“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.


5 Responses to ““35mm Equivalents”: A Plea For Clarity”

  1. Hastings Says:

    I totally agree with you. One thing camera manufacturers can do, is make all D-cams with 35e sensors. I would trade the useless “bells and whistles” for a full sensor any day.

    • petavoxel Says:

      Heh, although I’d prefer to call that “24 x 36mm sensors.” Because of course, nothing actually measures 35mm in a digital SLR any more.

      It’s even ahistorical to call them “full frame” sensors. Originally 35mm was movie film, and the long dimension of the frame went across it. The Leica first popularized 24 x 36mm —what was known then as the “double frame” format 😉

  2. […] that Ricoh is expressing all these focal lengths in “35mm equivalents.” As I’ve ranted about before, this is a needlessly-confusing convention which perverts the actual meaning of the word […]

  3. Kragom Says:

    Haha, this made me laugh. It surprises me you spend time to write this long article about this “issue”. There was actually a photo magazine in Sweden that started using angles instead of mm. they had to quit it when the readers rebelled against it, so it is quite clear where the proficient users stand. For anyone just wanting a camera, they will have a look in the camera and decide if they like it or not.

    For me, I am interested in the sensor size and its qualities. Based on the size I know what kind of lenses I want on it. 35mm equivalent does not bother me at all, but I can see how it confuses or annoys some people. We all have our foibles, I for one have a problem with people saying it is -40C “with the wind chill factor”. For me that was always bullshit, temperature is one thing and wind another, and you have different ways you can tackle them, but I guess it is nice if you want to give as high a number as possible :D. Ok, I am clearly digressing here.

    We will probably have to wait to see anything above “full frame” in DSLRs, as there is the constraint of the lens where any bigger sensors would need differently constricted lenses, and seeing how well an APS-C sensor such as that in the Canon 7D performs, I do not think that will happen any time soon, Leica S2 being an exception.

    • petavoxel Says:

      For older photographers who worked with different film formats (35mm, 120, 4×5) it was second nature to understand that focal lengths must scale up in proportion to the image diagonal. The problem is that today, focal lengths are being quoted in a chaotic mix of actual and “equivalent” millimeters.

      People have to guess at which meaning is intended from context—if they even understand the distinction. Recent discussions of the Noktor 50mm lens for µ4/3 (“is this going to be 50mm or 100mm“) show that the ambiguity is indeed still confusing people.

      About sensor formats… I once was among those who hoped all DSLRs would “grow up” to using 24×36 mm sensors. But as you say, APS-C has shown itself to be highly competent, at a significantly lower sensor cost. (Maybe those who do truly need 40 Mp can buy this.) So the “growing up” really must be going forward with lens options natively designed for that format.

      At that point, perhaps we can get used to a new way of expressing focal lengths: 30mm is a normal lens, 60mm is a portrait lens, etc.

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