Hugin Exposure tab
TODO explain why there is an Exposure tab.
Camera and Lens variables
The Camera and Lens variables are the photometric analog of the geometric lens correction model, hugin assumes that all input photos with the same lens number have identical values unless they are unlinked in the hugin Camera and Lens tab.
Vignetting is dependent mainly on your lens and the aperture. Usually the centre of the image is brighter with a falloff towards the edges, hugin can calculate this falloff curve as part of the photometric optimisation process or you can enter it manually in the hugin Camera and Lens tab as the three numbers shown here.
The centre of vignetting is rarely the exact centre of the photo, hugin can optimise this position or you can enter it manually in the hugin Camera and Lens tab. The scale is in pixels, with 0,0 indicating the centre of the photo (TODO is this relative to the d & e parameters?)
hugin can optimise the camera response curve by comparing differences between overlapping images. To do this your photos need to either have significant vignetting or have variable exposure. If your photos have perfectly even exposure and zero vignetting, then you would have to calibrate the camera response separately and then enter it manually in the hugin Camera and Lens tab.
The camera response curve is used both for mapping the images to a linear colourspace when creating HDR output, and for normalising the colourspace for internal vignetting, brightness and colour corrections when creating 'normal' LDR output. If your pictures don't require such corrections then you don't really need a calibrated response curve.
Hugin uses the EMoR response model from the Computer Vision Lab at Columbia University which simplifies the full response curve to five empirical coefficient numbers.