What does the dot between 24mm and 35mm on some zoom lenses indicate?

Question

On my Canon 18-55mm kit lens, there’s a small dot on the zoom scale between 24mm and 35mm, closer to 24mm. I noticed the lens also seems to be physically shortest around that position. What is this mark for?

user269 · Accepted Answer

In addition to the link dunxd posted, here's another one discussing the same sort of dot on a Sigma lens (I saw this on a couple other Sigma lenses, too). The DPReview examination of the 18-55 lens offers the same explanation. There's mention in dunxd's link about the 28mm explanation being the "official Canon response", but I can't see anything that traces this statement back to Canon.

Looking at other Canon lenses, this isn't a very common feature - I also found this mark on the following lens:

EF 24-105mm f/4 L IS USM

I was not able to find a 28mm mark on:

EF 8-15 L
EF-S 10-22 f/3-4.5 USM
EF-S 15-85 f/3.5-5.6 IS USM
EF 16-35 f/2.8 L USM (28mm is marked explicitly w/ a number)
EF 17-40 f/4 L USM - EF-S 17-55 f/2.8 IS USM (28mm is marked explicitly w/ a number)
EF-S 17-85 IS USM
EF-S 18-55 f/3.5-5.6 IS II (There's a review on The Digital Picture that shows the I-series and II-series side-by-side)
EF-S 18-135 f/3-5.6 IS
EF-S 18-200 f/3.5-5.6 IS
EF 20-35 f/3.5-4.5 USM (28mm is marked explicitly w/ a number)
EF 24-70 f/2.8 L USM (28mm is marked explicitly w/ a number)
EF 28-105 f/3.5-4.5 II USM
EF 28-135
EF-S 55-250
EF 70-300 IS USM
EF 70-300 DO IS USM

Note that I checked some lenses that don't cover the 28mm focal length. If any of these had shown the mark, that would certainly have ruled out the 28mm explanation. Also note that Canon appears to have removed this mark ob the II-series of the 18-55 lens, indicating that 28mm is an interesting, but not critically important measure.

I found observations supporting both for and against the "least extension" idea.

What does the dot between 24mm and 35mm on some zoom lenses indicate?

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