I have a mobile photo app that allows users to adjust things like brightness, contrast and saturation via sliders. The default value for all sliders is the center position between two extremes. While the values at the extreme ends of the scale are valid, and sometimes appropriate to what the user wants to do, in most cases cranking any of these things to the maximum or minimum produces ugly results. This is a problem because while doing UI testing I found that some users, to the extent that they use these sliders at all, only seem to bother trying the default (center), min and max values, and are then unsatisfied with the results.
I want to encourage them to mostly chose values in the range from half-way between the min and the center to half-way between the center and the max, while still allowing use of the min and max should the need arise. I'd rather not display any text to explain this because of limited screen real estate and also localization issues. I'd also rather not dodge the problem by introducing a weird non-linear response to the slider position or doing some automatic image processing magic under the hood to always make it look good no matter what the inputs, because it will make the interface less intuitive and predictable and some users might want the extreme behaviour.
How can I display a slider that visually conveys the meaning of "adjust in a range, but sticking mostly to the middle of the range"?
Edit: screenshot of current UI on small screen device.