Why are dark areas noisier than well lit areas?

Whenever I render scenes using cycles, it seems like I need far less samples to get a clean looking image in scenes where there are plenty of lights, but scenes where there are shadows require far more samples to have the same clarity. Is there a reason for this?

It’s because you have more samples getting to the brighter areas because it’s easier for the ray to find the light.

In darker areas, a ray might have to bounce many times and hope it finds the light, therefore you have less rays finding the light and less samples that actually contribute to the scene.

Ways of improving convergence include raising the min bounces value, clamping the value of the indirect light, raising the filter glossy value, and lightpath tricks that use the ray depth parameter.

Don’t forget that you can add more light in a render, compressing the tonal-range to make the problem easier to solve, and then re-adjust the contrast, hue and saturation “in post.” (A very similar concern exists in film-photography, in which you must be sure that detail exists in both highlights and shadows. If it is there, you can always adjust things in the darkroom.)

The Cycles algorithm obviously converges upon a solution, and light-levels are a big part of how well and how fast it finds one. But “the render” isn’t the last-step in creating “what you see.”

Okay, so can one of you point me to any good tutorials that talk about editing the darkness in post to make a scene look like it’s in the night time? I’ve tried this a few times and added a blue tint to the rendered image to make it seem like it’s night, but the problem is that just flat out tinting a scene gets rid of some of the light you would get from things like a fire or lamp for instance. Is there a technique that can darken a scene without destroying sources of light?

Do you mean something kind of like this?

I’d have to go back and look at my compositing set up to be completely sure, but I believe what I did here was that I had a separate layer for the phone screen that was mixed or multiplied (I’d have to look to see which I did) after the post tint. It was a year ago when I did this, so I don’t remember exactly where I found a tutorial on this, but it was most likely either on Blender Guru or CG Cookie or one of several YouTube channels.

There are a bunch of tuts out there for photoshop to turn day photos to night. Give that a shot. Your fires and stuff can just be rendered on separate layers.
Here is one:

Most movies with night scenes are actually shot in the day.

Hollywood was famous for shooting “night” scenes in broad daylight. Render the scene in layers, to separate files as needed, concentrating foremost on getting a good, clean, tonal-range in each shot. Make sure that there is detail in both the shadowed areas and the ones that are brightly lit. (Use MultiLayer OpenEXR files, which are capable of capturing “HDRI” values outside of the range 0.0…1.0.)

Now, use the compositor to produce night. At night, contrast is intensified, saturation is greatly decreased, and there is a bluish or yellowish hue on everything. All of these effects can be produced, and arbitrarily fine-tuned, using compositor filters. (The “rendering,” by this point, is over and done with.) The most important thing to be sure of is that all of the necessary detail is “numerically present,” and preferably, that the numeric data-value ranges are consistent from one component shot to the next. (A “histogram” of each component looks about the same, and is more-or-less bell shaped.)

One thing I’ve noticed about Cycles is that, if the image is going to look good at all, then it’s going to start looking good very soon, with a low number of iterations. Increasing the number seems to produce less and less return-on-investment. A well-lit and fairly evenly-lit scene seems to produce the best overall results. “So be it. You can take it from there.” Use the renderer to get the inputs that you require to feed into the compositing (“digital darkroom” …) that you use to build the final shot. As long as the detail is present, and reasonably noise-free, you can take it from there.