The situation is complicated by:
a) Magnification "7x" would mean about 5mm field width on 36mm sensor, but only about 3mm on a typical smaller one.
b) Microscope objectives are designed, as it happens, to cover the smaller sensors well, but not a 24 x 36mm. Some can be stretched better than others.
Commonly found possible contenders to fill the role include Nikon's 4x "160" (finite) (NA 0.2 and 0.13) objectives, and several macro lenses including the Olympus 20mm (f2.0 and f3.5) and 38mm (f2.8 and f3.5).
Time and kit limit the speed with which I can look at these, I'll add to the thread as resources permit.
However, here I have pushed the 4x 0.2 right out. On full frame, that's about 9x, on a 24 x 36mm sensor. It's physically on a full PB-4 bellows plus a PN11 tube (32mm?) and a protruberance to adapt to the rms thread.
The Oly 20f2 (at f2.0)does predictably well. The 4x 0.2, predictably struggles. It's very good when used within its intended scope, so I expect it to shine in other tests, to come.
Nikon D700, Flash, Zerene Pmax. 18 added in Photoshop Brightness to the Oly pic to match tone better.
Target vertical/horizontal lines are 1mm spacing, printed from Word by inkjet printer.
Left two are the Nikon Objective, lower are 100% crops from the centre of the images. Click for full size.
The image quality of the 4x objective goes off a lot, towards the edges, as would be expected. If you're using a smaller sensor, you won't use the worst of those edges though.
The images show about 4mm field width, on a 36mm sensor, so 9x.
About 2.5 mm of the field width would become 22.5mm therefore.
