The idea to test a new tube lens configuration was born reading the excellent blog of Robert OToole (https://www.closeuphotography.com/tube-lens-test ) and the blog on macrophotography (http://www.photomacrography.net/forum/viewtopic.php?f=25&t=37843&sid=384d0c0177d9563f5a37e023ecba7a61 ) in which it is tested a lens tube shorter than the canonical 208mm in association with a Mitutoyo 5X lens.
The set-up necessary to mount this tube lens requires, in addition to a corrective lens Raynox DCR-150, a tube of 50mm situated between the front lens of the Raynox and the objective (Mitutoyo 5x), and an extension tube of 144mm. The length of the tube must to be calculated between the camera sensor and the base of the corrective lens. In the following image the complete set-up with the different adapters:
B: 49 to 37mm
C: 48 to 49mm
D: 40mm extension (tube diameter 49mm)
E: 49 to 48mm
F: Rayox DCR-150 (not inverted)
G: 42 to 43mm
H: 43mm adapter ring for the extension tube
I: Extension tube (125mm)
The length of the extension tube varies function on the distance between the sensor position and the camera attachment flange.
According to Robert “Short focus, or using a lens with less extension than is needed for infinity focus means that it is “focused” beyond infinity. This would create an unusable blurry image, but in many cases this happens to work well with tube lenses – as described here. Short focus results in shorter “effective focal length” used here for magnification calculation purposes, so for example the Raynox +4.8, 208mm lens, with short focus is a 178mm effective FL lens, this would give you 0.859 x the Mitutoyo objectives marked magnification, for the Mitutoyo 5x M Plan lens this would be 4.29x. Compared to a down-sized normal focus image, the pushed down/short focus image is sharper, cleaner and more detailed.(…) This lens was tested in 9 different configurations, some of these, especially those with shorter extension, were excellent.”
The result you get is really interesting. I made the first test using as a subject a moth, acquired using the tube at 208mm with Raynox DCM-150 inverted. The image obtained with the short tube shows a slight reduction of the image, but the same is very well contrasted and sharp. I have not yet tried with other types of lenses (2.5X, 10x, and 20x), but the result obtained seems very promising.
The images below are unprocessed (no denoise or sharpen applyied). at left the image acquired with the 208mm tube length with inverted Raynox DCR-150, at right with the short tube length.