why is 50mm lens called 50mm?

I've read Brandon post "how many feets.." i'am wondering if any one

knows why 50mm lenses are called 50mm and 100mm are 100mm?

<p>That's the focal length of the lens. It comes from the physics behind how a lens works.</p>

Please check <a href="http://www.dpreview.com/learn/?/Glossary/Optical/Focal_Length_01.htm">here</a>.

Steve i know that! and i asked about "the physics behind" lol

Thanks Philip , that was very helpful <br>regards

A 100mm lens used to be called a 10cm lens. Why did that change? We're only using mm now.

So long as it focuses at 50mm (ie it works as it is supposed to), I could'nt care less what they call it :-)

 

- Harman

"Why did that change?"<p>Bad publicist?

<I>"A 100mm lens used to be called a 10cm lens. Why did that change? We're only using mm now."</I>

<P>

Yeah, and they used to be called a 2 inch or a 4 inch lens (50mm and 100mm respectively). Things just change, I guess.

<P>

Actually, I think that specifying 13.5cm, etc. didn't sound as good as saying 135mm. It's just marketing. Maybe someday we'll all go back to cubits. :)

A "50mm" lens has the same magnification at infinity as a pinhole 50mm from a plane.

<i>A 100mm lens used to be called a 10cm lens. Why did that change?</i><p>

 

100 is a lot bigger than 10 and sounds much more impressive.

Actually, I think you should call it a "5 times ten to the minus two" metre lens, metres being the standard international unit of distance. This way there is no confusion when discussing focal lengths with people from around the world.

Even old 100mm was once 10cm

Why they changed it I can not say

People just liked it better that way.

 

Sorry.

"The focal length of a lens is defined as the distance in mm from the optical center of the lens to the focal point, which is located on the sensor or film if the image is "in focus". The camera lens projects part of the scene onto the film or sensor."

 

How come the mirror doesn't hit the optical center of a superwideanglelens?

>> How come the mirror doesn't hit the optical center of a superwideanglelens? <<

 

It does, sorta. But the optical center of a super-wide SLR lens is located behind the lens' rear element, so there's nothing physically there. :-)

 

-Dave-

<I>"...the optical center of a super-wide SLR lens is located behind the lens' rear element..."</I>

<P>

I believe that's called a "retrofocal" design, and is often used in lenses designed for an SLR/DSLR just for the purpose of missing the swinging mirror. This is also one of the reasons a rangefinder camera is often a better tool for wide lenses, although lens makers have gotten quite good a designing retrofocus lenses.

<P>

The opposite is a "telefocal" design, and is where the optical node is forward of the physical center of the lens, possibly even ahead of the front element. This allows the long lens to be physically shorter than it is optically and is very commonly seen in zoom lenses.

I think it makes more sense to say 14mm than 1.4 cm lens, no?

 

The cm was not a good idea to begin with since we are talking about relatively small numbers/sizes, when we talk about photo lenses.

 

Telescopes are a different thing altogether. You measure the unit that makes the most sense for the task.

I used to take photografs since 1965 (when i was a child), and so far as I remenber always the focal distance was mesured in mm (never in cm or in)Perhaps because the first camera lens was build in Europa when we used metric system. I realize this is uneasy in some countries, but so it is the left wheel driving for them.

>>I think it makes more sense to say 14mm than 1.4 cm lens, no?<<

 

I think the idea is to use integral designations and get rid of the decimal altogether. That way everyone need not try and understand the unit behind the designation, but it would be implicit. After all your 1.4cm lens could be someone else's .014 meter lens or another's .000014 km lens ! (And we would have frequent lists arguing who is correct)

 

- Harman

>>I think the idea is to use integral designations and get rid of the decimal altogether<<

 

Of course. And yes, as far as I can remember, being born in Italy, we have always referred to lenses in "mm", NOT "cm". I don't know about anywhere else.

Prior to WWII most lenses of European origin were labeled in cm rather than mm. My Zeiss rangefinder lenses from the 1930s & early '40s are all in cm. The 1950s versions are all in mm.

 

-Dave-

Many early Nikon F lenses are in cm; like my 5.8cm F1.4 normal; or the smaller 5cm F2; or longer 13.5cm F3.5; or 10.5cm F4. In July 1959 the formal definition for the foot and cm and inch got changed a few parts per million; with respect to another. US Surveyors still use the pre 1959 definition

Sorry to butt into your jolly conversation.

The reason for this change was the introduction of the International System (SI) some years ago, except in the US and one other country I cannot remember right now.

Before that we had the CGS System (cm, gram, second) where cm was indeed a valid unit and this system is still wiedely used in atom physics.

 

Should anyone be interested in more details about this, there is a very comprehensive explanation to be found in

 

http://physics.nist.gov/cuu/Units/units.html

>>Before that we had the CGS System (cm, gram, second) where cm was indeed a valid unit<<

 

I think you missed it by a mile :-) Actually the cm and mm are part of the same SI system that you mention. It a metric system where quatities can be inter-expressed in any of the convenient suffixes which are decimal based, such as km, cm, m, mm - they are all part of the same system.

 

The other system that you mention is based on the Foot-Pound-Yard system which is the Imperial system. Most countries, except for the US have left the F-P system.

 

But trust me the cm is very much part of the same system that mm is - its just the bigger brother, so to speak. Take a look here:

 

http://convert.french-property.co.uk/

 

- Harman

Non USA folks here seem confused :) cgs was the metric system in many books in the 1950's; and the mks was mentioned at times too. with the older cgs system; in Mechanical Engineering we used the dyne; centimeter and the erg. 1 joule is equal to 10 million ergs. a dyne centimeter is an erg. Before 1959 the meter was 39.37 inches ; afterwards 1 inch became 2.54 centimeters. ; the change is 2 parts per million., With a surveyors EDM; the calibration is with the older LEGAL unit 1/39.37 . A hand calculatorwith "units" uses the newer convention. In super accurate surveys; one must not use the incorrect conversion for surveys; with a built in calculator. <BR><BR>In Chemistry and scales; the gram has been accepted as proper in the USA forever.<BR><BR>Legally in most places in the USA; a grocery store cannot legally sell produce or meat just in grams or Kg; they will be shut down and jailed. <BR><BR>In US money; the metric system goes back along time; each bill since the 1930's weighs 1 gram; each nickle 5 grams. <BR><BR>In gasoline; a US gallon is legally 231 cubic inches.