Stereo and Binocular Microscope Parts Explained

Binocular vs stereo: clearing up the terms

These two words cause a lot of confusion, so it’s worth settling them first.

• Binocular describes the viewing head: it has two eyepieces, one for each eye.
• Stereo describes the optics: there are two separate light paths, viewing the specimen from slightly different angles, so your brain combines them into a three-dimensional image. (For other ways microscopes produce depth, see what microscopes make 3D images.)

A binocular compound microscope has two eyepieces, but both eyes see the same image coming up through a single objective. A prism in the head simply splits the light. The view is more comfortable than a single eyepiece, but it isn’t 3D.

A stereo microscope (also called a dissecting microscope) almost always has a binocular head too, but behind it are two separate optical paths. That’s what gives true depth perception, which is why stereo microscopes are used for dissection, insects, rocks, electronics and anything you need to handle under the lens. They work at lower magnification, typically somewhere between about 5x and 50x depending on the model. If you’re deciding which kind to buy, see stereo vs compound microscope.

Parts of a binocular compound microscope

A binocular compound microscope has all the usual compound microscope parts (see our guide to the parts of a compound microscope). The binocular head adds:

• Two eyepiece tubes, each holding a matched eyepiece.
• Beam-splitting prisms inside the head that divide the light from the objective between the two tubes.
• Interpupillary adjustment: a way to change the spacing between the eyepieces. On a Siedentopf head, the tubes swing on a hinge like a pair of binoculars; on a sliding head, they move apart in a straight line. With a sliding head, the tube length changes slightly as the spacing changes, so you may need to refocus. Siedentopf designs avoid this.
• Diopter adjustment: a ring on one or both eyepiece tubes to correct for differences between your eyes.
• Head rotation: many heads can be rotated on the arm, often a full 360 degrees, and are inclined at about 30 or 45 degrees.

Parts of a stereo microscope

Head and eyepieces

The head holds two eyepieces, usually 10x, and contains the prisms that bring each light path to an eye. Diopter rings and interpupillary adjustment work the same way as on a compound microscope. Trinocular versions add a camera port.

Magnification changer: zoom or fixed steps

Stereo microscopes change magnification in one of three ways:

• Zoom: a knob on the side of the head (or two, one each side) turns continuously through a range such as 0.7x to 4.5x. Combined with 10x eyepieces, that’s roughly 7x to 45x.
• Turret or drum: a rotating drum clicks between fixed steps, such as 1x and 3x.
• Fixed: a single magnification, common on inexpensive student models. Some allow changing magnification by swapping eyepieces.

Objective: Greenough vs common main objective

There are two main optical designs:

• Greenough: two small objectives, angled toward each other like a pair of eyes. It’s compact, relatively inexpensive and gives a strong 3D effect. It’s used on most student and many lab stereo microscopes. At the edges of the field, the two images can look slightly tilted relative to each other.
• Common main objective (CMO): a single large lens at the bottom, shared by both light paths. The design is more flexible, and accessories such as cameras, beam splitters and coaxial lights can be added more easily. It’s usually found on higher-end instruments.

Auxiliary (Barlow) lens

Many stereo microscopes accept an extra lens that screws onto the bottom of the objective. Commonly called a Barlow lens, a reducing version (such as 0.5x) gives more working space and a wider field; a magnifying version (such as 2x) gives more power but less room. See our guide to Barlow lenses for microscopes for the trade-offs.

Focus mount and focus knob

The head sits in a focus mount (focus block or focusing rack) that slides up and down a post or rail. Turning the focus knob raises or lowers the whole head. Unlike a compound microscope, the stage doesn’t move. Many focus mounts have a tension adjustment to stop the head drifting down.

Stand: pillar, boom or arm

The stand holds everything up. Common types:

• Pillar (post) stand: a vertical post on a flat base. Compact and stable, suits most hobby and lab bench work.
• Track stand: a rigid rack-mounted column with the illumination built into the base.
• Boom stand: a horizontal arm on a heavy base, letting the head reach over large objects.
• Articulating arm: a flexible arm, often clamped to a bench, used in electronics and repair work.

Stage plate and clips

On a pillar or track stand, the base has a round stage plate. It’s often reversible black and white, for contrast with light or dark specimens, and frosted glass for use with the bottom light. Stage clips hold specimens or slides in place.

Illumination: top light and bottom light

Most stereo microscopes offer two lights:

• Top (incident) light shines down onto the specimen. It’s used for opaque objects such as coins, insects and circuit boards; see what reflected light is used for. Ring lights that fit around the objective give shadow-free light.
• Bottom (transmitted) light shines up through the stage plate. It’s used for translucent specimens such as thin leaves, pond water in a dish, or small aquatic creatures.

A control knob, or one for each light, adjusts brightness. For more on light sources in general, see our guide to microscope illumination.

Side-by-side comparison

Trinocular heads

Both compound and stereo microscopes come in trinocular versions, with a third tube on top for a camera. On some, a lever or slider sends all the light either to the eyepieces or to the camera. Others split it, so you can view and record at once. Our comparison of binocular vs trinocular microscopes covers whether the camera port is worth paying for. On stereo microscopes, a “simul-focal” head lets you view and photograph at the same time.

Setting up a binocular head for your eyes

Whether stereo or compound, two quick adjustments make a big difference to comfort:

1. Set the eye spacing. While looking through the eyepieces, move them together or apart until the two circles merge into one.
2. Set the diopter. Close the eye on the adjustable side and focus with the main focus knob using the other eye. Then switch eyes and use only the diopter ring to sharpen the image. On a zoom stereo microscope, do the main focusing at the highest zoom setting, then zoom out. That keeps the image in focus through the whole zoom range.

For a fuller walkthrough of the diopter and eye relief, see our guide to the eyepiece.