Autofocus is a standard feature on modern cameras and lenses, but many photographers do not understand the specifics of how focus changes interact with focal length. Gaining a deeper understanding of autofocus and focal length can help any photographer elevate their craft and create more impactful photographs.

Autofocus does change the focal length for most lenses to an extent. The effect that a change in focus has on a camera depends on the lens's focal length and the focus distance. Close autofocuses will have a more drastic effect on focal length, while medium to far lengths will be about the same.

For most photographs that do not employ an extremely close focus, the focal length difference will be minimal, but it should still be kept in mind. For extra information on how the change occurs and what it means, keep reading.

How Focusing Variations Affect Focal Length

When focal length changes occur due to autofocus, it is actually due to the change in focus in general; it will appear whether done manually or automatically. The difference in focal length is due to what it actually represents on a lens.

The listed focal length of a lens is for when the lens is focused at “infinity,” or far on the horizon. As the focal point of a photograph gets closer to the camera, and thus further from the infinity point, the focal length changes. Until the focal point hits a certain distance from the camera, the change is so minuscule that it is, for all intents and purposes, meaningless.

The distance at which a change in focal length becomes meaningful depends on the specific lens and listed focal length. Some cameras may begin to have a notable focal length difference at 6 feet, while others may only become noticeable when used for macro photography.

When the focal length changes as a result of autofocus, it only increases when it gets closer to the camera. If the listed focal length of a lens is, for example, 55mm, then that is the “minimum” focal length that lens will ever be. However, if used to photograph an object with an extremely close focus, it could increase to 60mm or even higher. However, because the listed focal length is for the “infinity” point, it will never decrease past the original 55mm.

Source: Photomodeler

Does the Difference in Focal Length Matter?

For most casual photographers or even professionals, the difference in focal length that occurs will ultimately not matter too much; it is more important to focus on capturing a great, in-focus photograph than maintaining the exact focal length across pictures.

However, there are some specific situations where learning about the difference in focal length is pivotal. Times when differences in focal length matter include:

  • Photogrammetry use
  • Macro photography
  • Videography

These are just three of the main areas when maintaining a consistent focal length matters; thus, using autofocus is not the best idea. If keeping it consistent is something that particularly intrigues you, then there is no reason not to explore that; just be aware that it will likely be more challenging to get in-focus photographs out of it.

Photogrammetry

Photogrammetry is the science of mapping out areas such as buildings or land through imagery, but it is also used for scanning 3D models, a process quickly gathering popularity.

Due to the exact calculations required for most of this craft's work, maintaining and adhering to the same focal length is essential for maintaining accuracy. An in-depth look at photogrammetry can be found here.

Macro Photography

Macro photography is when pictures are taken with an extremely close-up, or macro, focal point. This is most commonly used to photograph jewelry or capture details. Specific lenses exist that are meant for macro photography and have a tiny infinity line, meaning that the focal length will not change regardless of how the lens is used.

However, for anyone who attempts macro photography with a standard lens containing autofocus, the difference in focus point will result in a critically different focal length.

Videography

Finally, most professional video work is captured with a consistent focal length to maintain imagery and feel across the entire film. This is one reason why professional cinematography lenses are so expensive; they are specifically designed to not have a changing focal length even with focus point changes.

This is also why autofocus is rarely, if ever, used for videography work. Calculating and maintaining a specific focal length is essential for consistent video.

What Does a Change in Focal Length Mean for Me?

Changes in focal length relate directly to the viewing angle that will be captured in a photograph. Short focal lengths have wide-angle views, while longer focal lengths have smaller angles. When angles are small, the focused subject appears wider.

The measurement of a focal length represents the space between the camera's sensor and the point of convergence, or infinity line, of the lens. In prime lenses with only one focal length, this cannot change, but autofocus can slightly affect it.

Often, the terms “wide-angle” and “macro” describe different ends of the focal length spectrum. As the name suggests, wide-angle lenses are incredibly wide-angled lenses and are suitable for capturing landscapes or faraway places. Macro lenses are extremely small-angled lenses and do well capturing details and close objects. These represent opposite ends of the spectrum.

When the focal length of a lens changes, the viewing angle of the photo it takes changes as well. This is why focal length changes are essential, even for lenses that exist in the middle of the wide spectrum. If the difference is substantial at all, it can dramatically change the look of the photo. This page from Nikon includes example photos showing the difference that different focal lengths can have.

Autofocus will not change the focal length of a photograph too dramatically unless it is a zoom lens or incredibly out of its normal range. A single 50mm lens, for instance, cannot be autofocused to work wonderfully as a wide-angle lens or a macro lens. While the exact effect that autofocus has on focal length depends heavily on the lens itself, the differences are likely to be minimal.

Due to their ability to exist at a wide range of focal lengths, the zoom lenses' autofocus will have a noticeable difference in a photograph. If you are trying to achieve a specific look with an autofocusing zoom lens, you may need to temporarily disable the feature.

Focal length ultimately helps decide how far you have to be from the object you are capturing, and autofocus will not affect that. It is a tool that can dramatically help, and outside of a few specific circumstances, the minimal differences in focal length that occur as a result of it will not matter.

Recap

Autofocus does slightly affect the focal length, but the exact amount depends on the type of lens being used and the distance the photograph is being captured at. Most of the time, especially for the average photographer, changes are so minuscule that they do not matter.

Changes become more noticeable the closer that the autofocus point is to the camera.

Focal length represents the infinity line, or convergence, of the camera. The closer a focused subject is to the camera, the more it deviates from the listed length. This can result in dramatic changes when using a standard lens for macro photography, for instance, but most photographs will not see a difference.

Specific uses do require that focal length stay precisely the same at all times, such as for videography or photogrammetry.