Thank you for the comments
sometimes I wanted to push the limits to get a maximum of the precision
but I find myself limited grrrrr
may be there are some setting to get more quality
but this is true in some raison this is an acceptable result with about 30 to 50 min rendering
by the way
What's the difference between TIF, BMP and JPEG picture
why in TG we can't save it with Jpeg for example, i think with jpeg we can reduce the size ?
for example after render i save this image with TIF format (size = 2 Mo )

but with jpeg format its take only 137 ko

and I don't think there is loss of quality
So why not give directly the Jpeg format in TG ?
Sorry about my questions