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Lens dot, what's it for?

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Thread started 23 Apr 2008 (Wednesday) 17:57   
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parsnip
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I notice on my 18-55 there's a dot just after 24 on the focal length indicator on the lens. EXIF shows it has 27mm focal length, is there a significance to the dot?

Post #1, Apr 23, 2008 17:57:22


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Rubberhead
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Good question - I've wanted to know about the dot and the flag looking thing too.

Post #2, Apr 23, 2008 17:58:50


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Red ­ Dot
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it indicates where the lens is compacted most. at 55 it extends, and at the dot it is smalles. the flag is usually when the compactness is achieved at the end of the focal range.

Post #3, Apr 23, 2008 18:01:08


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cito17
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I looked on mine and it's the spot were the lens is most retracted.

Post #4, Apr 23, 2008 18:01:20 as a reply to Rubberhead's post 2 minutes earlier.


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parsnip
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yeah the flag is the beginning of the focal range on mine so it indicates 18

Mine isn't most retracted at the dot though, it's most around 30

Post #5, Apr 23, 2008 18:04:28 as a reply to cito17's post 3 minutes earlier.


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parsnip
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I just read a review saying 28 mm is the most common wide angle focal length, could this be a 28mm indicator?

Post #6, Apr 24, 2008 07:11:35 as a reply to parsnip's post 13 hours earlier.


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Rankinia
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28mm is a lovely focal length, but that is the equivalent for an ef-s lens is approx 17mm if i remember right.

Adam

Post #7, Apr 24, 2008 09:12:42


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timnosenzo
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parsnip wrote in post #5393755external link
I just read a review saying 28 mm is the most common wide angle focal length, could this be a 28mm indicator?

This would be my guess. My 24-105 has a dot at what looks to be 28, but its most retracted at 24.

Post #8, Apr 24, 2008 10:01:11


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Stocky
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I agree that its the (approximate) spot where the lens is its shortest physical length. I also realized that I don't have a manual for that lens. Apparently the kit didn't come with one. Unfortunately this is my only zoom lens (1 going on 5), but I see from pictures that the 24-70 and 17-55 both have the flags on them and the 24-70 has a dot as well (right next to the flag at the short end of the focal length). The 70-200 series do not seem to have any of these markings. I will be very interested to hear an explanation for all of this if there is one.

Post #9, Apr 24, 2008 10:49:12


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parsnip
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Rankinia wrote in post #5394296external link
28mm is a lovely focal length, but that is the equivalent for an ef-s lens is approx 17mm if i remember right.

Adam

That's true due to the 1.6 factor, it's about 17.5mm on an XSI although I guess the factor is camera dependent. So the dot may be more useful on a full frame camera

Either way the theory it is the shortest physical lengths doesnt seem to work across lenses, plus when you contract the lens focusing also affects the length

Post #10, Apr 24, 2008 15:10:05


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msowsun
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parsnip wrote in post #5396257external link
Either way the theory it is the shortest physical lengths doesnt seem to work across lenses, plus when you contract the lens focusing also affects the length

My Sigma 17-70 also has a white dot just between the 24 and 35. This lens only gets longer as it zooms out from 17 to 70. (the shortest physical length is at 17)

Post #11, Apr 24, 2008 16:32:23


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Chandler.
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Stocky wrote in post #5394790external link
I agree that its the (approximate) spot where the lens is its shortest physical length.... I see from pictures that the 24-70 and 17-55 both have the flags on them and the 24-70 has a dot as well (right next to the flag at the short end of the focal length). The 70-200 series do not seem to have any of these markings. I will be very interested to hear an explanation for all of this if there is one.

The 24-70L does not have any flag or dot markings. Besides, it's a reverse zoom lens and is longest at 24mm, and shortest at 70mm.

The 70-200Ls are all internal focus lenses, so they wouldn't need a dot to determine the shortest physical length.

Post #12, Apr 24, 2008 16:42:11


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20droger
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parsnip wrote in post #5390489external link
I notice on my 18-55 there's a dot just after 24 on the focal length indicator on the lens. EXIF shows it has 27mm focal length, is there a significance to the dot?

For the 18-55mm lens, the dot represents "28" on the focal-length scale. That means the scale reads "18 24 [28] 35 55". Even though the 28mm position has no real significance for an EF-S lens, it is still a very popular size, historically.

The lens instruction sheet came with our camera-lens kit, but does not specifically say anything about the dot. When I carefull set the lens to the dot, the measured focal length (on our lens) is 27.6mm. This lens has very high tolerances, however, and I would not be surprised at anything between 26mm and 30mm.

On the EF 28-135mm f/3.5-5.6 IS USM lens, there is a little "flag" just left of the 35mm marking, between the numbers "28" and "35". This flag is really a square dot indicating the position of 28mm, with a small line extending to the number "28".

These dots and similar tricks are used on some lenses to include numbers that simply will not fit, as numbers, in their proper positions. These dots or "flags" have absolutely nothing to do with the extension of the lens.

Hope this helps.

Post #13, Apr 24, 2008 17:04:59


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parsnip
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20droger was correct about the dot

Official canon response...

"The dot on the lens shows where 28mm is located."

Post #14, Apr 24, 2008 17:27:12


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number ­ six
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parsnip wrote in post #5397194external link
20droger was correct about the dot

Official canon response...

"The dot on the lens shows where 28mm is located."

And why do we care? Because 28 mm is the "normal" focal length on a 1.6 crop camera?

That's what I've been assuming...

-js

Post #15, Apr 24, 2008 19:22:26


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