Lens length

[dhugo3]

What is a 3 inch lens in terms of millimeters? How do you covert them?

Thanks

[Steve Feldman]

76.2mm

[Vaughn]

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]

inch = about 2.54 cm and 1cm = 10mm

[dhugo3]

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]

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

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]

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]

Can anyone explain the Kodak Ektars engraved 127mm & 203mm (great little lenses).
Pete.

[eddie]

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]

*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.
Regards
Bill