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Camera Exposure (2 replies and 1 comment)

GianniRanzuglia
2 months ago
GianniRanzuglia 2 months ago

Hey everyone,

Excuse the big newbie question but I've always been told that if you're shooting low-light on digital, you need some form of zebra/exposure that "carries the frame" otherwise you'll end up getting noise, despite being at an extremely low gain/ISO setting.

Does anyone know why this is true or have I gotten my information wrong? 

Thanks for clarifying! 

dmullenasc
2 months ago
dmullenasc 2 months ago

The short answer is no, if you are at a low ISO and using a standard gamma for viewing on a display and the overall image is not noisy, it isn't going to suddenly get noisy just because something dark or black comes into the frame. Otherwise, how could you have a black frame with white letter for titles? Or have someone turn off the lights in a scene? Or wear a black coat? 

This is easily tested, go outside where you have a lot of light for a low ISO and pan into an 18% grey card and then into a black card. Does the noise suddenly increase? (Well, if you were on AutoGain it would...). If you don't change your ISO setting, then the noise doesn't change just because the camera is looking into a darker shade rather than a lighter shade.

Now you said "shooting in low-light" which implies needing to use a higher ISO, otherwise how could you shoot in low levels of light?  So that's a bit of a contradiction there.  Let's say, for your particular camera, lens, and level of light, you need to shoot at ISO 400 at f/2 (assuming standard camera speed and shutter angle.) Does your camera produce a fairly clean image at ISO 400, at least, acceptable in terms of noise?  If so, then if you look at the recording with a standard gamma applied for viewing (if recording a log image for example), then you should be free to have darker shades in the frame and not see much noise.  Depends on the camera in terms of how noisy it is at different ISO settings.

Underexposing a sensor causes noise but that implies lifting the signal to closer to a normal brightness level, not leaving something dark, well, dark.  It really isn't underexposure if you have a shadow in the frame anymore than it is underexposure if everyone is wearing dark clothes in the scene. But as soon as you start trying to lift the shadows to see more detail, then you are defacto treating that area at a higher ISO, you are pushing the signal and getting more noise.  So there's no problem with darkness if you leave it dark and don't try and manipulate it in post.  But you do have to test your camera and see the test on a large screen to judge the noise at different ISO settings.  Only watching the image on a camera EVF while shooting and not seeing noise and then seeing noise later when you are editing it on a large computer screen just means you couldn't accurately judge your choice of ISO on the set. Which is why testing is important.

dmullenasc
2 months ago

As an earlier question brought up, I'm talking about image noise, not compression artifacts that come from recording and/or viewing a compressed 8-bit video signal that has a lot of dim shadow detail in the frame, that's a whole other problem, seeing macro-blocking and gradient artifacts in the shadows.

GianniRanzuglia
2 months ago
GianniRanzuglia 2 months ago

Thank you so much for your help. I'll definitely take your advice into account, and apply it whenever possible!!

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