I'm using a Nikon D5000 and I think I get more noise using Active D-Lighting on Auto. If I turn it off, how can I simulate A D-Lighting in post processing? I shoot RAW.
Active D-Lighting intentionally underexposes the image and then boosts the shadows (and applies other adjustments) hence the additional noise you're noticing. This is done to increase the highlight headroom and prevent losing information by blowing the highlights.
You can get a similar effect in most raw processing software by using custom tonecurves, or more advanced features such as fill light in Adobe Camera Raw, but if you turn D-Lighting off you must remember to underexpose as lost highlight information cannot be recovered in post!
If you are shooting in RAW then you can easily turn D-Lighting off and on at will as well as adjust the strength. This is done in Nikon NX2 software which you can download a 60 day trial for free from Nikon's web site ... http://imaging.nikon.com/products/imaging/lineup/software/capturenx2/
I use this and ONLY this for all my post processing. Once you understand how to use NX2 it is very powerful and you can replicate the effect of practically any camera setting when processing RAW images.
Good luck! Barry
I usually leave Active D-Lighting on at all times, though typically set to low or Auto. That extra 1/3 of a stop or so that the camera actively makes a decision to reduce tends to do well to preserve some of the highlight detail in bright scenes. The whole process is basically dynamic range compression and it does elevate a little bit of shadow noise, but I like the look of it. I have tried to simulate ADL in Lightroom and Photoshop, but it doesn't look as convincing. There's something particular about Nikon's curve that I haven't been able to emulate precisely.