for no noise in underexposed film... you can block shadows fairly easily if you under expose. Hitting correct exposure is important when shooting anything. No detail in the shadows is no detail. Trying to find it when it isn't there leads to bad things.

Jim

 

[David Mullen ASC]

Keep in mind the difference between deliberately allowing parts or all of the frame to be dark (underexposed) versus underexposure that has to be corrected in post back up to correct brightness.

Noise tends to occur when you are boosting the signal in one or more of the color channels to correct underexposure to look normal in brightness. This is why there is more blue channel noise in 3200K lighting, because technically the blue channel is underexposed relative to the other two channels since the sensor is balanced closer to daylight. But the blue channel noise is mild in 3200K if overall the exposure is good, so the most noise would be to underexpose accidentally in 3200K lighting, and most of that noise would be in the blue channel.

So in regards to your question, as long as you weren't lifting portions of the frame or the overall shadows or pushing one color channel heavily in post color-correction, the image should be fairly low in noise at 320 ASA.

Same thing happens in film -- if you underexpose, you are only exposing the largest of grains, the rest are unexposed and are washed away during processing, leaving only the bigger ones. But the graininess only becomes a problem if you attempt to print "up" the image. Also, faster films have more visible grain than slower films because larger grains are better collectors of light, hence faster films have larger grains.