Focal length and maximum aperture are the most common measures for summarizing the principle characteristics of a lens, but are they the best to use? Given the range and proliferation of different camera types, why don't we see field of view used more often?
Most photographers are familiar with the meaning of focal length and maximum aperture, and their impact upon the photographs they take and make. Focal length determines how much we can see in an image, whether it is wide angle, normal, or zoom. And we’ve become familiar with a "normal" nifty-fifty, street photography at 35mm, going wide at 24mm and shorter, the flattering effects of a portrait lens at 85mm, and then specialist zooms beyond 100mm. But what does 75mm on the Pentax 645Z, 10.4mm on the Sony RX100 V, 105mm on a Nikon D850, 129mm on the Panasonic Lumix ZS50, or a 4.47mm Google Pixel 2 really mean?
How much we can see is more commonly known as the field of view. As the lens opening is circular, it projects a cone of light rays onto the camera sensor; the further away the lens (i.e., the focal length), the narrower the field of view. As most cameras have a rectangular sensor, the horizontal and vertical angles of the cone (measured in degrees) define this.
However, it is the combination of focal length and sensor size that determines field of view. So when we talk about focal length, camera manufacturers (and reviewers) have defaulted to using this value for a 35mm (full frame) system. This is incorrect for non-full frame cameras and so we have to use the crop factor (usually assuming the same aspect ratio) as a multiplier to give the effective focal length, showing what you would see if it was a full frame system. So for example, the Nikon D7500, a DX sensor at 23.5 x 15.6mm, has a 1.54x crop factor when compared to the full frame (FX) D850 with a 35.9 x 23.9mm sensor. A 35mm lens on the D7500 therefore has an equivalent field of view to a 54mm lens on the D750.
Isn't it time for manufacturers and reviewers to use a physically meaningful measure? For over 50 years 35mm has had a virtual monopoly in terms of film sales so sensor size and focal length were synonymous. Only professionals used medium and large format and, arguably, understood the impact of focal length on field of view. But that's definitely no longer the case. We have APS-C, Micro Four Thirds, and 1-inch all being very common consumer formats, and that's before you get in to the myriad range of sensor sizes in smartphones and action cameras. Indeed, it's noticeable that some manufacturers, such as drone maker DJI, report field of view first for their cameras, then 35mm equivalent.
Sure, carry on using focal length as many people still understand it, but the plethora of sensor sizes makes this a redundant measure for what photographers need to understand and is irrelevant for the general public. And for the record, that would be 33 degrees, 65 degrees, 19 degrees, 3 degrees, and 75 degrees respectively.
Image by fernandozhiminaicela via Pixabay.