Alex,
Results from today.
I tried the 28mm reversed onto tubes but magnification was no increase on what you are currently getting and the viewfinder is really quite dim making it hard to even find the subject never mind focus on it.
I tried a different tack. Reverse the 28 onto a 2x converter. Now my converter is pretty rubbish ( recyled coke bottle job
), so look on these images from a magnification point of view only. This combo reduces the field of view to 5.5mm. Considerable magnification increase on 50 reversed on to a 200 at 11mm field of view.
A quality 1.4 converter would no doubt produce better quality at a bit less magnification. For anyone trying this, ensure all glass surfaces are clean. Any crud will show up very sharply on the image.
The downside of this degree of magnification is focusing. A focusing rail absolutely essential. I cheated and set this up on my milling table as I do not have a focusing rail but it proved the point.
Flash was as per your set up diffused about 8 inches from subject to get an aperture of f16. Then reduced to 1/2 power for the shot without converter.
The attatched images are all full frame. The first two are 2x converter with reversed 28 and the last just a reversed 28 for comparison. The subject is the dreaded green fly. Adults are around 2.8 to 3mm long. The warmth of the workshop seemed to have them multiplying at an alarming rate.
Don