28mm dot on Sigma zooms

Started Jan 29, 2007 | Discussions
kirbysdl
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28mm dot on Sigma zooms
Jan 29, 2007

On my Sigma 17-70, there's a little dot on the barrel's focal length scale at 28mm. There's also one on the 18-200 lens, and other Sigma DC zooms print the actual numbers instead of using a dot. Why is 28mm so important? On a Canon 1.6x sensor, its 35mm film equivalent length is only 44mm, so it's not even an indicator of the "standard film" 50mm view.

SNGX1327
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Re: 28mm dot on Sigma zooms
In reply to kirbysdl, Jan 30, 2007

not sure. on the canon kit lens, 28mm is marked as the focal length where the lens is the shortest physically. that particular lens extends when zoomed to any of the extreme focal lenghts

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nickleback
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Re: 28mm dot on Sigma zooms
In reply to kirbysdl, Jan 30, 2007

kirbysdl wrote:

On my Sigma 17-70, there's a little dot on the barrel's focal
length scale at 28mm. There's also one on the 18-200 lens, and
other Sigma DC zooms print the actual numbers instead of using a
dot. Why is 28mm so important? On a Canon 1.6x sensor, its 35mm
film equivalent length is only 44mm, so it's not even an indicator
of the "standard film" 50mm view.

28mm is the approximate diagonal of an APS-C sensor.

Standard view on a camera is where the diagonal measure of the capture medium equals the focal length. 35mm diagonal is 43.3mm, pretty darn close to 44mm. 50mm is used as the "standard" lens on most SLRs because that's about the shortest near-symmetrical design you can do on a 35mm SLR, which makes it inexpensive.

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kirbysdl
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Re: 28mm dot on Sigma zooms
In reply to nickleback, Jan 30, 2007

nickleback wrote:

Standard view on a camera is where the diagonal measure of the
capture medium equals the focal length.

A-ha, very concise. So I guess the reason old 35mm film cameras came with 50mm kit lenses is because 35mm lenses were too expensive to make well.

Thanks!

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nickleback
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Re: 28mm dot on Sigma zooms
In reply to kirbysdl, Jan 30, 2007

kirbysdl wrote:

nickleback wrote:

Standard view on a camera is where the diagonal measure of the
capture medium equals the focal length.

A-ha, very concise. So I guess the reason old 35mm film cameras
came with 50mm kit lenses is because 35mm lenses were too expensive
to make well.

The true normal for 35mm format is 43.3mm, so substitute "43 or 44mm lenses" for "35mm lenses" in your statement above and it is correct.

Thanks!

You're welcome!

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strikeback03
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except...
In reply to nickleback, Jan 30, 2007

lenses from long before digital also have the dot. Here is an EF 24-85 originally marketed in Sept 1996.

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nickleback
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Re: except...
In reply to strikeback03, Jan 30, 2007

strikeback03 wrote:

lenses from long before digital also have the dot. Here is an EF
24-85 originally marketed in Sept 1996.

Lack of space to write 28? Beats me. I was just commenting on 28 actually having a little bit of significance. Don't know if that's why it is a dot on the barrel.

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strikeback03
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Re: except...
In reply to nickleback, Jan 30, 2007

I'd assume that for whatever reason it is just tradition to mark 28 - if the numbers don't fit then with a dot. Also I don't believe it is the point of minimum extension on the kit, IIRC mine was shortest at around 33mm.
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kirbysdl
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Re: except...
In reply to strikeback03, Jan 30, 2007

Yeah, the 17-70 is shortest at 17mm grows continuously until 70. Thanks again for all the replies. I'm gonna shoot a bit at 28mm so I can grow accustomed to what a 28/30mm prime should look like.

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