Hugin Exposure tab
As explained on the Hugin Photos tab, hugin has a brightness and colour correction system using the default nona stitching tool.
Hugin comes with an extended set of predefined settings for photometric optimisation. If these predefined options do not give the desired results you can define your custom parameters in this Hugin Exposure tab.
The images list behaves very similar like the image list on the Photos tab. So can also in this tab add/remove images, change the lens or stack assignments, link or unlink image variables,... Beside the features of the list on the Photos tab it provides also in-place editing of single image variables. (Note: If you want to modify a value for a lens, you need to single-click the value, not double-click it.) If you want to change image variables from several images at once, select all affected images and choice Edit image variables... from the context menu.
With Ctrl+Left click you define which parameters will be optimised. The context menu provides short cuts to select the parameters of all images, of all images of a selected lens or of all image of a single stack.
In the Image variables part of the screen you can modify the exposure EV, the red multiplier Er and the blue multiplier Eb.
The Red multiplier and Blue multiplier settings are used to account for changes in white balance (also known as colour balance or colour temperature) between photos. Setting them both to 1 will result in no white balance change (the numbers are relative to the green channel which stays unaltered).
As with exposure, different photos in the same project are quite likely to require different white balance. Typically this will be caused by variations in lighting conditions between shots - For example a cloudy scene will contain considerably less red light than the same scene under direct sunlight.
Camera and Lens variables
The values that you can change in the Camera and Lens variables table is again dependent on the Simple, Advanced or Expert User interface.
Vignetting is dependent mainly on your lens and the aperture. Usually the centre of the image is brighter with a falloff towards the edges. The three numbers here represent a polynomial curve used by hugin to correct vignetting.
You are not expected to guess these values, they are generally loaded with a lens profile or calculated from two or more overlapping photos.
Set the values to 0,0,0 for no vignetting correction.
Usually all photos taken with the same lens will have the same vignetting, keep Link checked to ensure hugin applies the same vignetting correction to all photos with the selected lens number.
Vignetting Center Shift
The centre of vignetting is rarely the exact centre of the photo. The two numbers here indicate the position of the vignetting centre. The scale is in pixels, with 0,0 indicating the centre of the photo. The values are independent of the d & e parameters which specify the origin for projection and geometric distortion.
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.
hugin uses the EMoR response model from the Computer Vision Lab at Columbia University which simplifies the full response curve to these five empirical coefficient numbers. You are not expected to guess these values, they are generally loaded with a lens profile or calculated from two or more overlapping photos.
Set the five numbers to 0,0,0,0,0 to use a generic response curve or change the Type to Linear to indicate that your input photos have a scene-referred or HDR response.
Keep Link checked to ensure hugin applies the same response curve to all photos with the selected lens number.