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dhugo3
14-Dec-2010, 20:03
What is a 3 inch lens in terms of millimeters? How do you covert them?

Thanks

Steve Feldman
14-Dec-2010, 20:09
76.2mm

Vaughn
14-Dec-2010, 20:11
google for a converter, but otherwise:

1 inch = 25.4 mm

so

3 inches = 3 x 25.4 = 76.3mm

and divide millimeters by 25.4 to find inches

But I just try to remember the lenses I have: 150mm lens is about the same as 6 inches, so my 300mm is 12". Other lenses of mine are 19" is 480mm, the 24" is 600 (actually closer to 610mm, but close enough!), my 210mm is close to 8", and the 6.24" is 159mm.

Heespharm
14-Dec-2010, 20:12
inch = about 2.54 cm and 1cm = 10mm

dhugo3
14-Dec-2010, 23:48
OK, I should have also asked why. Why are some lenses in mm and others in inches? Does it have to do with the age of the lens or...?

Thanks

Emmanuel BIGLER
15-Dec-2010, 06:44
Why are some lenses in mm and others in inches? Does it have to do with the age of the lens or...?

Hello from the country where the Metric System was invented :D

Before World War Two, focal lengths were engraved in the unit of length actually in use in the country of manufacture.

Continental-European lenses, before WW-II, were engraved in centimetres. British lenses were engraved in Imperial units. And American lenses... well I do not know if "Imperial" is acceptable for anything manufactured in the US since 1774, so let's say : in American units. ;)

After WW-II, most Continental-European lenses switched their engravings to millimetres. I do not know why.
Probably some British lenses continued to be engraved in inches after WW-II. Dan Fromm ! Help !
American lenses for the domestic market, or US military lenses had no need to be engraved in millimetres. To the best of my knowledge, in the US only scientists of the civilian world use millimetres in their day-to-day use.
Plus some other citizen : when I stayed in the SF Bay area in 1990, I was very surprised to see Tour-de-France videos on display at the local bike shop as well as metric sets of wrenches for bike repair ! Metric sets labeled in millimetres, of course ;)

Post-scriptum : actually a focal length of 3 inches corresponds to a 75 mm "commercial focal length". The true focal length expressed within a fraction of a millimetre can be found in manufacturers' data sheet, and is neither 75 nor 76.2, and the "commercial-focal-lens-inch" is sometimes equal to 25 mm.
But not always, for example a 355 G-Claron is almost exactly a 14 inch lens ;)
And the "360 mm" Schneider tele-arton is in fact a 353....

Dan Fromm
15-Dec-2010, 08:12
Emmanuel, I don't know the date but sometimes in the 1950s the UK switched to metric.

I have 10.16" f/9 Taylor Hobson copying lens serial number 593611, 30 cm f/9 TTH Apotal s/n 670961, have had a pair of 12"/4 TTH Telephoto lenses serial numbers 668096 (I think Mr. Galli has this one now) and 689053 (I still have it). The 12"/4 teles were made for an aerial camera; air forces are very conservative, this may explain why the later one's focal length is in inches, not cm or mm.

Similarly, some years ago I saw a 152/1.9 Dallmeyer Super Six s/n 673640 on eBay. My 6"/1.9 Super Six (sold long ago, I needed the money it brought more than the lens) was s/n 495113.

Pete Watkins
16-Dec-2010, 01:41
Can anyone explain the Kodak Ektars engraved 127mm & 203mm (great little lenses).
Pete.

eddie
16-Dec-2010, 05:22
Emmanuel, I don't know the date but sometimes in the 1950s the UK switched to metric.

*sigh* i wish the USA switched when they threatened to do so in the 70s. fractions suck bad! inches, feet, miles, pounds, all suck bad!

1 liter of water weighs 1Kg. how nice is that....and then all is divisible/multiply by 10.

oh the simpleness it could be.....

cowanw
16-Dec-2010, 07:10
*sigh* i wish the USA switched when they threatened to do so in the 70s. fractions suck bad! inches, feet, miles, pounds, all suck bad!

1 liter of water weighs 1Kg. how nice is that....and then all is divisible/multiply by 10.

oh the simpleness it could be.....

True but for bellows factors 11", 16" etc is handy if you think that way.
I find that as I get more into older lenses I am converting my mm modern lenses to inches.:rolleyes:
Regards
Bill

David Lindquist
16-Dec-2010, 11:06
Can anyone explain the Kodak Ektars engraved 127mm & 203mm (great little lenses).
Pete.

I have a hunch Kodak's reasons for that are lost to the mists of time. Just checked my Kodak lenses:

The 1948 Wide Field Ektar is marked "250 mm." and in parentheses "(10 in.)"
The 1958 Commercial Ektar is simply marked "14 in.", with no metric equivalent
The 1959 f/7.7 Ektar is marked "203 mm." with no inch equivalent.

Not exactly a consistent pattern here.
David

Jack Dahlgren
16-Dec-2010, 13:28
*sigh* i wish the USA switched when they threatened to do so in the 70s. fractions suck bad! inches, feet, miles, pounds, all suck bad!

1 liter of water weighs 1Kg. how nice is that....and then all is divisible/multiply by 10.

oh the simpleness it could be.....

A pint is a pound, then all you need to do is divide by 16. :-)

domaz
16-Dec-2010, 14:44
A pint is a pound, then all you need to do is divide by 16. :-)

And a mile is 5280 feet. Of course! How elegant...

Leigh
16-Dec-2010, 16:13
1 inch = about 2.54 cm
Nope.

1 inch = EXACTLY 2.54 cm, by definition.

- Leigh

Brian C. Miller
16-Dec-2010, 20:28
A pint is a pound...

<joke> Could one of the lads over in the UK please go down to the pub and verify a pint? I see it's reportedly that way in Ruddles (http://www.pintprice.com/city.php?/ruddles/United Kingdom/gbp.htm), but they might've downed a few before reporting in. </joke>

Jim Jones
17-Dec-2010, 06:45
And a mile is 5280 feet. Of course! How elegant...

And a mile is also 8 furlongs, 320 rods, 1760 yards, 5280 feet, 21,120 hands, and 63,360 inches. The first four were commonly used in rural America when I was growing up. I've only seen hands used to measure the shoulder height of horses, and never seen inches used for really long measurements. Surveyers also used chains and links. There is more logic to the English system of measurements than is immediately apparent, just as there is some logic to the characters used to write Chinese. However, the Chinese could be very elegant in their caligraphy.

Ernest Purdum
17-Dec-2010, 09:50
Thomas Jefferson advocated use of a decimal system of measurements. We didn't follow his suggestion, so now everybody is out of step except us.

When I was making metal things for a living, we had to keep the ability to work in both centimeters and inches. If I was working to customer design it was usually inches, unless the customer was an auto maker. If my own design, it was to metric dimensions.

I think Americans are afraid of having to constantly convert. You don't. You just lay a metric rule on the object to be measured.

Leigh
17-Dec-2010, 10:11
I just can't fathom all the different units of measurement. ;) :p

- Leigh

Pete Watkins
17-Dec-2010, 12:18
Leigh,
You need to think more deeply (:-))
Pete.

Helen Bach
18-Dec-2010, 06:17
...There is more logic to the English system of measurements than is immediately apparent...

I agree, as long as you stay with one property (dimension). It's a natural sort of a system, with units being matched to their practical use at one time or another - an acre is a chain by a furlong, and this recalls a relationship with working the land before industrialisation.

Where it falls apart is in its failure to be systematic - when you start to combine fundamental properties (mass, length and time) to make others (power, force etc), and to compare. Then the advantages of SI (not just 'metric') really help with calculation and comparison.

Best,
Helen