The reason why it's often a good idea to state full-frame equivalent focal length (mm)

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Andersonm Contributing Member • Posts: 519
The reason why it's often a good idea to state full-frame equivalent focal length (mm)

A lot of talk in photography refers to the basic question of how much or little of the field of view is captured in the photo.

People care a lot about how much of the view is captured - whether a photo will be filled with a single face or with a crowd.

Often, this is simply referred to by focal length / mm / zoom level / magnification of the lens as the key question or topic. Hence you could see questions like:

"I'll be standing close to two people talking, and I want to take a picture of them with my camera. Which lens (or zoom level ) (or focal length) (or mm) should I use?"

"I'll be backpacking in the mountains, and I have been given a compact camera with lens (or zoom level) (or focal length) (or mm) X to Y, will this be good to take photos of both mountains and animals at a distance?"

This seems simple enough, like asking which magnification binoculars to use.

Crucially however, cameras have different size sensors, which are like looking through a window, where smaller windows lets you see less.

If we consider Full-frame a "Normal-sized", "Size 1" window, smaller sensors than this will be smaller windows, offering just a fraction of the view.

At the same time, the lens sits outside the window. Light first passes through the lens, then through the window, or hits the wall around it.

So if you're taking a photo of two people next to you, and want both their heads to be visible in your window, it's not just a question of the lens, but the size of the window.

If your window is "normal-sized" "Size 1" (Full-frame), let's say that a 50mm lens (meaning 1x magnification) could capture both their heads fully.

But if your window is only half the size, the half-size window would not capture both their heads fully. It would cut out and show only a smaller central part. The 50mm lens would function exactly as before, just that much of the light passing through it hits the wall around the smaller window/sensor.

If your sensor/window is half the size, you actually need a much WIDER ANGLE lens, a 25mm lens, so that when you cut out the central part of the wide angle view, you get both heads.

In discussions, stating the "full-frame equivalent" focal length / mm means that people don't have to know your sensor size to know what you're talking about

So back to the question: which focal length / mm lens to use for two people next to you? For any sensible answer, you would have to not only state the lens (50mm) but the sensor size.

There's some very common "standardized" sensor sizes (Full-frame, APS-C and Micro Four Thirds), but there's also a mass of smaller and custom sizes, particularly smaller for compact cameras and camera phones.

The "half-size" reference above isn't really precise. Size is rather described by a number, the "crop factor". Full-frame has a crop factor of 1, APS-C by around 1.6, MFT by 2. You multiply the focal length (mm) by the crop factor to get the full-frame equivalent.

So with a medium-angle 50mm lens on an MFT sensor, the window is small enough that you see the same as a 100mm lens on a full-frame sensor.

For all the obscure and varied smaller sensor sizes, speaking in full-frame equivalent terms is pretty much necessary. Advice like "Use 50-100mm for portraits most of the time" would apply to camera phones and full-frame cameras alike if speaking in FF equivalent terms.

So what does a "10mm lens" really mean? On a full-frame (crop factor 1) it's ultra wide angle. On a camera phone (crop factor ca. 7 usually) it's portrait length, 70mm equivalent.

And that is why, if you are on an MFT forum, saying you have an MFT camera, it's no problem to talk about your 25mm lens, which is actually 25mm.

But if you're on a general forum, it's usually much better to say you have a "50mm full frame equivalent" lens. And why camera phones and compact cameras with nonstandard sensor sizes will pretty much always and by default state the FF equivalent only.

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