“35mm Equivalents”: A Plea For Clarity
January 8, 2010
There’s one bit of digicam terminology that has always driven me batty: quoting lens focal lengths in terms of “35mm equivalents.”
I understand the reasons for it. But I always felt that in the end, these numbers just confuse everyone—because they are fictional.
The true optical focal length of a lens is an absolute quantity—it does not care what size or shape of imager you stick behind it. But for a particular lens focal length, the larger the sensor or film format, the wider the photo’s angle of view. Not too surprising, if you think about it.
With the rise of digital cameras, we left a familiar world where almost everyone used the 24 x 36 mm format of 35mm film. In came a plethora of new sensor sizes, differing even within the model lines of a single camera manufacturer. Almost no one knew the actual dimensions of their sensors, or how a true f.l. number would relate to that.
So how were we supposed to compare numbers for how “wide-angley” or “telephotoish” different lenses were?
One sensible idea would have been to stop right then and introduce a new industry-wide standard: Expressing lens coverage in degrees.
Let’s say you considered the angle diagonally from corner to corner of the frame: In that case a decent wide-angle would be 75°, a standard lens about 50°, and a moderate telephoto about 20°. Nice, simple-to-remember numbers. Doesn’t matter what the actual sensor size is.
But instead, the industry punted. With photographers already rattled by the film-to-digital transition, camera companies tried to soothe jangled nerves by recycling the numbering system people were most familiar with.
But there are several problems with this.
For DSLRs with less-than-full-frame sensors, do you mean 35mm equivalents on a sensor with a 1.6x crop factor (Canon), or 1.5x (Nikon and several others)—or some other oddball like the 1.3x Canon EOS-1D?
Or what about a digital camera that has a 3:4 aspect ratio, versus the 2:3 of 35mm film? Do you base equivalence on the diagonal measure, or on the height of the image? Depends whether you crop to make traditional 4×5 and 8×10 prints, or view shots onscreen in their native proportions.
Also, quoting equivalent focal lengths makes for some very clumsy wording: Shooting with a 21mm lens (equivalent to 32mm on 35mm film), Rusty moved closer to the model. How many times can we jam “mm” into the same sentence?
So now people have even started dropping the “35mm equivalent” qualifier. And that’s when the real confusion begins. Without that, the quoted number is actually incorrect (it is not the true optical focal length), and “millimeters” is being used in a new and meaningless way.
However, even someone as cranky as me must eventually cede to reality. I accept that it’s too late now to turn back the clock, and get everyone thinking in degrees (however accurate and sensible that would have been). The “35mm equivalent” is here to stay.
But can I propose something much more modest? Please? Lose the “mm.”
Just use “e.” Instead of 32mm equivalent, make it 32e.
You save some typing; it’s immediately clear that you mean equivalent and not true focal length; and you aren’t perverting the meaning of our faithful friend, the millimeter.
People seem perfectly happy learning new tech terms like 1080p and 3G; lets make “e” the new convention for expressing lens coverage.
Of course, optics for more technically-minded photographers could carry dual labeling, e.g. 21mm/32e. A third-party lens adapted for several DSLR systems would need a different “e” number for each different crop factor; but compared to swapping a whole lens mount, adding this number somewhere on the barrel seems trivial.
I’m not holding my breath. But I think photographers, both new and old, would find it much less confusing than the horrible system we’re using now.