It sound like you have a slider where the user can set some kind of parameter. And the parameter means different things depending on what pop-up item is selected. Is that right? So what I'd do is to have a separate function called AdjustImageContrast(), which would take handles as an input argument. Then in the callback for the popup, I'd check the selected item, and if it's "imadjust" I'd first initialize the slider min, max, and value properties since these might be different depending on what popup item was selected. Then after setting up the slider I'd call AdjustImageContrast(handles). Inside AdjustImageContrast, you'd get the slider value via get(handles.slider1, 'value') and do the contrast adjustment code.
Now, what if the user slides the slider instead of selecting a popup item? In that case, you'd do essentially the same thing as the popup callback - check which popup was selected EXCEPT that you don't need to reinitialize the slider since you need to take the current value the user set it to. So if the "imadjust" was already selected, you'd skip the slider initialization and immediately call AdjustImageContrast(handles).
I hope you followed all that.
