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35mm equivalents demystified

What do all of those focal length numbers on a camera lens really mean?

Canon%20EF%2024-70mm%20f/2.8L%20USM%20LensThere's a lot of numbers on this lens: what, exactly, does 24-70mm mean?

The “35 millimeter equivalent” explained

The concept of zoom with regard to camera lenses is simultaneously simple and complex. Simple, because the result of manipulating a zoom lens is easy to see: your subject is magnified or reduced in size in the viewfinder or LCD screen as you zoom in or out. Complex, because we have so many ways of describing magnification.

With a dizzying array of optical recipes available, it’s helpful to have a standard point of reference in order to compare “apples to apples.” Fortunately, manufacturers and camera professionals arrived at a solution a good while ago. In the heyday of the film-camera era, the most commonly sold cameras used 35mm film. Lenses and camera bodies were built to work with an imaging area of that size and so it became shorthand to refer to a lens’s characteristics by its focal length in millimeters when mounted on a typical 35mm film camera.

About crops: Apples, not oranges

Here's where things get complex. Imaging sensors aren’t all the same size. A lens projects a circle of light towards a sensor, and (for example) if that lens' circle of light is designed to fully illuminate a "full frame" sensor — one that is the same size as a single frame of 35mm film —  then a smaller sensor (like those on most consumer-level DSLRs and hybrids) paired with that same lens will be illuminated by just the center portion of that projected circle of light. The gives the apparent result of more magnification, and as the sensor size decreases, apparent magnification increases.

This relationship is defined by the ratio of a 35mm frame's diagonal measurement divided by the smaller sensor's diagonal measurement, and the resulting figure is referred to as the "crop factor," as you're cropping part of the sensor's projected circle of light. For example, an APS-C size sensor, found in many DSLRs and hybrids, is smaller than the full 35mm frame, and its diagonal measurement goes into the 35mm frames diagonal measurement approximately 1.5-1.6 times. This ratio is also often called a "focal length multiplier."

In the Digital Camera Glossary article on our site, there's a helpful illustration of crop factor's effect.

All things being equal

Because of this, we talk about lenses in terms of their “35-millimeter equivalents.” The angle of view of the human eye (and what we consider to be “the real world” with regard to magnification) is roughly equivalent to a 50mm lens on a 35mm film camera. If we were eagles instead of humans, no doubt our “real world” would be a higher 35mm equivalent number. But then again, eagles don’t see the “big picture” that we do.

So let’s look at a typical zoom lens and note its “35mm equivalent” rating. The specifications on a lens say it has a 35mm equivalent of 24mm-70mm. What is this telling us with relation to how our unaided eye sees? Well, at its widest it makes a 24mm (35mm equivalent) focal length. That means the amount of the scene in front of your face that it can fit into the frame is roughly twice what your unaided eye sees when it looks at the scene, because your eye is effectively 50mm and this lens is 24mm, and 24 goes into 50 a little more than two times. And when this lens is zoomed all the way in, it’s blowing the scene up – filling the 35mm film frame with less of the field of view than the typical eye, so the scene in front of you is blown up by approximately 40% (I’ll spare you the math). That makes it capable of a very light bit of apparent magnification.

Look to the 35mm equivalent first

So in our crop factor example above: a lens, built to project a circle that accomodates a full-frame (35mm film-sized) sensor, with a designation of 50mm gives you a 50mm equivalent when placed on a 35mm-sized sensor. But the same lens, placed on a camera with an APS-C sensor, will give you a 35mm equivalent length of 75-80mm (35mm x 1.5 - 1.6), because your sensor is just seeing the center portion of that circle of light.

Although it probably doesn't seem like it if you're new to the subject, this is a fairly simple overview of how lenses behave with certain sensors. There are many other factors that come into play, but for the purposes of seeing what kind of magnification to expect from a given lens on your camera, this is a good place to begin. There's no substitute for actually trying one out to see if it suits your needs, though. That's part of the fun of the hobby.

Here’s the lesson you can draw from this. Always look for the 35mm equivalent on a lens’s specs. It will give you a good idea of what magnification performance to expect no matter what kind of camera, system, lens or sensor you’re using. Often the lens will have its real focal length – the distance between the center of focus and the image sensor (or film emulsion layer), written on it, but don’t use that to set your expectations, use the 35mm equivalent. Yes, in some cases they’ll be the same thing, don’t worry.

The zoom-lens spectrum

Here’s a rough (and purely subjective) guideline of how 35mm equivalent focal lengths behave relative to your eye:

35mm equivalent Lens Type
35mm equivalents for common lens types
18mm and below: ultra wide-angle and fisheye lens
24mm-35mm: typical wide-angle lens

“normal” lens

70mm-110mm: “portrait” lens
135mm-300mm: telephoto lens
300mm-800mm: long telephoto lens
800mm and up:  super-telephoto lens

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