Photography 101: EFL Versus EFOV

Stephen F. Dennstedt

This post is mostly for the “Newbies” out there. So lets first start with the acronyms (not true acronyms of course) EFL and EFOV. EFL = Effective Focal Length and EFOV = Effective Field of View. What’s the difference?

These photography terms are being used within the context of sensor size, full-frame sensors (35mm film equivalent) and AFS-C crop-sensors (1.6x for Canon and 1.5x for Nikon). Google crop-sensors & crop factors for more info.

My Canon EOS 5D Mark IV has a full-frame sensor and my Canon EOS 7D Mark II has an AFS-C crop-sensor (1.6x). Canon Rebel-series cameras (such as the Canon Rebel T6i aka Canon EOS 750D) also fall into this later group of AFS-C crop-sensor cameras. This information is important to know because it significantly impacts your choice of lenses. Why? Because the same lens (or focal length) acts differently on different camera bodies containing different size sensors. The physical focal length of the lens doesn’t change but the field of view does.

On a camera with a full-frame sensor (like my Canon EOS 5D Mark IV) the lens acts as advertised—in other words a focal length of 100mm provides a field of view of 100mm. Not so on a crop-sensor—put that same 100mm focal length lens on my Canon EOS 7D Mark II and it acts like a 160mm lens (100mm x 1.6x crop factor = 160mm). Many photographers, when trying to describe this affect, refer to it as the EFL (Effective Focal Length) which is actually a misnomer. The physical focal length of the lens doesn’t change at all, it’s the EFOV (Effective Field of View) that changes.

Christ Church – Clifden (County Galway) Ireland

The net result is the 100mm lens on a crop-sensor camera body appears to have more reach (more telephoto effect) than when it’s on a full-frame sensor camera body, when in fact it’s just reducing the field of view. Focal length and field of view are not the same thing, the focal length is a physical attribute of the lens itself and the field of view is what the photographer sees through the viewfinder. See the illustration above to get a better idea of what I’m talking about. Like I said earlier, many photographers trying to explain this concept use the terms focal length and field of view interchangeably and that’s wrong.

Giant’s Causeway – Atlantic Coast of Northern Ireland

Just to thoroughly beat a dead horse: it appears as if the focal length is changing but it’s not because the focal is a physical attribute of the lens itself, what is changing is what you’re seeing through the viewfinder. This really comes into play when selecting a wide-angle lens for your camera. If you buy a 28mm lens for your full-frame sensor camera body it shoots like a wide-angle lens. Placing that same lens on an APS-C crop-sensor camera body and it shoots like a 45mm lens (which is a normal FL and not a wide-angle FL). To get the same wide-angle FOV you would have to buy an 18mm lens (18mm x 1.6x = 28mm EFOV).

The ruins of the Ross Errilly Friary (near the town of Headford) – Connemara (County Galway) Ireland.

I hope this makes sense to you newer photographers—you veterans should know all of this stuff. That’s one of the advantages of shooting with a full-frame sensor camera, the lenses shoot as advertised and you don’t have to do the math using the 1.6x (Canon) or 1.5x (Nikon) crop-factor. Other reasons include higher resolution and greater dynamic range but full-frame sensor camera bodies are more expensive than crop-sensor camera bodies. Many new photographers start out with the less expensive Rebel-series cameras or the Nikon equivalent. They’re great little cameras but you need to consider what lenses to use.

Field Notes: All of the photos included in this post were captured with my Canon EOS 5D Mark IV full-frame sensor camera and Canon EF 16-35mm f/4L IS USM ultra wide-angle zoom lens (shot at the 16mm end). It’s hard to get this kind of dramatic effect with an APS-C crop-sensor camera because lenses just don’t go wide enough when you factor in the 1.6x or 1.5x crop-factor. The opposite is true for telephoto lenses—the crop-factor can be an advantage when trying to reach farther. That’s why I will often use my Canon EOS 7D Mark II crop-sensor camera for wildlife and my 5D Mark IV for everything else. SFD

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