The problem is that we have only one blur radius per blur module, but if I understand well the more an object is in the distance the bigger blur radius has to be. So using the blur module with a gradient is ineffective. I think LR has more advanced blur functionalities which adapt blur radius to predicted depth map or something like this.
The blur module works well for easy cases were you need an accurate lens-like blur, for example masking an uniformly blurred background. But I don’t think you could achieve a graduated blur except with the solution you mentioned (multiple instances).
I appreciate that a lot of people were trying to add something, but I am disappointed that a question about processing specifically in darktable was used by some to free-associate about AI or whatever.
This makes asking questions here tiresome, because genuinely helpful answers get lost in the noise.
No convincing, how? You may want to post a picture with what you’ve done and see if we can do better. I would have used a blur + a gradient mask, but maybe you tried that?
I think with a real lens and real distances, the blur would become larger the further you are away from the focal plane.
Simply creating a blurred background and using a gradient as the “depth map” would mix the original image with the blurred image, keeping the blur diameter the same, only making the effect more or less pronounced. So, a point light source slightly behind/in front of the focal plane would have a large blur diameter, creating a faint, but large disc, where it should create a bright, small disc instead.
In darktable you can’t even mask blur manually, as the subject will be leaking into the background. In the end you will have to use other sw anyway. Luckily darktable has export mask feature so you will not have to redo masks.
As the manual says:
“The blurring process does not take scene depth and depth-of-field into account, but blurs the whole image as a flat object. It is therefore not suitable for creating fake depth-of-field. Using darktable’s general masking will only partially work to isolate the foreground of an image, since it will still be blurred into the background.”
Not a perfect solution but - what I did sometimes to improve “bokeh”-blur in portraits:
diffuse or sharpen module - bloom preset with higher count of iterations
drawn mask to protect main subject, use suitable mask feather plus feathering as needed (inverted drawn mask)
refine with drawn masks details treshold to bloom areas with little details more than detailed ones (which is hopefully already the case with background)
This should bloom/blur parts that are already out of focus and could reach the goal you want to achieve.