What Is Inside a Zoom Lens?

Many people are curious about the internal construction of zoom lenses. Common questions are:

How complicated Is the internal construction within a zoom lens?
How does a zoom lens work?
What are the various optical groups within a zoom lens?
How do these various optical groups function?
Do more optical elements mean that the lens will be sharper than other similar lenses?

Wow! When you think about it, all of these are really good questions. Trying to answer these questions is far from simple since there are many types of zoom lens designs. Lets try to answer these questions by looking at a fairly conventionally designed telephoto zoom lens. We will use Tamron's SP 70-210 model 19AH as a representative example of a conventional zoom lens. After all, telephoto zooms were the first types of zoom lenses which were ever developed and produced by virtually all OEM and after market lens manufacturers.

Internal Construction

Virtually every zoom lens, whether it is 30 years old or is a lens which you just purchased, features very complicated internal construction. Just take a quick look at the CAD cutaway of Tamron's SP 70-210 zoom lens. It is obvious that there are quite a number of both large and small parts. Zoom lenses generally have over 100 individual parts. If you look carefully at the CAD drawing, you will notice that a zoom lens necessarily has numerous threaded retaining rings and spacer rings for the various optical elements. Just one improperly machined retaining or spacer ring can turn a zoom lens into an optical lemon by causing either tilt or decentering of one or more optical lens elements. Suffice it to say that any zoom lens features extremely complicated internal construction and precise machining of all parts which support the optics.

Optical Groups Within a Conventional Zoom Lens

Conventional zoom lenses generally consist of three specific moving optical groups plus one stationary optical group. Each optical group usually includes two or more lens elements. Keep in mind that we are talking about the actual independently moving groups of lens elements within a zoom lens, and are NOT talking about the basic optical layout of the lens! Our SP 70-210 model 19AH, for example, optically consists of 15 lens elements in 11 groups. This optical specification means that there are 15 individual lens elements within this zoom lens, and that two or more lens elements are cemented together to create an overall optical configuration which is equivalent to 11 lens elements. In the case of the model 19AH example, two individual pairs of lens elements are cemented together with each cemented pair behaving as if it were a single lens element. The result is that this lens consists of 11 optical groups or 22 lens surfaces which need to be coated or multicoated in order to maintain proper light transmission, color balance, and to prevent internal reflections or lens flare.

Functions of the Optical Groups in a Conventional Zoom

The mechanical movement of the optics within this zoom lens can be broken down into three independently moving groups of optical elements and one stationary group of optical elements. Some zoom lenses actually move all four optical groups when the lens is zoomed. These four optical groups (from front to back) are:
the focus group,
the variator group,
the compensator group, and
the master group.

We have color shaded the CAD drawing to indicate the internal mechanical housings which hold the optics for each of these four optical groups. Nearly every zoom lens consists of these four types of optical groups. So, what do these four optical groups do?

The front or top group of lens elements is called the focus group. The focus group moves forward or backward as necessary for focusing the lens. In a conventional telephoto zoom such as the SP 70-210, this focus group does not move when the lens is zoomed. It only moves when the zoom's focus collar is rotated. Some telephoto zooms feature optical designs wherein the length of the lens increases as the lens is zoomed to its longest telephoto position. Some examples of lenses which change in length when zooming are Tamron's SP 60-300 model 23A and SP 35-210 model 26A. The front focus groups in these lenses move not only when focusing but also when zooming these lenses.

The variator group of lens elements, like its name implies, varies the magnification power of the focus group. The variator group is the moving group which is mostly responsible for changing the focal length of a zoom lens when it is zoomed. Note that the variator group alone isn't enough to achieve a true zoom function and maintain focus. Many readers may recall the varifocal lenses which were introduced in the late 1970s. These lenses were not true zooms because the user had to refocus the lens after "zooming".

Something else, optically, has to be used to maintain proper focus when zooming a true zoom lens. This "something else" is the compensator group of lens elements. True zoom lenses have a compensator group which moves in the same direction but at a different rate compared to the focus group. The compensator group's sole purpose is to maintain correct focus whenever the zoom lens is zoomed. We mentioned that the variator group is mostly responsible for changing the focal length of a zoom lens when its is zoomed. The compensator group is also partly responsible for changing the focal length of the lens when it is zoomed. The variator and compensator groups work together to create the change in focal length when the lens is zoomed, and the compensator group also assures that the lens remains in focus while being zoomed. As mentioned, some zoom lenses also move the focus group when zooming. In such lenses, all three groups are responsible for creating the change in focal length when zooming.

You would think that the three aforementioned lens groups would be enough in order to construct a zoom lens, but they usually aren't! They would be if we didn't have to deal with the space which the mirror in a SLR or digital camera occupies, if we didn't care how big and bulky the zoom lens was, and if we didn't care about off-axis lens aberrations. Thus virtually all zoom lenses also feature a master group of lens elements. The master group serves to relay the variable magnifications from the other groups to the camera's focal plane. The master group, aside from relaying the optical path to the camera's focal plane, is also optically constructed to correct aberrations from the other optical groups. The master group also serves to make the zoom lens more compact by allowing the focus group to be positioned closer to the film plane and by requiring less overall movement of the variator and compensator groups in order to achieve the inherent zoom range of the lens. The aperture diaphragm is almost always located either directly in front of or within this master group of optical elements. The aperture diaphragm in our SP 70-210 example is located directly in front of the master group.

Functions of the Optical Groups in an Unconventional Zoom

Some zoom lenses, particularly very wide angle to moderate telephoto zooms, actually move all four optical groups as the lens is zoomed. Such zoom lenses break all design rules for conventional zoom lenses because even the normally stationary focus and master lens groups and the aperture diaphragm move when zooming. Thus every optical group in these complex lenses is part of the overall zooming function, and each optical group may perform more than one individual function. The focus group, for example, not only is used for focusing but also may move at varying rates when zooming to also function as either a variator or compensator group. Likewise the rearmost master lens group may move when zooming to function as a variator or compensator group. Many modern zoom lenses feature internal focusing. The front optical group in these lenses no longer is the focus group as would be the case for a conventional zoom lens. Just how complicated are the movements within these advanced zoom lenses? Have a look at the following diagram which is taken from one of Tamron's patents:

You will note in the above illustration that all four groups of optical elements move when this lens is zoomed. You will also note that the second group G2 is used for focusing rather than the first or front group G1. Thus this lens also features internal focusing. This lens also features an aperture diaphragm (STP in the above illustration) which moves in concert with the third group G3. Also note that groups G3 and G4 move at almost the same rate. Thus they can be thought of as the master group but with a slight built-in zooming effect to act as a mild compensator group.

Zoom Lens Mechanics

You have to be wondering just how complicated are the internal mechanics within a zoom lens. The answer quite simply is that zoom lenses feature complicated to extremely complex internal construction. The zoom's mechanics not only must move the various optical groups with the correct rates of movement, but also must do this precisely while also maintaining optical alignment of the optical elements. Shown below is an illustration of the variety of parts and nested sleeves which are required in a modern wide angle to telephoto zoom. We have colored the four optical groups yellow for clarity.

The movement of the various optical groups is accomplished via a series of nested barrels or sleeves. These sleeves feature machined cam-slots and cam-followers. In zoom lenses, some sleeves remain stationary while others rotate or move back and forth. The operation of these sleeves can get really complicated. Just have a look at the following illustration which shows several sleeves along with the multitude of necessary cam-slots and cam-followers for moving the various optical groups:

Total Number of Optical Elements

Many photographers assume that more optical elements means that the lens will be sharper. This rarely is the case. Optical engineers have a variety of options available to them for optimizing lens designs. Some options are rare earth optical glasses which feature high refractive indices, low dispersion (LD) glasses which minimize chromatic aberration and lateral color, anomalous dispersion glasses which feature compressed and uneven dispersion of colors, and finally composite aspherical lens elements. A good optical engineer or design team can combine, as necessary, some or all of these options to create a lens with extremely good performance while using fewer lens elements.

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