Backlit mayfly spinner
Frontlit mayfly spinner
The use of extension tubes allows for greater "magnification" but greatly reduces the depth of field essentially making the focus plane extremly narrow. On a full frame sensor like the Nikon D3S, using extension tubes also causes some vignetting--darkening of the corners. These images were taken handheld using an iso of 1,600 in order to get a shutter speed fast enough and an aperture around f10 for a reasonable level of depth of field. On cold mornings mayflies and bugs in general are far more still and much easier to capture...
Camera: Nikon D3S
Lens: Nikon Micro 105 with extension tubes