The traditional layout of a complete 'compound' microscope is shown here. The objective forms a real, magnified and inverted image because the sample is further from the lens than its focus. The image is 'real' because it can be projected on a screen - a slide projector produces a real image in this way (and we therefore have to put slides in upside down). The eyepiece is placed quite close to this real image - too close to form a real image of it. Instead the rays which reach the eye appear to come from a magnified 'virtual image' located further away. The virtual image is not inverted, so in the end we always see an inverted image of the specimen.

Modern research microscopes modify this layout a bit. The problem with the simple arrangement is that the distance between objective and eyepiece must be absolutely fixed, since spherical aberration can only be corrected for one position of the image. If we want to add in components for fluorescence, polarization and so on we are in trouble. Modern objectives put the specimen at the focus of the lens, so they will form an image 'at infinity' - that is, the rays from any one point on the sample leave the lens parallel to each other. This won't form an actual image, so an additional lens, the tube lens brings the rays to a focus just in front of the eyepiece, as before. The diagram below shows this layout, and indicates where each component is in the actual microscope.

The great advantage of this plan is that it doesn't matter (within reason) what the distance is between the objective and the tube lens - the rays are parallel and so the SA correction is unaffected. There is a limit, of course, or rays from objects at the edge of the field of view will get cut off. Nevertheless the few centimetres of free space we gain are very valuable.

The other feature of a modern research microscope is that the illumination system is built in. Abbe showed that when we view an object with transmitted light diffraction at the sample, not just the objective, limits our resolution. We therefore need a condenser lens to illuminate the sample with an NA matching that of the objective. Since the illuminator has to be aligned with the condenser it makes sense to build this into the microscope as well.

Just because the condenser and illuminator are built on does not absolve the user from the need to adjust them correctly, and the next section explains how to optimise the system. Do not fall into the trap of assuming that if you are just doing fluorescence or confocal you don't need this. You will almost always want to capture a phase or DIC image to match your fluorescence, and if you are going to use the (non-confocal) transmission detector built into most confocal microscopes the condenser and illuminator must be accurately set up or the image will be terrible.