What Is 35mm Equivalent

In the article Low Light And Heavy Lenses I wrote about a 300mm lens and I said “a 300m lens (that’s the true focal length, not the 35mm equivalent)”, and you may wonder what that means exactly.

It’s a meaning with a lot of history.

In 1913 Oscar Barnack built a small film camera that became world famous. It was the first in a line of Leica cameras that were produced for over a century and then transitioned to digital cameras. The film carrier in the camera engaged with sprocket holes at top and bottom of the film, and the actual size of the frame of film that was exposed to the light was 24x36mm. And that is the size that is nowaday described as 35mm, or full frame.

35mm film cameras are the most popular size film cameras by far. With the new digital cameras, manufacturers were free to make sensors any size and shape they wanted, but to begin with they stuck the same 2:3 ratio that is 24x36mm. But they could make smaller sensors in the same ratio. Smaller means cheaper to make – and with digital, the increase in quality meant that nothing was sacrificed in making smaller sensors.

So now we get to APS-C sensors that are two thirds the size of 24x36mm. Actually, Canon camera sensors are described as APS-C but they are slightly smaller still.

Leaving that aside, what it means is that lenses with an image circle that will cover an APS-C size sensor don’t have to cover such a large area as a lens that has to cover a full frame sensor. Smaller means less weight and cheaper to manufacture. And that means more customers.

Let’s bring this into the real world. A 300mm lens on a full frame (that means 24x36mm) digital camera will be 300mm. Put that lens on an APS-C camera and it will reach one and a half times as far – namely 450mm. That’s why wildlife photographers like APS-C cameras – because the lenses ‘reach’ further than they would if they used them on full frame cameras.

So now we know what I meant when I said “a 300m lens (that’s the true focal length, not the 35mm equivalent)”. I meant a lens that has 300mm stamped on the barrel. It will behave like a 300mm lens on a full frame camera and it will behave like a 450mm lens on an APS-C camera.

APS-C lenses versus Full Frame lenses

There is always a ‘but’ in photography. And the ‘but’ here is that some lenses are made smaller to specifically fit APS-C cameras. They have smaller lens elements of less diameter. That means they are cheaper to manufacture and cheaper to buy. And here is the but – if you try to use them on full frame cameras, the image circle will be too small and the outer edges of the frame will be dark because no light or very little light will reach there.

I have a Nikon D500, which is an APS-C camera. But I have a Nikon 70-300mm f4.5-5.6E ED VR AF-P lens, which is a full frame lens. Why? Well, it could be because I know I could then use it on a full frame camera. In fact, though, the reason is that it is simply a better lens than the APS-C size lens. And final word, APS-C lenses – at least in the Nikon line – are called DX lenses. So if you come across a lens and you see DX stamped on the barrel – understand that it cannot be used on a full frame camera.