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I noticed on my Canon 18-55mm kit lens there is a dot on the zoom scale between 24 & 35mm (though skewed very much towards 24mm).

This position appears to be the spot where the lens is physically the most compact, but I have a hard time believing that there would be a marking just for that. What is this dot for?

18-55mm lens

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1  
What about the flag next to 18mm? –  dunxd May 16 '11 at 20:50
3  
@dunxd I think that just marks where the 18 mm really is (IIRC, it isn't straight down the "18") –  koiyu May 16 '11 at 20:56
1  
Yeah, the flag is because it doesn't quite turn all the way to where the 18 is at. –  speshak May 17 '11 at 2:06
3  
They have corrected the problem in version II of this lens, the dot is no longer there and you can freely get back to taking pictures ;) –  Itai May 17 '11 at 3:24

4 Answers 4

up vote 6 down vote accepted

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.

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3  
+1 for bringing some data to the table! Someone with a copy of the 24-105mm ought to be able to test the least extension at 28mm hypothesis - hopefully they can post a comment here with the result. –  fmark May 17 '11 at 16:24
    
I would say that some lenses having the mark, but it not being at 28mm, does not necessarily rule out that it means 28mm on the lens pictured in the original question (which, given my experience, it definitely does.) I think the reason for a dot, rather than the full number 28, was ultimately a space issue. The 28mm focal length is pretty common (unlike 27mm), however it is very near to 24mm...and printing '28' on the lens might end up confusing users to thinking they are at 28mm, when they are actually at 30mm. –  jrista May 18 '11 at 0:23
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Even if this dot did show up on other lenses without a 28mm focal length, I would assume it was meant for the same purpose...to indicate a common and useful focal length that couldn't otherwise be printed in an accurate location due to space limitations. –  jrista May 18 '11 at 0:23
    
Sorry - is there a lens that's got the mark where it's not at 28mm? The only two where I saw the mark both have it at 28mm, plus the Sigma lenses, from what I understand, too. The mark on the 24-105 L is a little misleading at first glance, but remember that the little flag icon actually represents 24mm, so the dot right next to it does, indeed, appear to be at 28mm. –  D. Lambert May 18 '11 at 1:38

Some discussion suggests it marks 28mm focal length.

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Just for reference, I have removed several comments here that were not useful in answering this question. –  chills42 May 23 '11 at 19:50

I believe this mark is intended to indicate 28mm (just zoom to the dot and take a photo, you should see 28mm in EXIF.) In general, I think the mark is also intended to indicate the point of least extension (based on anecdotal evidence I have from using three of the pictured lenses), however there does appear to be some variation, with least extension occurring between 27mm and 31mm.

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1  
@jr If that is the case on your copy of the lens, then there must be considerable manufacturing variation, because the point of least extension on mine is 31 mm--the setting shown in the photo. It's roughly 1 mm longer at the mark, a visible difference. Do you have a reference for "by design"? I looked on the Canon site but couldn't find any such information. –  whuber May 17 '11 at 2:43
    
I've had two copies of this lens and used a friends that came with his 500D, and both encountered least extension at that dot, or ever so slightly after (28.5-29mm?) I know its 28mm because if I take a photograph and check the exif, it says 28mm when I zoom to the dot. I am pretty sure the dot specifically means 28mm by design, however regarding the extension, my information is anecdotal. It does seem that the lens "bottoms" at the mark, and remains there till 30-31mm, then extends again. The photo appears quite a bit LESS extended than my lenses have ever reached...not sure why. –  jrista May 17 '11 at 3:05
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I just had a though, and checked the back element. Between 28mm and 31mm, the back element on my current copy moves quite a bit, while the front element stays "bottomed"...then both the front and back elements move towards the front of the lens. There may indeed be manufacturing variation, although so much variation seems unusual. –  jrista May 17 '11 at 3:11
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@jr nice observation about the back element. The lens has a gritty feel and doesn't move easily, so some variation wouldn't be surprising at all. Out of curiosity, why do you suppose Canon would want to mark the point of least extension? To make it easy to pack up? (All my other zooms are fixed length and don't have any such dot, which is suggestive...) –  whuber May 17 '11 at 5:22
    
@Whuber: Not really sure, but thats as good an explanation as any. All of my other lenses except a telephoto are all internally focusing/zooming, and none of them have a dot either. –  jrista May 17 '11 at 5:53

The dot I think is where the lens is the most compact. or the smallest. notice that in 55mm it extends at its longest, then when you go down to 28mm, it's the shortest and extends a little bit at 18mm.

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