Why millimetres?


HarisF1

Link Posted 29/10/2018 - 16:48
Here's a random thought I've just had - it's peculiar that we refer to focal lengths in millimetres instead of centimetres. I've got a couple of ideas as to why, but it's still funny.

I mean, doesn't anyone else want to buy a new D-FA 7-20cm f/2.8 lens?

derek897

Link Posted 29/10/2018 - 16:57
Too many inbetween focal lengths, I'd say.
Just off the top of my head, there's
14
15
16-
17-
18-
19-
21
24-
28-
35-
43
55
75
77
105
What were they before everyone went metric ?
I know what i like, If not always why.

johnriley

Link Posted 29/10/2018 - 17:17
A 50mm lens is a 2 inch lens, a 150mm lens would be a 6 inch lens. Or we could use 5cm and 15cm, and usually on pre-war lenses you are likely to find focal lengths in cm. Post-war we went to mm.

It doesn't really matter as long as we have consistency and we all know what we are talking about.
Best regards, John

MrB

Link Posted 29/10/2018 - 17:49
And print sizes are often quoted in inches, e.g. - 16x12, 12x8, 10x8, 8x6, 7x5 and 6x4.

Philip

petrochemist

Link Posted 29/10/2018 - 18:00
Millimeters are the preferred SI unit of length that is appropriate for typical lenses. Centimeters while still a recognized SI unit have not been a preferred unit since the old CGS reference was replaced by MKS back in the 1940s. The full SI system being adopted in the 1960s
Older lenses are often marked in cm or inches. Using the base unit of the meter would give us nasty decimals for nearly all lenses. (Less than 1% om my 'lenses' have round figure focal lengths in meters, both of these are actually telescopes one 1m-4m with the eyepiece & the other fixed at 1m without an eyepiece).

Preferred multipliers are multiples of 1000, giving us:
Kilo, Mega, Giga, Terra, Penta... going up
& milli, micro, nano, pico, femto... going down.
Mike
.
Pentax:K5ii, K7, K100D, DA18-55, DA10-17, DA55-300, DA50-200, F100-300, F50, DA35 AL, 4* M50, 2* M135, Helicoid extension, Tak 300 f4 (& 6 film bodies)
3rd Party: Bigmos (Sigma 150-500mm OS HSM),2* 28mm, 100mm macro, 28-200 zoom, 35-80 zoom, 80-200 zoom, 80-210 zoom, 300mm M42, 600 mirror, 1000-4000 scope, 50mm M42, enlarger lenses, Sony & micro 4/3 cameras with various PK mounts, Zenit E...
Far to many tele-converters, adapters, project parts & extension tubes etc.

.[size=11:].Flickr WPF Panoramio
Last Edited by petrochemist on 29/10/2018 - 18:03

pschlute

Link Posted 29/10/2018 - 18:13
Going off on a tangent here , but it always amuses me that newspapers (and online media) in the UK when reporting on an aircraft story, almost always report the altitude in metres. This is despite the fact that the entire world, with the exceptions of Russia and China fly to the altitude measurement of feet.
Peter



My Flickr page
Last Edited by pschlute on 29/10/2018 - 18:13

johnha

Link Posted 29/10/2018 - 18:20
Most consumer zooms don't match their descriptions anyway - many of the xx-300 zooms from the '90s only went to about 265mm when tested. As John points out, many pre (and possibly post) war lenses were identified in centimetres (and are still advertised as 2.8, 3.5 or 5cm lenses by some dealers). Many Russian lenses are known by their names rather than focal lengths - Jupiter-8, 9, 11 or Helios M44-2 (which was 58mm) for example.
PPG Flickr
Last Edited by johnha on 29/10/2018 - 18:41

johnha

Link Posted 29/10/2018 - 18:29
pschlute wrote:
Going off on a tangent here , but it always amuses me that newspapers (and online media) in the UK when reporting on an aircraft story, almost always report the altitude in metres. This is despite the fact that the entire world, with the exceptions of Russia and China fly to the altitude measurement of feet.

Aviation is a funny one, altitude is in feet, speed in knots & Mach, visibility in metres, distance in Nm, fuel volume is measured by weight and drinks are often measured by the Oz.

Similar examples are when they use both but round one up/down incorrectly: "1000lb (455Kg) WWII bomb destroyed by controlled explosion in . The 455Kg (986lb) bomb was detained by the Army's bomb disposal team..."
PPG Flickr

stub

Link Posted 29/10/2018 - 18:32
Not heard any mention in the current Brexit stategy to make any amendments....!!
K-1Gripped K-1 ungripped K-5ii K7 Various lenses

Stuart..
Last Edited by stub on 29/10/2018 - 18:32

pschlute

Link Posted 29/10/2018 - 18:40
johnha wrote:
Most consumer zooms don't match their descriptions anyway - many of the xx-300 zooms from the '90s only went to about 265mm when tested.

Indeed and even zooms of today can have a much shorter FL when close focussing due to focus-breathing
Peter



My Flickr page

Kim C

Link Posted 29/10/2018 - 19:03
pschlute wrote:
Going off on a tangent here , but it always amuses me that newspapers (and online media) in the UK when reporting on an aircraft story, almost always report the altitude in metres. This is despite the fact that the entire world, with the exceptions of Russia and China fly to the altitude measurement of feet.

Maybe the flying has to do with the fact that the aviation industry worldwide use English as a language.

Kim C

Link Posted 29/10/2018 - 19:11
johnha wrote:
pschlute wrote:
Going off on a tangent here , but it always amuses me that newspapers (and online media) in the UK when reporting on an aircraft story, almost always report the altitude in metres. This is despite the fact that the entire world, with the exceptions of Russia and China fly to the altitude measurement of feet.

Aviation is a funny one, altitude is in feet, speed in knots & Mach, visibility in metres, distance in Nm, fuel volume is measured by weight and drinks are often measured by the Oz.

"

Not funny really. Actually there is a great deal of logic behind it. Knots are nautical miles per hour. This makes it much easier in long distance flight because the way lines of latitude are drawn. Separating altitude in feet and vis in metres means there is much less confusion on both the departure and arrival especially when English is not the native tongue. On a modern airliner, nearly half the weight on take off is fuel. The weight of the aircraft is critical the speeds etc. At altitude flying mach is critical not only because most fly close the the transonic region. It also make navigation much easier. 0.8 Mach = 8 miles per minute.

Never drank whilst flying so that one is useless. lol

DOIK

Link Posted 29/10/2018 - 19:19
johnha wrote:
The 455Kg (986lb) bomb was detained by the Army's bomb disposal team..."

I wonder what the charge was.....Boom Boom

John
My Photobucket

pschlute

Link Posted 29/10/2018 - 20:09
Kim C wrote:
Actually there is a great deal of logic behind it. Knots are nautical miles per hour. This makes it much easier in long distance flight because the way lines of latitude are drawn.

The main reason is that early aircraft navigation relied on the same ideas as navigation at sea. Thus knots as the standard measurement of speed. Aircraft up to the VC10 still had a sextant installed for navigation if so required. Sometimes it was.
Peter



My Flickr page

Kim C

Link Posted 29/10/2018 - 20:14
pschlute wrote:
Kim C wrote:
Actually there is a great deal of logic behind it. Knots are nautical miles per hour. This makes it much easier in long distance flight because the way lines of latitude are drawn.

The main reason is that early aircraft navigation relied on the same ideas as navigation at sea. Thus knots as the standard measurement of speed. Aircraft up to the VC10 still had a sextant installed for navigation if so required. Sometimes it was.

So did a few others. Always a great source of fun for pilots. As soon as the nav said to keep it steady so he could take a "shot", the autopilot came out.
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