My Nikonscan workflow to deal with the curvature of slides is to measure the focus points at various places on the slide using Ctrl-focus, and then, based on the range of focus distances my measurements returned, make several scans using focus bracketing by manually setting a series of focus distances (set in the Nikonscan software), and then stack and mask in Photoshop to give me grain-sharp scans across the entire image. I wonder if this workflow can be replicated in Vuescan? I see how to set the focus point using the x & y offsets, but don't see a way to get a numerical value for the focus distance at the selected offset and then manally enter that value to perform a "manual focus" scan. Is that possible?http://www.hamrick.com/vuescan/html/vuesc28.htm#inputautofocus
You typically autofocus on the spot you want, then switch to manual focus (which should give the focus position found) and vary the focus distance. Due to potential heating up of the film inside the scanner, if you wait too long the focus plane may have moved. Just let the film acclimatize a bit inside the scanner before doing the actual setting of the focus position.