The preview isn't a good indicator of how your final scene will look, particularly when it comes to lights.  Often, a light of high intensity may give the impression that it's illuminating the whole scene but it's actually only working on a small section.  Objects casting shadows can also have a dramatic effect on the amount of light hitting the parts you want to be bright.

OTOH, if your preview is too dark, you can take advantage of this shortcoming by adding in extra lights which don't render, but will make your preview scene easier to see.

Coppula eam se non posit acceptera jocularum.

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