The role of the condenser is to provide the illumination that will be diffracted by the specimen. As we have seen from the Abbe equation the condenser must have a similar N.A. to the objective. A condenser with to low of an N.A. will reduce the resolution of the system.

Condensers are classified by their optical type and by the techniques that they have built in. The two basic types are Abbe and the Achromatic-Aplanatic. The most popular is the Abbe condenser. This is a simple condenser designed by Dr. Abbe that is adequate for most bright field and phase purposes. The Abbe condenser is not highly corrected for flatness of its field or for color, but then it doesn't have to be. Mostly for visual use the Abbe is just fine. Abbe optics are used in all swing-out condensers and most phase condensers.

A swing-out condenser is corrects one problem with all condensers. A condenser that will perform well at forty may not be able to fill the specimen field at 4X. The problem is greater at 1X and 2X. The swing out allows the user to remove the top set of lenses by swinging them out of the way when the lower powers are used. This provides full field illumination over a wide range of magnifications. What this does is to reduce the N.A. of the condenser and increase the field size.

A.O. its remnants went at the problem differently. They have mostly used a swing in lens. This is located under the condenser and is swung in at low powers. This lens reduces the N.A. of the condenser and must be removed at higher powers. While this type of lens and condenser works it doesn't work as well as a swing out condenser.

Most phase condensers use Abbe optics and the "pancake" design. They are called "pancakes" since they are round and flat. The flat, round part is used to house a ring containing the phase rings. Each manufacturer structures there phase system differently so phase objectives from one maker will not work with phase condensers from another.

A lot of phase condensers contain simple dark field rings. While this may seem useful I do not know a lot of users who actually use this feature. The dark field that you get from this type of set up is dim and hard to work with. This is more a sales feature than a user feature. In fact most phase condensers can be used for low power, dry dark field. Usually the phase ring for the highest phase power if used with a 2X to 10X objective will give very nice dark field. This can be a help if you are trying to find the specimen. A unstained specimen really shows up under dark field.

All phase condensers will have some way of centering the phase ring. The better makers allow you to center one phase ring and all the rest will be centered. Some make you center every ring. The exact centering procedure is different for each make so don't lose those instructions!

Long working distance condensers are built for some types of industrial uses and for inverted microscopes. The longer working distance is needed on an inverted microscope so that large vessels and titre plate can be used. Long working distance condensers have lower NA's than short working distance condensers. This is a good trade off for a lot of applications were you are not going to be using high performance objectives any way.

Be aware though that this is a limiting factor on the performance of many inverteds. If you try to use an oil 100X on an inverted the image may not look to good. This is caused by the lack of NA in the condenser. If you change to a high NA condenser you may loose the working distance you need.

Achromatic-Aplanatic condensers (AAC) are much better corrected for flatness of their field and for color. If you are doing photography or video through the microscope you should use an Achromatic-Aplanatic. These condensers also have a very high NA. This is very good for high performance microscopy such as cyto-genetics but it means that the condenser won't provide full field illumination at powers under 10X in most cases.

Achromatic-Aplanatic optics are used in most Nomarski condensers. Nomarski condensers are usually of the pancake type. The pancake contains a ring that holds the various Wollaston prisms. Since each manufacturer varies in how they make their prisms you need to use a condenser and objectives from the same manufacturer.

A Nomarski condenser must also have a polarizer that can be moved in and out of the light path. Usually the polarizer can be adjusted. The easier this is to use the better. However since the polarizer has to be under the condenser it usually isn't very easy to use.

Achromatic-Aplanitic Nomarski condensers make very good condensers for photography but you do have to remove all Wollaston prisms from the light path if you want to use bright field. A Wollaston prism will cause a shading of the image from one side to the other if left in. Usually you eye can't pick this up but a camera, film or video, will.

Dark field condenser come in two types; dry dark field and oil dark field. All really usable dark field condensers are Ultra condenser types that use semi-circular mirrors to direct the light. This makes them much more efficient. Since dry dark field condensers are limited in their NA they can not be used with high NA objectives. Usually the highest NA objective that can be used with a dry dark field condenser is a 40X with a .70 NA. This means that dry dark field is a commercial non-entity. It just doesn't sell, however all good manufacturers make it so if you need it you can get it.

Oil dark field is the tool of choice is you have to do dark field. The only draw back with oil dark field besides the general problems with dark field is that oil dark field condensers will not work very well with low power objectives. Usually they can cover down to 20X and then you will start getting dark field in the center of the field only.

Most manufacturers make a universal condenser. These condensers are usually pancake types that contain phase, Nomarski and simple dark field. This may be the only Nomarski condenser the manufacturer has. The phase and Nomarski will work quite well, the dark field won't. You can't put a modern dark field condenser in a pancake type condenser. If you need real dark field and bright field you will need two condensers.

Changing condensers is really no big deal, unless you own a Reichert 410 with the simple condenser carrier and then it is. Most manufacturers allow you to remove the condenser with out changing the centeration. You may only need to change the height of the carrier to get the condenser out. Make sure the rep. show you how to change condenser.

If you do a lot of low power photography through the microscope you will need a low power condenser. While is possible to use a swing out condenser or just remove the condenser all together a low power condenser will provide better resolution, evenness of field and controllable contrast. These condensers are used up to approximately 4X. They are a real help for low power work.

What this means to you Condensers are rated on their optical type and whether they can do optical staining or not.