I just got this email from fixation. The buyer really needed to do his research and so did I lol.
J*** here, the person behind the Fixation twitter page.
I spoke to a technician about your images and we looked at them together. In his opinion your lens is performing as we would expect it to as there are a few important things to be aware of with the 50mm.
1. At maximum aperture f1.2 the lens will deliver lower contrast in your images. This is clear in your last picture of the young girl, we zoomed in on the picture and there is plenty of sharp detail on her hair and definition in her eyebrows and the velvet bow however the detail doesn't "pop" as the colours do not have much contrast. This should be possible to bring back with some subtle contrast adjustment and sharpening in photoshop or similar post-production software.
2. Very well demonstrated in your series of jewellery images; as you open the aperture the plane of focus moves slightly. At smaller appertures this does not often affect image quality as the depth of field is wide enough to compensate but with the lens wide open this can cause significant focus inaccuracy.
3. The depth of field is incredibly shallow. At the lens' closest focusing distance 45cm the depth of field is only 3mm, any detail outside of that narrow range will begin to blur and foreground detail may further soften the area inside your plane of focus.
I can see all 3 of these issues causing an effect on your images, especially the progression of jewellery images, which range from higher contrast with a broader depth of field at 2.4 to a softer more ethereal look at 1.2 which still retains edge sharpness on a small slice of the image visible in the fourth bead from the left.
For your studio, tripod set-up the technician had one recommendation, rather than relying on the cameras autofocus system you may get better results focusing manually with live-view, this can be zoomed in 10x and you can see the image as it is projected on the sensor so you can be sure that the right point is truly focussed, this will give you a greater amount of control.
I hope this has helped, if you have any further queries don't hesitate to email me direct on: