Internal Focusing Primes Uncompensated Varifocal Lenses Discussion

Hey guys! Ever wondered about the inner workings of your favorite lenses? Specifically, have you ever pondered the fascinating relationship between internal focusing (IF) primes and uncompensated varifocal lenses? It's a question that dives deep into the heart of lens design, and it's something we're going to unpack today. Let's get started!

Delving into the Fundamentals of Focal Length and Flange Distance

To truly grasp the concept, we first need to understand the basics of focal length and flange distance. The focal length of a lens, measured in millimeters (mm), dictates its magnification and angle of view. A shorter focal length gives a wider angle of view and less magnification, while a longer focal length provides a narrower angle of view and greater magnification. Think of it this way: a 24mm lens captures a vast scene, while a 200mm lens zooms in on distant subjects.

Now, flange distance comes into play. The flange distance, also measured in millimeters, is the distance between the lens mount on the camera body and the image sensor (or film plane). It's a crucial measurement because it determines the correct positioning of the lens to achieve sharp focus at various distances. Each camera system has a specific flange distance, and lenses designed for that system must be manufactured to meet that specification. For example, Sony's E-mount has a flange distance of 18mm, while Canon's EF mount has a flange distance of 44mm. These differences are why you can't directly mount an EF lens on an E-mount camera without an adapter.

Now, let's consider what happens when you shorten the focal length of a lens without making any other adjustments. As you shorten the focal length, the required flange distance for infinity focus decreases. This means that if you don't change the actual flange distance, the lens will need to be positioned closer to the sensor to achieve a sharp image at infinity. If the actual flange distance remains unchanged, the lens will no longer be correctly positioned for infinity focus. This is where the concept of internal focusing comes into the picture.

Internal Focusing (IF): A Clever Mechanism

Internal focusing (IF) is a clever design mechanism used in many modern lenses, especially prime lenses and zoom lenses. In an IF lens, the focusing is achieved by moving one or more internal lens elements, rather than moving the entire lens assembly. This has several advantages. Firstly, it allows for faster and quieter autofocus, as the internal elements are typically smaller and lighter than the entire lens group. Secondly, it maintains the overall lens length during focusing, which is beneficial for handling and balance. Thirdly, it can improve optical performance by minimizing aberrations that can occur when the entire lens barrel extends or retracts.

However, there's a trade-off. When an IF lens focuses closer, the effective focal length of the lens changes slightly. This is because the internal movement of the lens elements alters the overall optical path and magnification. This phenomenon is more pronounced in zoom lenses, but it also exists in IF prime lenses, albeit to a lesser extent. It's this change in effective focal length that leads us to the core of our discussion: are IF primes actually uncompensated varifocal lenses?

Varifocal vs. Zoom: Understanding the Nuances

Before we answer that question, let's clarify the difference between varifocal and zoom lenses. Both types of lenses allow you to change the focal length, but they do so in different ways. A zoom lens is designed to maintain focus throughout the zoom range. This means that as you zoom in or out, the lens elements move in a coordinated manner to keep the image in focus. In a true zoom lens, the focal plane remains constant as the focal length changes. This complex movement is achieved through intricate mechanical linkages and precise optical designs.

A varifocal lens, on the other hand, does not maintain focus throughout the zoom range. When you change the focal length on a varifocal lens, you typically need to refocus the lens to achieve a sharp image. Varifocal lenses are simpler and often less expensive to manufacture than zoom lenses. They're commonly found in CCTV cameras and some older photographic lenses. The key difference is that varifocal lenses do not compensate for the shift in the focal plane that occurs when the focal length is changed.

The Connection: IF Primes as Uncompensated Varifocals?

Now, back to our original question: are internal focusing primes actually uncompensated varifocal lenses? The answer, as with many things in optics, is a nuanced one. In a strict sense, yes, an IF prime lens does exhibit varifocal characteristics. When you focus an IF prime lens at different distances, the effective focal length changes slightly, just like a varifocal lens. This change is typically small, but it's measurable and can be observed in certain situations, such as close-up photography or when making precise measurements.

However, it's crucial to understand the context and magnitude of this effect. The focal length variation in a well-designed IF prime lens is usually minimal, often within a few percent of the nominal focal length. This is why it's generally considered a prime lens, as the focal length remains relatively constant. The key difference between an IF prime and a true varifocal lens is the degree of compensation. A varifocal lens makes no attempt to maintain focus as the focal length changes, while an IF prime lens is designed to minimize focal length variations and maintain sharpness across the focusing range.

The term “uncompensated” is crucial here. A true varifocal lens is intentionally designed without compensation for focal plane shift, while an IF prime lens experiences focal length variation as an unavoidable consequence of the internal focusing mechanism. The lens designer's goal is to minimize this variation as much as possible.

Real-World Implications and Considerations

So, what are the real-world implications of this subtle varifocal behavior in IF primes? For most photographic applications, the effect is negligible. The change in focal length is so small that it's unlikely to be noticeable in typical shooting scenarios. However, there are some situations where it can become relevant.

• Close-up photography: When shooting macro or close-up subjects, the focal length variation can be more pronounced. This is because the focusing distance is a larger proportion of the overall focal length. In these cases, the effective magnification of the lens may be slightly different than what's indicated by the nominal focal length.
• Precise measurements: If you're using a lens for applications that require precise measurements, such as photogrammetry or scientific imaging, the focal length variation needs to be taken into account. Calibration procedures may be necessary to compensate for the change in effective focal length at different focusing distances.
• Focus breathing: A related phenomenon known as “focus breathing” is often more noticeable than the actual focal length change. Focus breathing refers to the change in the angle of view as the lens is focused. IF lenses can exhibit focus breathing, which can be a concern for videographers as it can cause distracting shifts in the framing during focus pulls.

Conclusion: A Matter of Degree

In conclusion, the idea that internal focusing primes are essentially uncompensated varifocal lenses holds a degree of truth. IF primes do exhibit varifocal characteristics due to the change in effective focal length during focusing. However, the extent of this variation is typically small and well-controlled in a well-designed lens. For most photographic purposes, it's not a significant concern. But, understanding this behavior is essential for specialized applications where precise measurements or consistent framing are critical. So, the next time you're using your favorite prime lens, remember that there's a bit of varifocal magic happening inside! Keep those cameras clicking, guys!