Wednesday, August 24, 2011

Microscope Objective Tube Length

Microscope objectives are typically either a fixed tube length, or Infinity Corrected. A fixed tube length objective might have the number 160 printed on the objective, while an infinity corrected objective lens would have this: ∞ printed on it.

A fixed tube length objective lens has a finite tube length. If 160 is printed on the microscope objective, this means that 160mm is the distance from the opening of the nosepiece (where the microscope objective is screwed into the microscope) to the top of the observation tube (where the eyepiece is inserted). If this distance were to be lengthened (for example by inserting accessories in the light path above the nosepiece), spherical aberration would result.

Infinity corrected optics are more common, and allow the distance between the nosepiece and the top of the observation tube to be altered. Infinity corrected optics project emerging rays of parallel bundles projected toward infinity.

Infinity corrected microscope objectives can not be used with fixed tube length microscope systems because the fixed tube length microscope system does not include a tube lens to bring the parallel rays of light into focus. Alternatively, using a fixed tube length objective lens on an Infinity optical system microscope may produce an image, but it most likely will be a deteriorated image.

You can learn more about mixing microscope objectives here.