I invented this term to express how loosely or tightly zoom rings space focal lengths.
An octave is a doubling. It comes from music, where it represents a doubling of frequency. In photography we have a similar unit of doubling called a stop. I didn't use stop, since we use that for exposure and it would confuse things.
A cm is a centimeter. It's a unit of length. There are 2.54 cm in an inch.
Measure the distance between two focal lengths double (or half of) each other on a zoom ring, for instance, between 50mm and 100mm. Measure it in centimeters, and that's cm/octave.
The more cm/octave, the easier it is to set small differences in focal lengths.
Some lenses are good and have a lot of spacing. Others are bad, and cram them together at one end or the other of the zoom range.
Most lenses, like the Nikon 18-200mm VR, run about 2cm/octave. This is fine, especially if the entire zoom range runs at this spacing without bunching up at either end, as it does on the 18-200mm VR.
Things become very cramped at less than 1cm/octave, like the Nikon 18-70mm at the wide end. Lenses like that drive me up a wall because it's almost impossible to set exact focal lengths (cropping) when they are so close together.
Zoom spacing is a significant factor for me when I buy a lens. I won't buy a lens that's difficult to operate.
For those of you who love calculus, you can plot this versus focal length. Ideally the plot is flat, showing the same spacing along the zoom ring. Poorer lenses change the zoom rate along the ring.
Zoom rings should not have equal spacing per millimeter of focal length. That's a big mistake. If they do this, the wider end becomes very cramped the the longer focal lengths become too spaced out. For you familiar with math, the best zoom rings run logarithmically: equal spacing per the same percentage change in focal length.
If you find this as helpful as a book you might have had to buy or a workshop you may have had to take, feel free to help me continue helping everyone.
Thanks for reading!
Caveat: The ads below come from a third party and I don't see or approve them. They are sent to your screen directly from a third party. They don't come from me or my site. See more at my Buying Advice page. Personally I get my goodies at Ritz, Amazon and Adorama.