I work in a lab which deals with a significant amount of scientific photography. My current situation calls for an increase in magnification, however, there is a physical limit to how close the lens can be to the object. This imposed working distance is far greater than the minimum working distance of the lens itself.
Lens: Nikon AF Micro 60 mm; Working distance at closest focus: 90 mm; Physical minimum working distance: 280 mm; Magnification at physical minimum distance: 0.2
In previous applications with a Nikon 210 mm lens, the close focus distance was several feet, so it was feasible to use a close-up lens and increase magnification by moving the camera closer. This is not the case here. My primary questions are:
1) Would extension tubes be of any benefit here, since the camera/lens cannot be moved closer to the object? Stated in a general way, if the distance between the lens and object remains fixed and an extension tube is added, would the magnification increase? I am aware of the rule that magnification increases by the length of the tube divided by lens focal length, but does this assume that the lens is moved closer to the object?
2) If extension tubes will not help, are teleconverters the only other option? I have used a 3x teleconverter in the past with the above mentioned 210 mm zoom lens.
I would like to get the magnification close to 1:1, and if I was able to fine-tune the magnification, this would be an asset. The camera and lens have plenty of room to move away from the object, if necessary.
Edit: additional details on the application below:
The camera is tasked with capturing laser-illuminated oil droplets suspended in moving air (a technique known as particle image velocimetry). This airflow is contained in a sealed spherical chamber with 4 windows. The camera must be placed on the outside of this chamber for obvious reasons, and therefore it can only be moved up so far before the front lens element bumps into the window. When the camera hits the window, the front lens element is 280 mm away from the "object" (the flow field of interest).