Quote - "when I change to other atmospheres I really feel like I lose the realism."

Many books could be written about what makes an image realistic… :-) It's not only about atmospheres, it's also about materials, scale, light, shadow, ...

Regarding render settings, which you refer explicitely, here's what I typically do:

I always use "user settings", I never use any of the standard modes because I think they are not balanced (for example, Ultra has a too high advanced effects quality for most purposes but a poor anti-aliasing; others have other unbalanced settings).

I change my render settings depending on the image I'm doing. If I have complex clouds or many plants, I typically raise AA quality to 95%, min subrays to 8 and max subrays to 20 or so. If I have simple or no clouds and no plants (or too far away), I go with 80%, 4, 12, respectively (good for portraits, for example).

I never use texture anti-aliasing, except when I detect artifacts in textures (which happens rarely).

I have "optimize volumetric lights" and "optimize last render pass" always off, as they decrease the quality of the final render, IMO.

Normally, I have advanced effects quality at 46%, except if I have radiosity, in which case I raise it if I detect the so called "light leaks" (I raise it gradually until they disappear).

I typically never check "blurred reflection" nor "blurred transparency" nor do I use materials with those effects in my images because the render time increases dramatically. If I need to have blur, I simulate it with a very fine bump, it's much faster and the final effect is almost the same (and I have more precise control over the effect).

In AA settings, I typically use "Sharp" mode.

A very important "companion" setting to the render settings is the quality slider in the light tab of the atmosphere settings. I typically leave that at around -0.5 or -0.7 for the final render (-2 for test renders). The difference in render time between -0.5 and 0 is dramatic. If I detect blotches or other irregularities in the shadows, for example, I increase that until it disappears. With radiosity, I sometimes have to increase to 3 or 4 to get rid of those.

 

About stuff to try to make an image realistic, here's some of the things I do:

Lighting model: I never use standard nor global ambience. I always use AO, GI or (rarely) GR.

Atmosphere model: I never use standard. Sometimes (rarely), I use volumetric (colors and textures seem to be richer), but most of the times I use spectral.

Sky dome lighting gain: I typically never set it to zero. Most of the times a value between 0 and 1 is enough, other times I go for 4 or 5, depending on the image.

Secondary light sources in landscapes: I very rarely use them (almost never). I try my best to use only the sunlight but if I absolutely cant get the effect I want, I consider using a secondary light, very dark. I think that realism is heavily affected if in a lanscape there is lighting inconsistency, as if there are two or three suns. Even if you dont see it at a conscious level, subconsciously you see it.

Ambient light: in a cloudy day, I leave it at 50 or 60. In a sunny day, I set it at 80 or 90. In this latter case, I increase the sky dome lighting gain a lot to make the shadows richer.

Shadow softness: in a sunny day, I leave it at zero or very low (0.1), like in real world. In a cloudy day I set it between 1 and 5, depending on the effect. If I have shadow softness, I always increase the quality slider of the effect in the light source properties.

Shadow: I typically set the sun shadow at 85%.

Clouds: Vue has many non-realistic clouds. It's very typical to see images in the galleries here that have non realistic clouds (although beautiful). I built my own cloud material which I use all the time, with variations. Lately, I very rarely use other clouds. I can share the settings I use, if anyone's interested.

Details: if you want a realistic landscape, you must have details. Those are the kind of things that you almost dont notice but, if it's not there, you feel something is wrong. I'm talking of pebbles spread around, variations in the grass and mountains, etc.

References: I typically compare my images with photo references from the net, before, during and after the creation process. For example, a very useful site is www.istockphoto.com. Even if you dont register in the site, you can still see the images (with watermark, but for reference purposes it's not important).

Scale: to have a sense of scale, a good use of haze and fog is very important. The distant objects should partially fade in the haze or fog.

That's what I can remember for now that I do, typically. I hope they're useful but, like I said in another thread, every Vue'er will have a different opinion about many of these things. :-)